Plasmodium species (Malaria)

Parasitology

Malaria is the most important parasitic infection of man. It is caused by the protozoan parasites Plasmodium falciparum, P. vivax, P. malariae or P. ovale. Malaria is transmitted by the bite of a sporozoite-bearing female anopheline mosquito. After a period of pre-erythrocytic development in the liver, the blood stage infection, which causes the disease malaria, begins. Illness and fever do not occur until a critical density of parasites has been reached in the blood (60). In non-immune subjects this "pyrogenic density" is reached at an average of 10,000/µL for P. falciparum and 200/µL for P. vivax (60,62,76), although in individuals much lower densities may be symptomatic.

Life Cycle: Following inoculation by the female anopheline mosquito during a blood meal the injected sporozoites target the liver. Hepatocytes are invaded within 60 minutes of inoculation. The intra-hepatic stage of development lasts between five and seven days before rupture of the infected hepatocytes and liberation of large numbers (circa 30,000/cell) of merozoites into the blood stream (41). These merozoites then immediately invade circulating erythrocytes. The asexual cycle lasts approximately 48 hours in P. falciparum, P. vivax, and P. ovale infections, and 72 hours in P. malariae infection (42). In the initial phase the infection expands exponentially at approximately 6-20 fold per cycle. In most cases of falciparum malaria and all infections caused by P. vivax, P. ovale or P. malariae the blood stage infection stops expansion and parasite numbers stabilize when less than 3% of the erythrocytes are parasitized. After approximately one week of blood stage infection a sub-population of parasites undergoes sexual differentiation and sexual stages (gametocytes) are formed. These will transmit the infection if a feeding female anopheline mosquito takes up mature gametocytes. The male and female gametocytes fuse in the mosquito's mid-gut to form a zygote. The zygote then encysts in the gut wall of the mosquito, forming an oocyst. This expands as the parasites multiply and finally ruptures, liberating myriads of sporozoites into the coelomic cavity of the mosquito. The sporozoites then migrate to the mosquito salivary glands to await inoculation and transmission of the infection to another individual and thereby to complete the transmission cycle.

Video: Wehi. 

Video: Malaria Life Cycle in the Human Host. 

Video: Malaria Life Cycle in the Mosquito Host. Walter & Eliza Hall Institute of Medical Research

Epidemiology

Approximately 250 million people suffer from malaria, and between one and two million die each year. The majority of deaths occur in African children and nearly all are caused by Plasmodium falciparum. Malaria occurs throughout most of the tropical regions of the world. P. falciparum predominates in Africa, New Guinea, and Haiti while P. vivax is more common in Central America and the Indian subcontinent. The prevalence of these two species is approximately equal in South America, and eastern Asia. P. malariae is found in most endemic areas, especially throughout sub-Saharan Africa, but is much less common than other species mentioned. P. ovale is relatively unusual outside of Africa and, where it is found, comprises <1% of isolates.

Clinical Manifestations

Pathological processes in malaria relate entirely to the intravascular erythrocytic infection (204). The first symptoms are non-specific. There is often a prodrome during which the patient feels vaguely unwell with anorexia, lassitude, headache and muscle aching. These worsening symptoms and the accompanying fever are non-specific and resemble an influenza-like illness. In some patients abdominal pain is prominent and, although usually the bowel habit is normal, occasionally diarrhea may be a presenting feature. The regular spikes of fever and rigors, which were prominent in early descriptions of malaria, are rarely seen today. These represent synchronization of the blood stage infection. More usually the initial fever is erratic. The clinical features of malaria depend on the level of background immunity. In areas of high stable transmission where infectious bites may be received as frequently as several times each day, the clinical signs and symptoms are confined to childhood. At lower levels of transmission a broader age range becomes susceptible, and at low or unstable levels of transmission, or in non-immunetravelers, malaria is symptomatic and P. falciparum is potentially lethal at all ages (68).

Severe Malaria

The vast majority of cases of severe malaria are caused by P. falciparum. Occasionally debilitated patients with the other three human species of malaria may die, but nearly all malaria-attributable mortality results from P. falciparum. Severe falciparum malaria is a multi-system disease. Cerebral malaria, a diffuse symmetrical encephalopathy associated with intense sequestration of parasitized erythrocytes in the cerebral microvasculature, is associated with an overall mortality of approximately 16% in children, and 20% in adults (77,99,158,183,210). Cerebral malaria presents as unrousable coma, often following convulsions. Overt or covert seizures occur in the majority of children (20). Patients with severe malaria become anaemic as the result of accelerated destruction of parasitized and unparasitized red cells. The bone marrow is dyserythropoietic. Severe anemia is a particular problem in young children, and also in pregnant women. Metabolic acidosis is a major cause of death in severe malaria (77,163). Acidosis results from accumulation of lactic acid and other unidentified organic acids as a result of tissue ischaemia, compounded by an increased glycolytic rate and reduced hepatic clearance and also from renal impairment (27). Lactic acidosis is often associated with hypoglycaemia, which occurs in 20% - 30% of children and 8% of adults with severe malaria (196,197). Other severe manifestations of severe malaria include acute pulmonary edema (ARDS), acute renal failure resulting from acute tubular necrosis (common in adults but rare in children), disseminated intravascular coagulation (rarely), and liver dysfunction resulting in jaundice and impaired metabolic clearance of antimalarial drugs (136,210).

Laboratory Diagnosis

Malaria is diagnosed by examination of thick and thin blood smears stained with supravital dyes. These are usually the stains described by Wright, Field, or Giemsa. Under light microscopy at a magnification of x1,000 the stained malaria parasites are seen readily within erythrocytes. At low parasitaemias (< 0.1%) the thin film examination may be negative. On the thick film, in which many layers of red cells are overlaid and lysed by water in the staining procedure, parasitaemias as low as 30-50/mL may be detected. In P. vivax, P. ovale and P. malariae malaria all stages of parasite development may be seen in the peripheral blood film, whereas in P. falciparuminfections only parasites in the first 24 hours of the 48 hour asexual life cycle are usually visible. Sexual stages of the parasites (gametocytes) may also be seen but do not indicate acute infection, as their period of maturation and subsequent clearance is considerably slower than that of asexual stages. In general the higher the parasite count, the more severe is the infection, although in falciparum malaria there is wide variability because of the sequestration of the mature pathogenic parasites in the microvasculature (192). Although parasitaemias over 5% are not uncommon in endemic areas in patients who are mildly ill, in a non-immune patient this indicates a potentially serious infection. Parasitaemias in P. vivax, P. malariae or P. ovale infections rarely exceed 2%. Both HRP2 and malaria pigment phagocytosed by monocytes clear more slowly than malaria parasites and indicate recent infection in a parasite negative patient (26). In severe malaria the stage of parasite development on the peripheral blood smear also has prognostic importance (149). The blood slide should be examined for parasites and for malaria pigment, a characteristic coal-colored refractile material, inside peripheral blood leucocytes. In severe malaria finding over 5% of peripheral blood neutrophils containing visible pigment indicates a poor prognosis (100). Although the examination of the blood smear is the cornerstone of diagnosis, recently rapid stick or card tests for Plasmodium falciparum histidine-rich protein II, and parasite specific lactate dehydrogenase (PfLDH) have been developed. In experienced hands these give results equivalent to those of light microscopy (7). Specificity, however, is lower in areas of high transmission, for instance around 88% in Uganda, because of the persistence of HRP2 in the blood.

Pathogenesis

The growing parasite, invades the erythrocyte, and progressively consumes and degrades intracellular proteins, particularly hemoglobin and also alters the red cell membrane making it irregular in shape, more antigenic, and less deformable. P. falciparum is the only malarial parasite that causes microvascular disease. Membrane protuberances appear on the erythrocyte's surface in the second 24 h of the asexual cycle and these "knobs" mediate attachment to receptors on venular and capillary endothelium-an event termed cytoadherence. The infected erythrocytes stick inside the small blood vessels of brain, kidneys and other affected organs. The P. falciparum infected red cells may also adhere to uninfected red cells to form rosettes. The processes of cytoadherence and rosetting are central to the pathogenesis of falciparum malaria. The sequestered red cells containing mature forms of the parasite interfere with microcirculatory flow and metabolism of vital organs. Sequestered parasites also evade the host defense mechanism: splenic processing and filtration. Severe falciparum malaria is also associated with reduced deformability of the uninfected erythrocytes, which compromises their passage through the partially obstructed capillaries and venules and shortens red cell survival. Sequestration does not occur in the benign malarias due to P. vivax, P. ovale, and P. malariae. While P. falciparum invades erythrocytes of all ages and may be associated with very high levels of parasitemia, the other plasmodium species show a marked predilection for either old red cells or reticulocytes and produce a lower level of parasitemia seldom exceeding 2%.

Malaria and Pregnancy

Pregnant women living in low-transmission areas with little premunition are at increased risk of developing severe malaria. Malaria in pregnancy is associated with maternal mortality and anaemia, low birth weight (LBW), stillbirth and severe anaemia of the new born. In high transmission settings mothers are usually asymptomatic but in primigravidae, birthweight is reduced (9). Hypoglycaemia and pulmonary edema are more common manifestations of severe falciparum malaria during pregnancy. Sequestration of parasites in the placenta through binding of infected red cells to chondroitin-sulphate can be a source of recrudescent infection, which is an important problem in the treatment of malaria in pregnancy. Selective multiplication of this subpopulation of parasites, with a chondroitin-sulphate phenotype, elicits a specific immune response, which could explain reduced vulnerabilityafter the first pregnancy. Women who experience even a single parasitaemic episode of either P. falciparum or P. vivax during pregnancy have an increased risk of anemia and low birthweight (108,111). Although more common than previously reported, vertical transmission from mother to child at delivery is relatively rare and often has an asymptomatic course in the neonate. But severe disease in the neonate does occur, and the clinician should be aware of this possibility.

In areas of multidrug-resistant P. falciparum malaria, the treatment options for malaria in pregnancy are increasingly limited (107). The declining efficacy of available drugs results in multiple treatment failures and prolonged parasitisation of the feto-placental unit with the accompanying adverse consequences for the fetus.

Impact of HIV/AIDS on malaria during pregnancy

There is substantial evidence of the effects of interactions between malaria and HIV/AIDS in pregnant women. HIV infection impairs the ability of pregnant women to control a Plasmodium falciparum infection (167). HIV positive mothers are more likely to develop clinical and placental malaria; more often have detectable malaria parasitaemia, and have higher malaria parasite densities in peripheral blood (4). Compared to women with either malaria or HIV infection, co-infected pregnant women are at increased risk of anaemia, preterm birth and intrauterine growth retardation (54). As a result, children born to women with dual malaria and HIV infection are at high risk of low birth weight and death during infancy.

The presence of HIV/AIDS may result in a poorer response to treatment with antimalarials and to intermittent preventive treatment for malaria during pregnancy. Furthermore, there is a risk of adverse reactions if sulfadoxine-pyrimethamine for the prevention of malaria in pregnant women and cotrimoxazole (a coformulation of trimethoprim and sulfamethoxazole) for prophylaxis against opportunistic infections are taken together, as both are sulfa-containing medicines.

Malaria in neonates

Although more common than previously reported, vertical transmission (parasites in neonate within 7d of birth) from mother to child at delivery is relatively rare and often has an asymptomatic course. But in neonates the disease course can deteriorate quickly, and severe disease does occur. The clinician should be aware of this possibility, and treat every case of neonatal malaria with parenteral antimalarial drugs. Clinical features include fever, irritability, feeding problems, hepatosplenomegaly, anaemia and jaundice. Every child born to a malaria smear positive mother (P. falciparum or P. vivax) or a mother receiving treatment within a week of delivery should have an assessment of the peripheral blood slide for the presence of malaria parasites at birth and at 7 days after delivery or if clinically indicated. Since severe anaemia and hypoglycaemia are frequent features of neonatal malaria, every neonate with a positive malaria smear should be checked for these conditions. Continuation of feeding in the neonate is important.

Although no data from clinical trials are available, it can be advised to treat neonatal malaria with parenteral artesunate in the same doses as recommended for the treatment of severe malaria, for a total course of 7 days. If artesunate is not available, artemether can be used. Parenteral quinine is a third option, but blood glucose concentrations should be regularly checked, because of the high incidence of hypoglycaemia.

SUSCEPTIBILITY IN VITRO AND IN VIVO

Of the four human malarias only Plasmodium falciparum can be cultured reliably and easily in vitro. Plasmodium falciparum will also infect Saimiri and Aotus monkeys. Plasmodium vivax will infect chimpanzees. Methods for Plasmodium falciparum in vitro susceptibility testing of antimalarial drugs have been developed, and rapid microtest methods have been adapted for field conditions (143,186). These simple tests are based on the inhibition of maturation of parasites in peripheral blood samples, which are placed in culture medium in plastic multiple-well plates in the presence of different concentrations of antimalarial drugs. The proportions of parasites, which mature to the schizont stage, are counted. The original Macro-test, which used venous blood samples, has been largely superceded by a microtest requiring only a capillary blood sample and uses 96 well plastic microtiter plates pre-coated with a range of antimalarial drug concentrations. 5µL of capillary blood, diluted in 50µL of glutamine-supplemented RPMI 1640, is incubated in each well at 37.5°C for 24-30 hours (depending on the developmental stage of the parasites) and the proportion of parasites that have matured to schizonts at each drug concentration is counted (143). A "satisfactory (i.e. sensitive) antimalarial response is indicated by complete schizont inhibition by 4 pmol/well of chloroquine, 2 pmol/well of amodiaquine, and 128 pmol/well of quinine. "Resistance" is indicated by growth of schizonts at 8 pmol/well of chloroquine, 4 pmol/well of amodiaquine, 64 pmol/well of mefloquine, or 256 pmol/well of quinine (211). These in vitro criteria for resistance correspond very loosely with the clinical response.

Alternatively the parasites are co-incubated with radio-labeled hypoxanthine (or sometimes another labeled substrate) and inhibition of hypoxanthine uptake by the antimalarial drug is measured (189). Malaria parasites are unable to synthesize purines and have an obligatory requirement for exogenous purines for nucleic acid synthesis (16). Recently non-radio-isotopic methods of susceptibility testing based on drug inhibition of PfLDH have been developed. Whereas the relationship between in vitro susceptibility and in vivo therapeutic response in an individual is variable, for epidemiological purposes in vitro susceptibility testing provides a good indication of the prevailing level of antimalarial drug resistance and is useful in monitoring its evolution (12). Cut-off values for resistance have been used widely. However, in the individual patient in vitro susceptibility tests often does not predict the clinical response to antimalarial drug treatment. This is because the in vitro test takes place in the absence of serum factors, and cellular host defense mechanisms. Additional factors such as previous antimalarial drug treatment, plasma protein binding, and variable adsorption of the antimalarial drug to plastic further confound the relationship. More recently an in vitro model for drug susceptibility testing of Plasmodium vivax has been developed using short-term culture (14).

Chloroquine Resistance

Chloroquine and amodiaquine are 4-aminoquinoline compounds. Chloroquine is concentrated at least 1000 fold inside the food vacuole, the organelle responsible for parasite haemoglobin digestion. The malaria parasite consumes haemoglobin using the globin as a food source and disposes of the potentially toxic haem partly as malaria pigment. Drug concentration results from binding to ferriprotoporphyrin IX (FP) and the acid milieu in the vacuole, which traps the diprotic base. Chloroquine exerts its antimalarial activity by preventing the detoxification of FP. It interferes with FP polymerisation to produce inert malaria pigment haemozoin (154,155), and it prevents FP destruction by glutathione dependent and peroxidative mechanisms. FP is left to accumulate in parasite membranes where it peroxidates lipids and disrupts their barrier function. It has been known for several years that chloroquine resistance in Plasmodium falciparum could be genetically mapped to chromosome 7 (the malaria parasite has 14 chromosomes). Recently mutations in the polymorphic gene (cg 10 orcrt) encoding a putative chloroquine transporter (CRT) and located on chromosome 7 have been closely linked with chloroquine resistance (30). Linkage and transfection studies also clearly associate a separate gene Pgh (located on chromosome 5) as a modulator of resistance. Two homologues of the MDR gene family, which are part of the ATP binding cassette gene super-family, have been identified in Plasmodium falciparum. The gene Pgh encodes for a 162 kDa homologue of the ATP-dependent P-glycoprotein vacuolar transmembrane pump, that was identified originally in mammalian tumor cells. In general the association with amplification or increased expression of MDR genes is closest with mefloquine resistance (19), and is also strong for lumefantrine resistance. Mefloquine and chloroquine resistance show an inverse relationship (49). Recent transfection studies show unequivocally that mutations in Pgh do modify chloroquine resistance (140). Yet another, as yet unidentified, gene on chromosome 13 has also been implicated emphasizing the multigenic nature of the resistance process (34,45). 4 aminoquinolines also competitively inhibit the degradation of haem by glutathione allowing it to accumulate in membranes where it disrupts their barrier function (44). Resistance is associated with reduced concentrations of these compounds in the parasites' food vacuole. Both increased efflux of these drugs and reduced influx have been documented (10,39,40,185). Chloroquine resistance can be reversed in vitro by a variety of structurally unrelated pump inhibitors such as verapamil, fluoxetine, amlodipine, cyproheptadine, desipramine, and the phenothiazines (29,78). Unfortunately none of these has proved unequivocally to be of clinical benefit (8). The mechanism of chloroquine resistance in Plasmodium vivax is not known.

Mefloquine Resistance

As described above, mefloquine resistance appears to be associated with amplification or increased expression of MDR genes (19,130,206). Amplification of the wild type Pfmdr is strongly associated in epidemiological studies with mefloquine resistance, and is associated with reduced cytosolic mefloquine concentrations. Mefloquine resistance can be reversed by penfluridol but is affected little by the drugs which reverse chloroquine resistance (116). Mefloquine, halofantrine and to a lesser extent quinine resistance are all linked epidemiologically and appear to share a common mechanism although resistant lines can be selected in-vitro which do not exhibit MDR amplification. Furthermore transfection studies show clearly that certain Pgh mutations increase mefloquine and halofantrine resistance but decrease quinine resistance (140). As mutation in Pfmdr (particularly at position 86) is associated with chloroquine resistance, back mutation to the wild type is generally required before amplification. Amplification occurs readily throughout the P. falciparum genome. Mefloquine and halofantrine resistance develops relatively easily in experimental rodent malarias, whereas quinine resistance is more difficult to induce. In Thailand (where mefloquine was first deployed alone) resistance developed over a six-year period between 1984 and 1990 despite excellent control of drug deployment (109). By mid 1994 on the Western border 50% of infectious treated with high dose mefloquine recrudesced and 10% did not clear parasites at all. Halofantrine is intrinsically more active than mefloquine in vivo and has been shown to be significantly more effective against mefloquine-resistant parasites (166), but the doses required for treatment of multi-drug resistant malaria have been associated with an unacceptable risk of cardiotoxicity (112,113). Despite the inverse relationship between mefloquine and chloroquine resistance, highly mefloquine-resistant parasites are nearly always resistant to chloroquine too.

Quinine Resistance

There has been a gradual decline in the sensitivity of P. falciparum to quinine in some areas, notably SE Asia. Cure rates with quinine alone in uncomplicated malaria have fallen, but used in combination with tetracyclines, quinine still retains good efficacy (71,73,188). In Thailand there is evidence of declining efficacy in recent years in clinical and parasitological responses in severe malaria, but mortality rates have not changed significantly (137). Quinine resistance is usually correlated with mefloquine resistance, and is caused partly by Pfmdr amplification but there are well-documented exceptions to this rule.

Dihydrofolate Reductase Inhibitor Resistance

Pyrimethamine and the active cyclic triazine metabolites of proguanil and chlorproguanil (cycloguanil and chlorcycloguanil respectively) all selectively inhibit plasmodial dihydrofolate reductase. The antibiotic trimethoprim also has significant activity against plasmodial dihydrofolate reductase and has been used clinically to treat malaria. The antimalarial activity of these compounds is potentiated considerably by the addition of dihydropteroate synthase (DHPS) inhibitors, the sulfonamides or sulfones. These block the conversion of para-aminobenzoic acid to dihydropteroate and, together with the DHFR inhibitors, provide sequential inhibition of the folate biosynthesis pathway. Recently some isolates of P. falciparum have been shown to utilise exogenous folate and thus bypass sulphonamide inhibition. The sulfonamides most commonly used are the long-acting compounds sulfadoxine and sulfalene. Although amplification of the DHFR genes and thus increased expression of the enzyme can be induced experimentally, resistance in wild parasite isolates is always associated with mutations in the DHFR genes which confer reduced susceptibility of the enzyme product to inhibitors (39,152). Serial point mutations in DHFR and DHPS confer a stepwise reduction in the affinity for these antimalarials (39,142,153). For pyrimethamine resistance the initial mutation is usually at position 108 (SER to ASN). This confers pyrimethamine resistance but only slightly reduced sensitivity to cycloguanil. Interestingly, the SER to THR mutation at position 108, when combined with ALA to VAL at position 16 confers cycloguanil resistance but not pyrimethamine resistance (123). Additional mutations at positions 46, 51, 59, and 164 confer increasing resistance to both classes of drugs (141). "Triple mutants" (mutations at positions 108, 51, 59) now prevalent in many areas retain some susceptibility to sulfadoxine-pyrimethamine (SP) and are treated effectively by chlorproguanil-dapsone. The164 (ILE to LEU) mutation confers complete resistance to currently available antifols. Thus in some areas resistance to one but not the other class of compounds may occur, whereas in areas, such as South East Asia and much of South America, multiple DHFR mutations which include the 164 mutation are now prevalent, and these confer high-level resistance to both pyrimethamine and cycloguanil. Unfortunately the 164 mutation has now arrived in Africa. Similar mutations are found in antifol resistant P. vivax. Resistance in both Plasmodium falciparum and Plasmodium vivax to proguanil and later to pyrimethamine both developed rapidly after the drugs were introduced as monotherapy in malaria endemic areas (36,122). These drugs are now prescribed in combinations and no longer used alone for treatment. Synergistic combinations with sulfonamides are now used widely and were the next choice when chloroquine resistance precluded further deployment of chloroquine. Unfortunately resistance has also developed rapidly to the pyrimethamine-sulpha combination preparations in many areas.

Atovaquone Resistance

Although the atovaquone-proguanil combination has yet to be deployed widely, during initial clinical studies with atovaquone, high-level resistance occurred in one third of patients treated (64). Resistance is related to single point mutations in the cytochrome b gene of Plasmodium falciparum, which mediates up to 10,000-fold reduction in susceptibility to the drug (63). These mutants occur at a frequency of approximately 3 per 1012 parasites, which translates to emergence of this mutant in one third of infections treated with atovaquone alone. Although the atovaquone-proguanil combination reduces the frequency with which such mutants emerge, it does not abolish it altogether, and this drug must be considered highly vulnerable to the selection of resistance (202,204).

Primaquine Resistance in Radical Treatment

The efficacy of primaquine in preventing relapse of Plasmodium vivax varies. In South East Asia and Oceania higher doses of primaquine are required than elsewhere to prevent relapse of P. vivax (18). The mechanism for this variable susceptibility is not known.

Other Antimalarial Antibiotics

Many antibiotics have antimalarial activity. These include the tetracyclines, rifampicin, the macrolides, clindamycin, the fluoroquinolones, chloramphenicol, and trimethoprim-sulfamethoxazole (134). Only for trimethoprim-sulfamethoxazole is antimalarial activity sufficient for the drug to be used alone. Otherwise the therapeutic response is too slow and these drugs should be combined with a more effective antimalarial drug. In practice tetracycline and doxycycline are the most widely used compounds and amongst the remainder there is sufficient evidence only for clindamycin in combination with quinine (which appears to be equally effective compared with doxycycline) (65,66).

Combination Drugs In Vitro

The current standard of antimalarial treatment is with a combination of antimalarial drugs. Although using conventional isobolograms both synergy and antagonism can be demonstrated, this is clinically important in only two circumstances. The first is the marked potentiation of the DHFR inhibitors (pyrimethamine cycloguanil, chlorcycloguanil) by sulfonamides or sulfones, and the second is the marked synergy of atovaquone with proguanil. It should be noted that it is the parent drug proguanil, and not cycloguanil (the DHFR metabolite), that is acting in synergistic combination with atovaquone-and its mode of action in this combination is not known. The rationale for the use of combination therapy is further discussed below.

Antimalarial Drug Resistance

Global Distribution of Antimalarial Drug Resistance

Resistance of Plasmodium falciparum to chloroquine now occurs throughout most of the tropical world. Chloroquine sensitivity is still retained in Central America north of the Panama Canal, Haiti, North Africa and parts of the Middle East, and some parts of Asia such as peninsular Malaysia, and parts of the Philippines. In many areas resistance is low-grade and partial clinical responses to chloroquine treatment are observed. In many areas of the tropics chloroquine is still used as a first-line treatment despite resistance even though clinical responses are unsatisfactory or the drug is ineffective. This is partly because of its low cost (approximately five US cents for a treatment course). High-level chloroquine resistance is now prevalent in many areas of South America and East Asia and increasingly in Africa, and in these areas there is usually no therapeutic response at all to chloroquine. The Mannich-base compound amodiaquine is more active than chloroquine against low level chloroquine resistant parasites (118) but in areas with high-level chloroquine resistance amodiaquine is also not effective. Resistance to sulfonamide-pyrimethamine combinations is widespread in South America and East Asia, and focal areas of East, South and West Africa. Resistance to both chloroquine and sulfonamide-pyrimethamine is worsening across Africa.. The triazine metabolites of the antimalarial biguanides (proguanil and chlorproguanil ) are, like pyrimethamine, dihydrofolate reductase inhibitors, but the mutations of the DHFR gene which confer pyrimethamine resistance do not necessarily confer biguanide resistance, and vice versa (40,123). Thus in some areas where pyrimethamine-resistant parasites are prevalent the antimalarial biguanides are still effective. However mutations conferring resistance to both groups of compound are now common in resistant areas. Mefloquine resistance is still relatively unusual. Parasites from West Africa appear to be intrinsically mefloquine-resistant (117). In South East Asia, where mefloquine has been deployed in recent years, high-level mefloquine resistance has developed on the eastern and western borders of Thailand and the adjacent countries (106,109,114) and sensitivity is declining in Southern Vietnam. Mefloquine resistance is usually associated with reduced susceptibility to halofantrine and quinine (12,162). Although quinine resistance was first documented in 1910, P. falciparum sensitivity to quinine is still retained throughout most of the world. Quinine may still be relied upon to treat severe falciparum malaria everywhere. Although there is evidence of a decline in the efficacy of quinine both in some areas of South East Asia and South America (191), there is still no well-documented case of high-level (R3) resistance with adequate blood levels of quinine (137) In Thailand where the world's most drug-resistant parasites are to be found, the efficacy of quinine alone in the treatment of uncomplicated falciparum malaria has declined in recent years, but the combinations of quinine with a tetracycline or clindamycin still retains over 85% efficacy (73,135,199). There are no well-documented cases of resistance to artemisinin or its derivatives (12).

Prevention of Antimalarial Drug Resistance

Resistance to antimalarial drugs, as with other micro-organisms, occurs when selective drug pressure is applied to the parasite population. Resistance occurs most rapidly when a high proportion of infections are exposed to the antimalarial drug, if only one or two point mutations confer a marked reduction in drug susceptibility, if elimination from the body is slow, the dose response (concentration - effect) relationship is flat, and use of the drugs is unregulated or dose regimens inadequate (202,204). Resistance to proguanil, pyrimethamine, and atovaquone involves only one or two point mutations. In endemic areas resistance to pyrimethamine and proguanil (used alone) developed rapidly in both P. falciparum and P. vivax. In contrast mutations conferring resistance to the quinoline antimalarials involve multigenic changes and develop slowly mefloquineresistance results from gene amplification (Pfmdr), which happens readily. This developed within six years of mefloquine deployment in Thailand (109). Several strategies may be employed to retard the development of resistance. If antimalarial drug dosing is correct and prescribing is well regulated, then uncontrolled self-medication and exposure of parasites to sub-therapeutic concentrations of the drugs can be minimized. Resistance only spreads when infections persist long enough for sexual stages of the parasite (gametocytes) to appear in the blood and to be transmitted by mosquitoes. Recrudescent infections have a higher likelihood of carrying gametocytes (131) it is this transmission advantage that drives the spread of resistance. If the transmission advantage can be reduced or eliminated by using artemisinin combinations (which are more effective and reduce gametocyte carriage) then the spread of resistance in P. falciparum may be retarded. There is evidence that this happens when artemisinin derivatives are deployed for treatment in low transmission areas (114).

Drug combinations are well established in the treatment of tuberculosis, HIV infection, and cancer. The principle is simple. If drug resistance develops by a spontaneous point mutations or gene amplification then the chance of resistance developing to two structurally unrelated drugs with different modes of action is a product of the two mutation frequencies. As these frequencies are very low in malaria parasites (less than one in 1012) parasites, which are spontaneously resistant to two unrelated drugs, are exceeding rare or never occur (202,204). For three drugs the chance of a spontaneously resistant viable mutant is the product of the three mutation frequencies.Sulfadoxine and pyrimethamine were combined originally with mefloquine when it was deployed in Thailand in 1984. This was done specifically to retard the development of mefloquine resistance, which had been demonstrated to occur readily in rodent models. This strategy was sound in theory but did not work because P. falciparum in Thailand in 1984 was already highly resistant to both sulfadoxine and pyrimethamine. Furthermore by the time that mefloquine had declined to sub-therapeutic concentrations in a patient several weeks after drug administration, both pyrimethamine and sulfadoxine had been eliminated from the blood because of their more rapid systemic clearance. They could not "protect" mefloquine from exposure during the elimination phase. Thus in order for antimalarial drug combinations to be effective, either the two or three components must be pharmacokinetically well-matched, or the parasite biomass must be reduced sufficiently by one of the drug components that the chances of mutation to the other, more slowly eliminated, drug are greatly reduced (204). The latter scenario describes the rationale behind the combination of artemisinin derivatives with mefloquine and other drugs. The artemisinin derivatives are the most active of the available antimalarial compounds and produce a fractional reduction in parasite biomass of approximately 104 per asexual cycle (202). Thus three days treatment, involving two cycles, usually produces a 108-fold reduction in biomass. This leaves a maximum of 105 parasites for the other antimalarial drug (usually mefloquine or lumefantrine) to clear. This considerably reduces the exposure of the parasite population to mefloquine and thus reduces considerably the chance of an escape-resistant mutant arising from the infection (204). The artemisinin derivatives also have the advantage of reducing gametocyte carriage and thus transmissibility of the infection (131).

ANTIPARASITIC THERAPY

General Principles

Antimalarial treatment should give a rapid reduction in parasite load, which is especially important in the treatment of severe malaria. Moreover, in an attempt to reduce the rapid spread of antimalarial drug resistance, and to prevent recrudescence of the infection, is the World Health Organisation now recommends that P. falciparum infections everywhere be treated with a combination of antimalarial drugs. The combinations of choice areartemisinin based combination treatments or ACTs. Combinations of drugs will mutually protect each other against selection of resistant clones, since the mutant strain will still be sensitive to the partner drug. The choice ofantimalarial treatment is determined by the likely susceptibility of the infecting parasites, and the pharmacokinetic and pharmacodynamic properties of the drugs. The gametocytocidal properties of the drug, which will reduce transmission, can also be taken into consideration. Where diagnostic facilities exist, antimalarial treatment should be given only to symptomatic patients with positive blood smears. In non-immune or semi-immune subjects any parasitemia associated with fever indicates that malaria is the likely cause of that fever, but in areas of higher transmission asymptomatic parasitemia is common. In this context parasitemia may co-exist with fever caused by another infection. Different thresholds are therefore used to differentiate malaria from incidental parasitaemias, e.g. in areas of very high transmission a threshold of 10,000 parasites/µl is often used to distinguish fever caused by malaria and that caused by other infections (145).

Antimalarial Drug Pharmacokinetics and Pharmacodynamics

The pharmacokinetic properties of many antimalarial drugs are altered during acute disease. This results from impaired absorption in acute illness, alterations in tissue and plasma protein binding which alter the volume of distribution, and reductions in hepatic biotransformation or elimination, which lowers systemic clearance.

The pharmacokinetic properties of antimalarials that are relevant to the therapeutic effect are the stage specificity of activity, which is of relevance to severe malaria and the maximum effect on inhibition of multiplication, which determines parasite clearance in uncomplicated malaria. Drugs which act earlier in the asexual life cycle of P. falciparum, are preferable in severe malaria e.g., the artemisinins prevent ring form development and thus cytoadherence, but quinine does not (171,187). In uncomplicated malaria activity may be described by the fractional reduction in parasite numbers per asexual life cycle or parasite reduction ratio (PRR). This ranges from 10,000 fold (artemisinins) to 10 fold (tetracycline) per asexual life cycle (134,202). As there may be up to 1013 parasites in an infection it is evident that at least four asexual 48 hour life cycles must be covered by treatment to clear all parasites. Thus if rapidly eliminated drugs are used (artemisinins or quinine alone) then at least seven days treatment is required, and drugs with lower parasite reduction ratios need to "cover" (i.e. provide maximal parasite reduction ratio values) for longer periods to ensure cure.

Severe Malaria

Falciparum malaria is defined as severe malaria when the disease presents with one ore more severity signs, including coma or severe prostration, severe anaemia (< 20 % with parasite count >100,000/ mm3), severe jaundice (bilirubin > 2.5 mg/dl), acute renal failure (> 3 mg/dl with urine <400 ml/24 hours), respiratory failure, hypoglycaemia (venous glucose < 40 mg/dl), shock (systolic blood pressure < 80 mm Hg with cool extremities), hyperparasitaemia (peripheral asexual stage parasitaemia > 10 %) or metabolic acidosis (peripheral venous lactate > 4 mmol/l, peripheral venous bicarbonate < 15 mmol/l).

The objective of treatment in severe malaria is to save life. Prevention of recrudescence is of secondary concern. Patients with severe malaria require intensive care treatment (see below). The mortality of severe malaria is high; approximately 20% of adults and 16% of children with cerebral malaria will die despite optimum treatment. It is essential, therefore that therapeutic antimalarial drug concentrations are achieved as soon as possible once the diagnosis is made. As most deaths occur within the first 48 hours following the start of treatment, fears over significant antimalarial toxicity are secondary to the risks of inadequate treatment. Undertreatment is seldom recognized and may be fatal.

Chloroquine-resistant P. falciparum has reached almost all areas of the tropics, and Chloroquine is therefore no longer recommended for the treatment of severe malaria. Two classes of drugs are currently available for the parenteral treatment of severe malaria: the cinchona alkaloids (quinine and quinidine) and the artemisinin derivatives (artesunate, artemether, and artemotil or arteether). In adult patients with severe malaria randomized trials comparing artesunate and quinine show clear evidence of benefit with artesunate. In the largest ever multicentre trial of severe malaria, which enrolled 1461 patients, mortality was reduced from 22% to 15%, a relative reduction of 34.7% (95% CI: 18.5 to 47.6%, p=0.002) (31). Moreover quinine was more frequently associated with hypoglycaemia (RR=3.2, p=0.009). The artemisinin derivatives are thus more effective, safer, and also easier to use than quinine. The large randomized trials comparing the oil soluble artemisinin derivative artemether with quinine have not shown a significant reduction in mortality for artemether over quinine. This is likely to be explained by the less favourable pharmacokinetic profile of artemether compared to water-soluble artesunate. Erratic absorption of artemether (and the related compound artemotil) after i.m. injection has been documented, especially in severely ill patients. Artesunate should be the treatment of choice for adults with severe malaria. There are, however, still insufficient data for children, particularly from high transmission settings to make the same conclusions, but randomized trials are under way. Until more evidence emerges, children with severe malaria can be treated with either quinine, artesunate or artemether, but, of these three, the authors prefer water-soluble artesunate.

Artesunate, Artemether and Arteether

Artesunate is formulated as artesunic acid powder and is prepared immediately before injection by dissolving this powder in 1 ml of 5% sodium bicarbonate solution (provided as an ampoule together with the powder). This is then diluted in 5 ml of 5% dextrose and injected over 1 - 2 minutes intravenously, or given by deep intramuscular injection to the anterior thigh. Artesunate is given in an initial dose of 2.4mg/kg followed by the same those after 12 hours, 24 hours and then daily. To prevent recrudescence of the infection, follow on medication should be given once the patient is able to take oral medication. Several regimens are possible, such as a full course of oral artemether-lumefantrine (Co-artemR), or a combination of oral artesunate (2 mg/kg per day, total course 7 days including the parenteral form) and doxycycline (4 mg/kg per day for 7 days, contraindicated in small children ≤ 8 y/o and pregnant women). Mefloquine is not recommended as maintenance antimalarial drug, because of its association with post-malaria neurological syndrome (101).

Artemether is given by deep intramuscular injection to the anterior thigh in an initial dose of 3.2mg/kg followed by 1.6mg/kg daily. Artemotil (arteether) is very similar to artemether, differing only in the substitution of an ethyl for a methyl group, and that it is suspended in sesame seed oil whereas artemether is suspended usually in groundnut oil. Dose regimens provisionally are similar to those for artemether (24,94). Artesunate is absorbed very rapidly after intramuscular injection whereas the absorption of artemether or arteether is slow and erratic, particularly in shocked children (97). The parenteral artemisinin derivatives are safe and very easy to use; they have no evident local or systemic toxicity, and no dose modification is required in renal failure or with severe hepatic dysfunction.

In Vietnam suppository formulations of artemisinin have proved as effective as parenteral artesunate or artemether in the treatment of severe malaria (13,52). These can be used in rural tropical areas where parenteral treatment is not possible, pending transfer to hospital. Formulations of artesunate for rectal administration are also satisfactorily absorbed after rectal administration. A randomised study in African children and adults with moderately severe malaria showed more rapid parasite clearance after rectal artesunate compared to parenteral quinine (6). Very large community based evaluations have just been completed.

Quinine

Quinine should be given by careful rate-controlled infusion and, as for chloroquine, should never be given by intravenous injection. In order that therapeutic concentrations are reached as soon as possible in the course of treatment an initial loading dose should be given, either of 20mg salt/kg infused over 4 hours or 7mg salt/kg infused over half an hour, followed immediately by 10mg salt/kg infused over 4 hours (25,195). The initial dose is the most important of all, and should only be reduced if there is good evidence of previous quinine treatment (>40mg/kgin 48 hours for an adult). The safety of this approach has been established in recent large randomised trials (170,173). A lower loading dose of 15mg salt/kg has been proposed for the treatment of African children (207), but this has been disputed (200). Most debate has centered on the risks of significant cardiovascular or nervous system toxicity resulting from quinine - but this is very rare (198,200). A treatment history is often unavailable, or there may be uncertainly whether the patient has received quinine treatment before admission to hospital. Quinine toxicity might result if a full loading dose were given in addition to the earlier treatment. If 40mg salt/kg or more has been given over the preceding 48 hours then a loading dose should not be given, but if less has been taken, or, most importantly, there is uncertainly, then the loading dose should be administered. As there is now extensive evidence in nearly one thousand carefully followed patients of safety with the 20mg/kg dose, and reducing the initial dose increases the risk of undertreatment in the critical initial phase of treatment of this life threatening infection, the initial 20mg/kg dose should only be reduced if there are compelling reasons. The maintenance dose of quinine is 10mg salt/kg given 8-hourly (210). Quinine doses are similar at all ages, and in pregnancy. The infusion should not be given faster than over a 2-hour period (nor greater than 5 mg/kg/hour). Although quinine is light sensitive, photodegradation is not significant over a 24-hour period and there is no need to protect solutions from light. Quinine may be given in either normal saline or 5% dextrose. In general quinine is well tolerated provided the administration rates are not too rapid (which, as for chloroquine, may produce potentially lethal hypotension). If intravenous infusions cannot be given, or supervised properly, then quinine should be given by intramuscular injection to the anterior thighs (182) (Table 7). The principal adverse effect of quinine in severe malaria is hypoglycaemia (197). This results from quinine-induced insulin secretion. It usually occurs after quinine has been given for 24 hours and is more likely in patients with severe malaria and, in particular, in pregnant women. Fifty percent of pregnant women with severe malaria treated with quinine will become hypoglycaemic. Hypoglycaemia is usually recurrent and all patients receiving quinine should have their blood glucose monitored frequently. Even if an artemisinin derivative is substituted, the slow elimination of quinine means that the risk of hypoglycaemia will persist for up to two days.

As soon as the patient can take solids by mouth oral treatment should be substituted to complete a 7-day course of treatment. A 7-day course of tetracycline, doxycycline (for children >8 years and for non-pregnant adults) or clindamycin should be started as soon as renal function has returned to normal. If there is no improvement in the overall condition by 48 hours after starting intravenous quinine treatment the total daily dose should be reduced by 30 - 50% to avoid accumulation to toxic levels (210). The same applies if the patient requires dialysis because of acute renal failure. Doses should not be reduced before 48 hours unless there is clear evidence of toxicity. If plasma concentration monitoring is available the total plasma concentration should be kept between 8 - 15mg/L (this corresponds approximately to free quinine concentrations between 0.8 and 1.5 mg/L). Although quinine produces an average of 10% prolongation of the electrocardiogram QT interval (mainly as a result of QRS lengthening) electrocardiographic monitoring is not mandatory during quinine treatment. However monitoring is advisable in the very young and very old, and in the management of acute renal failure where electrolyte disturbances are also likely. Where intravenous infusions cannot be given quinine may be given by deep intramuscular injection. Parenteral quinine is usually formulated as the dihydrochloride salt. This is acidic (pH 2) and painful if injected undiluted. In tropical areas intramuscular quinine administration has been associated with subsequently lethal tetanus (213). Intramuscular quinine should be given to the anterior thigh (never the buttock or arm) with scrupulous aseptic technique. The quinine should be diluted 1:1 to 1:5 in normal saline before injection. Subsequent doses should be given into alternate thighs. Untreated severe malaria is usually fatal, and antimalarial drugs save life by killing and arresting the development of malaria parasites. Although both quinine and chloroquine have "anti-inflammatory" or "anti-cytokine" activities there is no evidence that these contribute to the therapeutic effect of these drugs in vivo (67).

Quinidine

In situations where quinine is not available but quinidine is, the latter drug may be used although there is some controversy over the correct dosage regimens (57,194). In comparison with the quinine treatment of severe falciparum malaria for which there is extensive information, there are few data on which to base treatment recommendations for quinidine. Both drugs share each other's properties, although there are significant pharmacokinetic differences between them and quinidine has greater cardiac and antimalarial effects. It is therefore more potent and more toxic. The therapeutic range for the antiarrhythmic activity of quinidine at steady state has been estimated at between 3 and 8 mg/L when measured by the extraction-fluorescence method (which also measures the bioactive metabolites) and approximately 2 to 5 mg/L when measured by specific HPLC. The therapeutic range for antimalarial treatment with quinidine is not known precisely, but considering the experience in cardiovascular disease, and the 2-3 fold greater activity against P. falciparum in-vitro compared with quinine, it has been suggested that total plasma concentrations between 4 and 8 mg/L should represent a reasonable compromise between efficacy and toxicity (compared to 8-20 mg/L for quinine, which corresponds to a free concentration of 0.8-2 mg/L).

The dose recommendations for quinidine in severe malaria are based on two studies. In the first, conducted in 14 adults Thai patients with severe falciparum malaria, intravenous quinidine gluconate was given in a dose of 15 mg base/kg over 4 hours followed by 7.5 mg base/kg eight hourly until oral treatment could be substituted (i.e. 52.5 mg/kg in 48 hours) (125). Mean (SD) total plasma quinidine concentrations, measured by the extraction fluorescence method, were 9.2 (2.7) mg/L with a range of 4.4 to 14.3 mg/L at the end of the initial loading dose infusion. Post infusion plasma concentrations generally declined thereafter reflecting increased clearance and expansion of the apparent volume of distribution with recovery. A similar pattern over a nearly two-fold higher plasma concentration range was observed for quinine treatment. In the second study conducted in the United States (92) intravenous quinidine was given in a dose of 6.2 mg base/kg infused over 1-2 hours followed by 0.75 mg base/kg/hour by constant infusion (i.e. a dose 22.5% lower than in the Thai study). Five patients with acute falciparum malaria (4 adults 1 child) and >5% parasitemia received this infusion alone, and ten (7 adults 2 children) received this together with exchange transfusion. Compared with the first study plasma concentrations of quinidine, measured by specific HPLC, were lower (although precise details are not given) and only two had concentrations exceeding 7 mg/L whereas in the Thai study all patients' plasma concentrations exceeded 5.8 mg/L after the initial infusion, a concentration considered to correspond with the maximum MIC value for Thai P. falciparum isolates at the time. Also exchange transfusion would be expected to result in an increased free fraction of quinidine because of the normal alpha 1-acid glycoprotein (AAGP) concentrations in the exchanged blood - and this would change the relationship between total plasma quinidine levels and toxicity. These two studies were sufficient to justify use of quinidine as an alternative to quinine in North America, as quinidine was widely available whereas quinine was not, but they were both relatively small and certainly cannot be considered definitive. Plasma quinidine levels vary considerably between individuals (three-fold in the Thai study), and the individual concentration-effect relationships are not predictable. In order to avoid cardiotoxicity patients with severe falciparum malaria receiving quinidine require careful monitoring. Reversible hypotension and significant QT prolongation are relatively common (whereas they are rare with quinine treatment). Frequent monitoring of blood pressure and continuous EKG monitoring is necessary to avoid cardiotoxicity. The higher dosage recommendation is an initial loading dose of 10mg base/kg given over 2 hours followed by constant infusion of 0.02 mg base/kg/min until the patient is able to take oral medication (205). As with quinine, quinidine blood concentrations may continue to rise in patients who do not respond rapidly to treatment, so that if there is no improvement in the overall condition by 48 hours the total daily dose should be reduced by 30 - 50% to avoid toxic levels. Some authorities recommend the lower initial dose regimen of 6.25mg base/kg followed by 0.0125mg base/kg/min. The debate has centered on the relative risks of toxicity with the higher dose regimen versus the risks of under treatment with the lower dosage (57,125,194). As argued earlier, undertreatment is difficult to recognize and may be fatal whereas overtreatment is easily detected and easily reversed. If the QRS interval widens by >50%, the QT interval exceeds 0.60 sec, or the QTc interval is prolonged by more than 25% of the baseline value, the infusion should be stopped until the QRS or QT interval prolongations falls below these values. Quinidine is usually formulated as the gluconate salt. There are no data on intramuscular quinidine in severe malaria.

Uncomplicated Malaria

Uncomplicated malaria is defined as malaria infection in the absence of any signs of severity or evidence of vital organ dysfunction, including coma or severe prostration, convulsions, severe anaemia (< 20 % with parasite count > 100,000/ mm3), severe jaundice (bilirubin > 2.5 mg/dl), acute renal failure (> 3 mg/dl with urine < 400 ml/24 hours), respiratory failure, hypoglycaemia (venous glucose < 40 mg/dl), shock (systolic blood pressure < 80 mm Hg with cool extremities), hyperparasitaemia (peripheral asexual stage parasitaemia > 10 %) or metabolic acidosis (peripheral venous lactate > 4 mmol/l, peripheral venous bicarbonate < 15 mmol/l)

Treatment of P. vivax or P. ovale Malaria

For most P. vivax and all P. ovale infections, the drugs of choice are a combination of chloroquine plus primaquine. Chloroquine resistance is increasingly reported from Papua New Guinea and Indonesia where high-level resistance is prevalent, and South America. There are also reports from India and Burma.

Chloroquine

Chloroquine (total dose 25mg base/kg) should be given in standard doses and combined with primaquine given in an adult dose of 15mg base (0.25 mg base/kg) daily for 14 days. In South East Asia and Oceania higher doses of primaquine are recommended (0.375 to 0.5mg base/kg/day). Primaquine should be taken after a meal not on an empty stomach. Chloroquine is available in tablet and liquid (pediatric) suspensions. It is cheap, well absorbed, well tolerated, and effective in a treatment course of 2 - 3 days. Chloroquine may be used in very young children and in pregnant women. The traditional treatment regimen has been an initial dose of 10mg base/kg followed at 6, 24, and 48 hours by further doses of 5mg/kg. This may be compressed into a 36-hour administration of 10mg/kg initially, followed at 12-hour intervals by 5mg base/kg (139). Oral chloroquine is generally well tolerated although it has a bitter taste, which children may not like. Nausea and dysphoria may occur, and in dark-skinned subjects pruritus is common and may be sufficiently severe to prevent usage of the drug. Chloroquine is dangerous in overdose, producing cardiovascular and neurological toxicity, although these are both very rare with therapeutic doses. Diazepam is a specific antidote. Occasional neuropsychiatric reactions follow antimalarial treatment. Cerebellar dysfunction has been reported particularly from the Indian sub-continent. These reactions resolve spontaneously without treatment. Long-term prophylactic use for 5 years or more has been associated with retinopathy, but this is not relevant to acute treatment use. Chloroquine is still amongst most widely used drugs in the world and still in some parts of Africa the majority of the population has detectable levels of chloroquine in their urine.

Primaquine

Primaquine is an oxidant drug and may cause haemolysis, particularly in subjects who are G6PD deficient. In patients who have mild variants of G6PD deficiency, primaquine 45mg base (0.75mg/kg) should be given once weekly for six weeks. In parts of SE Asia and Oceania, the exoerythrocytic (liver) stages of P. vivax are more resistant to primaquine and higher doses are required; 22.5-30mg base/day (0.375-0.5mg/kg) for 14 days. Primaquine also has weak asexual stage activity against P. vivax (138). Relapses with most tropical "strains" of P. vivax occur at intervals of 3-6 weeks (61,134). If a short acting drug is used (quinine, artemisinin derivative) then relapses tend to occur within one month. Except in those few areas with significant resistance, chloroquine levels in blood following treatment are sufficient to suppress this first relapse. Therefore, in most cases the first relapse after chloroquine (or mefloquine) treatment occurs around 6 weeks following treatment. Relapses should be treated in the same way as the primary infection, with both chloroquine and primaquine. Relapse intervals over six months are unusual in tropical vivax infections. The newly introduced 8-aminoquinoline, tafenoquine, is at least ten times more active compared with primaquine, and preliminary clinical trials with courses as short as a single dose of this compound have shown encouraging results. Its long half-life can however be a disadvantage in case of G6PD deficiency since it will cause prolonged exposure of the sensitive erythrocytes to the oxidative properties of tafenoquine.

For chloroquine-resistant vivax malaria relatively few data are available. Amodiaquine is efficacious, and there is some evidence that mefloquine and quinine can also be used. Artemisinin combination therapies, like artemether-lumefantrine, are a good alternative. High-level pyrimethamine-sulphadoxine resistance is common in P. vivax and should therefore not be used.

P. malariae (Quartan) Malaria

For P. malariae, the drug of choice is chloroquine at standard doses. Radical treatment with primaquine is not necessary as there are no persistent exoerythrocytic stages in this infection. Chloroquine resistance has to date only been reported in one study from Indonesia.

Uncomplicated Falciparum Malaria

Resistance of P. falciparum against chloroquine has reached most parts of the tropics and resistance against the usual next alternative, sulfadoxine pyrimethamine, is rapidly spreading. To ensure efficacy and to limit the chances of de-novo appearance and spread of antimalarial drug resistance, P. falciparum infections should be treated with combinations of two or more blood schizontocidal drugs with independent modes of action, as in the treatment of AIDS and tuberculosis. The use of an artemisinin derivative as one of the combination partners has become the standard recommendation, since its potent antimalarial capacity quickly reduces the total body parasite number, which will not only relief symptoms rapidly, but also reduces the chance of emergence of clones resistant against the partner drug (201). Artemisinin combination therapies (ACT) are now recommended by the WHO as the first line treatment of uncomplicated falciparum malaria. The different artemisinin derivatives available in oral formulations (dihydroartemisinin, artesunate, and artemether) are all suitable for use in ACT, although they do differ slightly in oral absorption rates bioavailability and disposition. The partner drugs have longer elimination half-lives in order to kill the parasites that remain after three days of artemisinin (about 103 to 105 of the initial 1011 to 1013). Possible partner drugs in ACT regimens include amodiaquine, atovaquone-proguanil, chloroquine, clindamycin, doxycycline, lumefantrine, mefloquine, piperaquine, pyronaridine, proguanil-dapsone, sulfadoxine-pyrimethamine, and tetracyclines. Of these partner drugs, lumefantrine and mefloquine have proven efficacy in areas of multidrug resistant P. falciparum, whereas the combinations with amodiaquine or sulfadoxine-pyrimethamine have shown to be effective in areas where monotherapy failure rates following of amodiaquine and sulfadoxine-pyrimethamine did not exceed 20%.

The artemisinins and the recommended ACTs are discussed below.

Artemisinin and Its Derivatives

Although highly active against all Plasmodium species, the short half-life of these drugs have always limited their use as a single agent, since a minimal treatment course of seven days is needed in order to cover at least 3 erythrocytic cycles of the parasite. However, their potent action and short half-lives reduces the risk of resistance development, and makes the drugs an ideal candidate for the use in combination therapies. Artemisinin and its derivatives are remarkably well tolerated, with very rare serious adverse effects (51,132,174,175). The only evidence of toxicity documented in large clinical trials to date has been a temporary depression in reticulocyte counts, which does not translate into anemia, acute (type 1) hypersensitivity reactions, and in common with quinine, an association with blackwater fever (58,170). The artemisinin derivatives are the most rapidly acting of all antimalarial drugs and produce the fastest clinical responses to treatment. They have a broad spectrum of antimalarial activity acting against the young ring form parasites and preventing their development to the more mature pathogenic stages (171). Oral artemether and oral artesunate have equivalent antimalarial activity and dose regimens of the two drugs are similar (129). In combination with mefloquine, the addition of artemether or artesunate (4mg/kg/day) consistently improves cure rates and has the advantage of producing a more rapid clinical response to treatment, prevention of malaria transmission (51,74,131,169), and, where deployed extensively in low transmission areas, a reduction in malaria incidence. A three-day course of the artemisinin derivative in combination with a partner drug is required for optimum cure rates. Two and one day courses are insufficient (74,106).

Artesunate plus sulfadoxine-pyrimethamine (table 3)

Sulfadoxine-pyrimethamine is a fixed combination of a long-acting sulfonamide (25 mg sulfadoxine) and the antifolate pyrimethamine (500 mg). Minor adverse effects are unusual. Serious sulphonamide toxicity is also unusual with a single-dose treatment of malaria. The anti-folate properties of pyrimethamine rarely produce toxicity. The combination with artesunate is available as separate scored tablets containing 50 mg of artesunate and tablets containing 500 mg of sulfadoxine and 25 mg of pyrimethamine. The total recommended treatment is 4 mg/kg bw of artesunate, given once a day for 3 days and a single administration of sulfadoxine-pyrimethamine 1.25/25 mg base/ kg bw on admission. The combination has been evaluated in children with uncomplicated malaria in Sub-saharan Africa and is sufficiently efficacious in areas where 28-days cure rates with sulfadoxine-pyrimethamine alone exceed 80% (33,179). Since sulfadoxine-pyrimethamine, sulfalene-pyrimethamine and trimethoprim-sulfamethoxazole (co-trimoxazole) are still widely used, resistance is likely to worsen.

Artesunate plus amodiaquine (table 2)

Amodiaquine, like chloroquine, is a 4-aminoquinoline, but it is effective against chloroquine resistant strains of P. falciparum, although there is some cross-resistance. It is generally well tolerated and more palatable than chloroquine. The serious adverse effects that have been associated with its prophylactic use (agranulocytosis and severe liver toxicity) are rare when amodiaquine is used in malaria treatment. The combination of amodiaquine with artesunate is currently available as separate scored tablets containing 50 mg of artesunate and 153 mg base of amodiaquine, but co-formulated tablets are under development. The total recommended treatment is 4 mg/kg bw of artesunate and 10 mg base/kg bw of amodiaquine once a day for 3 days. It has proven to be an efficacious combination in areas where 28-day cure rates with amodiaquine monotherapy exceed 80%.

In those areas where resistance to sulphonamide-pyrimethamine, chloroquine and amodiaquine is prevalent, then artemisinin combinations with either lumefantrine or mefloquine are the alternatives.

Artemether – Lumefantrine (table 4)

This is the first fixed combination of an artemisinin derivative and a second unrelated antimalarial compound. Lumefantrine (formerly benflumetol) is an aryl amino-alcohol in the same general group as mefloquine and halofantrine. It was discovered in the Peoples' Republic of China. Lumefantrine is active against all the human malaria parasites including multidrug resistant P. falciparum (although there is some cross resistance with halofantrine and mefloquine). Artemether-lumefantrine has been used mainly at an adult oral dose of 80/480mg given at 0, 8, 24 and 48 hours. This has given satisfactory cure rates in semi-immune subjects, but in non-immunes has proved inferior to artesunate-mefloquine. Pharmacokinetic-pharmacodynamic (PK-PD) studies indicated that the principal PK determinant of cure was the area under the plasma lumefantrine concentration time curve (AUC), or its surrogate, the day 7 lumefantrine level. Day 7 levels over 500ng/mL were associated with >90% cure rates (35,133). Lumefantrine absorption (like that of atovaquone and halofantrine) is critically dependent on co-administration with fats and thus plasma concentrations vary markedly between patients. To increase the AUC and thus cure rate, a six-dose regimen (adult dose 80/480mg at 0, 8, 24, 36, 48, 60 hours) was evaluated. This has proved highly effective and remarkably well tolerated. Against multidrug resistant falciparum malaria the six dose regimen of artemether-lumefantrine was as effective and better tolerated than artesunate-mefloquine (175). Artemether-lumefantrine is becoming increasingly available in tropical countries. The rapid and reliable therapeutic response, the high level of efficacy, and the mutual protection provided by each of the drugs against resistance selection makes combinations such as this ideal antimalarial treatments.

Artesunate plus mefloquine (table 5)

Mefloquine is a quinoline methanol compound related to quinine. Several different mefloquine formulations are now available with different oral bioavailability. Employment of mefloquine as monotherapy for the treatment of malaria has lead to rapid spread of resistance, mediated at least in part, by an increase in copy number and expression of the P. falciparum multi-drug resistance (MDR) gene. There is theoretical evidence to suggest that initial use of the lower dose of mefloquine encourages resistance (151), and that high doses, preferably in combination with an artemisinin derivative deployed de-novo are less likely to fall to resistance. The dose should be split at 15mg/kg initially followed 8 - 24 hours later by a second 10mg/kg or as 8.3mg/kg daily for 3 days to improve bioavailability and reduce vomiting (132,151). There is no liquid formulation of mefloquine for children. Despite earlier restrictions there is no reason to withhold mefloquine from young children (75). Limited information suggested that mefloquine was probably safe in pregnancy, although the observation in Thailand of an increased stillbirth risk when mefloquine was used in treatment at any stage of pregnancy has cast uncertainty over its use in pregnant women (115). Mefloquine commonly induces nausea, dysphoria, and dizziness, and in approximately 1:1,000 Asian patients, and up to 1:200 Caucasians or African subjects, mefloquine treatment induces a self-limiting acute neuropsychiatric syndrome manifest by coma, convulsions, or psychosis (126). Suidcide has been reported. The risks of this acute neuropsychiatric syndrome are increased if the patient has a previous history of psychiatric illness or epilepsy, and may be increased if mefloquine follows quinine administration. There is a considerable increase in the risk if mefloquine is used following severe malaria. Approximately 1:20 patients with severe malaria given mefloquine following recovery will have an acute reaction and therefore mefloquine should not be used in this group (101). Neuropsychiatric reactions are also more common if mefloquine has been used in the previous month, and therefore mefloquine should not be used to treat recrudescent infections occurring within one month of treatment. However in practice the principle adverse effect of mefloquine is vomiting. This is more likely in young children, and even if the drug is administered again, low blood levels and an increased risk of treatment failure result (165). Combining artesunate or artemether (4mg/kg/day for 3 days) with mefloquine has many advantages. The cure rate is increased, resistance is prevented, and if mefloquine is split as 8.3mg/kg/day for 3 days or not given until the second day of treatment then absorption is increased and adverse effects are lessened.

Non-artemisinin based combination therapies

Atovaquone plus proguanil (MalaroneR)

Malarone is a fixed combination of two blood schizonticides (Atovaquone and Proguanil, 15/6 mg/kg, usual adult dose is 4 tablets once a day for 3 days) that demonstrate synergistic activity against multi-drug-resistant Plasmodium falciparum. Either used alone or in combination with artesunate in areas of drug resistance it is a highly effective treatment.

Generally, atovaquone/proguanil (AQ/PG) is very well tolerated. Treatment limiting adverse events occur in <1% of patients, whereas and no serious adverse effects have been reported. Its high costs limit widespread use in tropical countries, but it can be recommended for travelers returning to non-endemic countries (table 6).

Amodiaquine plus pyrimethamine

Amodiaquine plus sulfadoxine-pyrimethamine combinations has shown to be effective if there is not significant resistance to either drug (212). The combination is generally well tolerated, although some studies report a higher incidence of sinus bradycardia and vomiting compared to amodiaquine or sulfadoxine-pyrimethamine alone.

Quinine plus tetracycline, doxycycline or clindamycin

Although short courses of quinine (3-5 days) are sometimes considered effective in areas of high transmission in semi-immune subjects, as explained previously, quinine should be given for at least seven days for optimum cure rates (54). As quinine is extremely bitter and reliably induces the symptom complex of cinchonism (tinnitus, dysphoria, high-tone deafness nausea, dizziness) compliance with 7-day regimens is notoriously poor and treatment failure rates in practice are usually higher than those reported in carefully observed clinical trials. Quinine should be combined with tetracycline or doxycycline (in non-pregnant adults and children over 8 years old) or clindamycin, also given for 7 days. Even in areas with multi-drug resistant falciparum malaria cure rates with the quinine-tetracycline or quinine-clindamycin combination exceed 85% (71,73,91,135,187). Although minor toxicity is common with quinine, more serious toxicity is unusual. Occasional idiosyncratic or allergic reactions such as urticaria, immune thrombocytopenia and, rarely, haemolytic uraemic syndrome have been reported. Quinine reliably augments pancreatic insulin secretion and in pregnancy may cause hypoglycemia even in uncomplicated falciparum malaria (197). The tetracyclines may be associated with diarrhea, candida infection, and photosensitivity rashes. With the exception of doxycycline they should not be given to patients with renal impairment. Clindamycin combined with quinine has the advantage that it can be used in young children and pregnant women although it is relatively expensive (64-66). There is no advantage to the use of quinidine in uncomplicated malaria, and this should not be used.

Combinations with halofantrine

Halofantrine is intrinsically better tolerated and slightly more effective than mefloquine. Conventionally three doses (total 24 mg base/kg) are given in 24 hours. The manufacturer recommends that non-immune patients should receive a second dose after 7 days. However, in areas of mefloquine resistance the single day treatment with halofantrine usually recommended is insufficient and a longer initial course was found to be required for optimum therapeutic responses (164). Nausea, vomiting or diarrhea is unusual and there are usually no subjective adverse effects. Unfortunately halofantrine and its desbutyl metabolite predictably induce concentration-dependent prolongation of the electrocardiograph QT interval, reflecting delayed ventricular repolarisation (112). Delayed atrioventricular conduction also occurred. Halofantrine use is associated with an increased risk of ventricular tachyarrhythmia and sudden death. This precludes the use of increased doses of halofantrine and has even led the manufacturer to recommend that standard doses of the drug should not be given with fats (which augment absorption). Halofantrine should only be used in patients, with a normal electrocardiograph QT interval before drug treatment, which is not taking other drugs known to prolong the QT interval. Because of these safety concerns, halofantrine is not recommended either as an ACT partner drug or alone. There is no evidence that the combination of chloroquine with sulfadoxine-pyrimethamine provides any additional benefit over pyrimethamine-pyrimethamine alone, and because of the increasing resistance to chloroquine in all settings this combination is not recommended.

Immunocompromised Patients

There is no reason to treat malaria in immunocompromised patients any differently to immunocompetent patients. Patients with previous splenectomies are more vulnerable to severe malaria, and will have longer parasite clearance times - but treatment regimens should not be altered. The pharmacological interactions between antimalarials and antiretrovirals are largely unexplored. Patients receiving rifampicin for tuberculosis eliminate quininemore rapidly and have a higher treatment failure rate.

(Printable Version of General Principles of Antiparasitic Therapy)

Treatment of malaria in pregnancy (table 9)

Pregnant women with symptomatic acute malaria are a high-risk group, and must receive effective antimalarials. There is insufficient information on the safety and efficacy of most antimalarials in pregnancy, particularly for exposure in the first trimester when organogenesis occurs and teratogenesis is of particular concern. The antimalarials considered safe in the first trimester of pregnancy include quinine, chloroquine, and proguanil. Sulfadoxine-pyrimethamine is considered to be safe during the 2nd and 3rd trimester. The artemisinins are considered safe in the 2nd and 3rd trimesters, but there are insufficient data to recommend these highly effective drugs in the 1st trimester, and they are currently not recommended (unless there are no alternatives). Doxycycline, tetracycline, halofantrine (embryotoxic and cardiotoxic), primaquine (haemolysis of fetal RBC) and tafenoquine are contraindicated for use during pregnancy. Mefloquine has been associated with an increased risk of stillbirth but can be used if there is no available alternative.

First trimester

Quinine plus clindamycin to be given for 7 days remains the recommended treatment for falciparum malaria during the first trimester of pregnancy. If clindamycin is unavailable or unaffordable because of its high costs, then monotherapy with quinine should be given. Quinine has been associated with teratogenic effects and damage to fetal optic and auditory nerves, but only at very high (abortifacient) doses (23) and is generally considered safe in pregnancy when taken at normal therapeutic doses (127). In a total of 376 Thai malaria cases, quinine for P. falciparum and chloroquine for P. vivax malaria in the first trimester of pregnancy, caused no increase of congenital abnormalities, stillbirths or low birthweight babies as compared to background levels (90). Clindamycin, a lincosamid antibiotic, has weak antimalarial properties and is only used in combination with other more potent drugs. It has been routinely used for bacterial infections in pregnant women without any evidence of adverse effects (15). The drug can cross the placenta and accumulate in fetal tissues perhaps to the point of therapeutic levels (124), but to date there is no evidence that this accumulation is harmful.

Second and third trimesters

Uncomplicated falciparum malaria in the second and third trimesters of pregnancy can be treated with an artemisinin-based combination therapy (ACT) known to be effective in the country/region. A seven-day course of artesunate plus clindamycin or quinine plus clindamycin is an alternative. Artesunate plus atovaquone/proguanil is also considered safe and effective, but is very costly. Systematic summaries of safety suggest that the artemisinin derivatives are safe in the second and third trimesters of pregnancy. The choice of the combination partner is more difficult. Mefloquine has been associated with an increased risk of stillbirth in large observational studies in Thailand, but not in Malawi. Amodiaquine, lumefantrine, and piperaquine have not been evaluated sufficiently to permit confident recommendations. Chlorproguanil-dapsone is a fixed combination, which, due to altered pharmacokinetics during pregnancy of the chlorproguanil, could have decreased efficacy during pregnancy (see below). Sulfadoxine-pyrimethamine (SP) is safe but may be ineffective in many areas because of increasing resistance. SP should also not be used for treatment where it has already been given for IPD (79). Clindamycin is also safe, but both medicines (clindamycin and the artemisinin partner) must be given for 7 days.Primaquine, halofantrine and tetracyclines should not be used in pregnancy.

The artemisinins and partner drugs, as well as the use of atovaquone-proguanil in relation to pregnancy are further discussed below.

Artemisinin derivatives

Artesunate has proved to be more effective then quinine in preventing recrudescent disease in malaria in pregnancy. Treatment failure is an important problem in the treatment of malaria in pregnancy. A study on the Thai-Burmese border showed that treatment with artemisinin derivatives, (mainly artesunate), resulted in a cumulative failure rate of 7.8%, compared to 33% after a 7 day course of quinine. Treatment with artesunate of recrudescent infections showed a cumulative failure rate of 21% compared to 37% with quinine and 38% with mefloquine (83). Safety data for the artemisinins for use in the first trimester are still limited. A total of 287 pregnant women, of whom 80 were in the first trimester, in The Gambia were exposed to a single artesunate dose (4mg/kg) in combination with SP during a mass drug administration; no differences between exposed and non-exposed pregnancies were observed in the rates of abortions, stillbirths, or infant deaths (28). In a study of 28 Sudanese women, intramuscular artemether was used as salvage therapy (6 injections, total 480mg) after failure ofchloroquine and quinine-based regimens. Parasitemia was successfully cleared after 3 days, no maternal side effects, abortions, stillbirths or congenital abnormalities occurred. A study in 83 women in Thailand treated with either artesunate or artemether, of whom 16 were accidentally exposed during the first trimester, found no increase in rates of spontaneous abortion or stillbirth. Follow-up of this cohort found no developmental delay in the exposed children (82).

Another Thai study in 66 pregnant women treated with the combination of artesunate plus mefloquine (81) or artesunate alone (85) showed no increase in adverse birth outcomes, and no developmental retardation compared to quinine treatment. The results of a study using artesunate (n=528) and artemether (n=11) to treat 539 episodes of acute falciparum malaria in a total of 461 pregnant women, including 44 first-trimester episodes, showed that the artemisinins were well tolerated, and there were no increased rates of abortion, stillbirth or congenital abnormalities. Mean gestation at delivery was unaltered (84). These data suggest the artemisinin derivatives are safe in the second and third trimesters.

It has recently been shown, that the kinetics of antimalarials can be modified in pregnancy, as plasma levels of the active antimalarial metabolite Dihydro-artemisinin were found to be lower than reported previously in non-pregnant adults (89). Further dose-optimisation studies involving pharmacokinetics in pregnant women are needed and will directive for future regimens.

Mefloquine

Safety of mefloquine during pregnancy has been evaluated through post marketing surveillance, retrospective and prospective studies. An early dose-finding study in Thailand found no increase in adverse pregnancy outcomes among women (n=20) who took either 125 or 250 mg/week for prophylaxis during the third trimester (104). In a prospective study in pregnant women, mefloquine chemoprophylaxis was associated with an increase in stillbirths compared to sulfadoxin-pyrimethamin (9.1% vs. 2.6%), but this rate was not significantly different from the non-exposed population (7–10%) (128). Among US soldiers in Somalia, 72 women used mefloquine as a chemoprophylaxis before learning of their pregnancies, resulting in an above average rate (16.7%) of spontaneous abortions (157). Data from a considerably larger retrospective study in Thailand (n=3,587) suggested a significantly increased risk of stillbirth among women exposed to treatment doses of mefloquine (n=208) during pregnancy compared to those exposed to quinine (n=656) [odds ratio (OR) 4.72], other treatments (n=909, OR 5.10) or pregnancies with no malaria episodes (n=2470, OR 3.50) (115). No specific patterns of physical abnormalities or defects were found in the stillbirths associated with mefloquine use. Other studies confirm the absence of a teratogenic effect of mefloquine. Data from two studies in Thailand have shown no developmental changes in infants born to mothers exposed to mefloquine as chemoprophylaxis during pregnancy compared to those given placebo (110), or born to mothers treated for malaria infection with mefloquine plus artesunate compared to quinine (81). Post-marketing data collected by the manufacturer showed that among 1627 women exposed to the mefloquine drug during pregnancy (95% for chemoprophylaxis), there was no increase in congenital malformations over the expected background rate (176). Because of the association with stillbirth in the Thai studies, mefloquine cannot be recommended during the first trimester, but can be used in the second and third trimester if no safer alternative is available.

Lumefantrine

The safety profile of the artemether-lumefantrine combination has proven to be excellent in both children (178) and adults (181), and the drug has a good safety profile from animal studies. The only evaluation so far for pregnant women is an ongoing pharmacokinetic study on the Thai-Burmese border. It is currently contraindicated in the first trimester of pregnancy.

Proguanil and Chlorproguanil

Proguanil and chlorproguanil are used only in combinations in malaria treatment and are considered safe throughout all trimesters of pregnancy. In a cohort of pregnant travelers taking proguanil in combination withchloroquine, rates of spontaneous abortions or congenital anomalies were no higher than the expected background rate (128). A study in Nigeria found no increase in adverse outcomes among pregnant women receiving daily proguanil (100 mg) in addition to weekly chloroquine (37). Another study from Tanzania demonstrated no increase in adverse outcomes among women receiving proguanil alone or in combination with weekly chloroquine as compared to those receiving chloroquine alone (98).

Amodiaquine

Amodiaquine is generally used in combination with other drugs. Little information is available of its use during pregnancy (168).

Dapsone

Dapsone has been extensively used in pregnant women with leprosy, without reported adverse effects (56). In studies where Dapsone was given in combination with pyrimethamine (Maloprim) for malaria chemoprophylaxis during pregnancy no increase of adverse pregnancy outcomes were documented (47). Lapdap™ is a fixed combination of dapsone and chlorproguanil, of which the chlorproguanil component is likely to have decreased absorption during pregnancy (88). If the dose is increased, the risk of side effects related to dapsone will also be increased. It has been used thus far mainly in Africa. In a small study involving 158 pregnant women 62 were treated with Lapdap (59). No adverse effects were documented, but details on pregnancy outcome were not reported. If further safety data become available, it could be used in combination with an artemisinin derivative during pregnancy.

Sulphonamides and Pyrimethamine

Sulphonamides and pyrimethamine are generally considered safe in the second and third trimesters of pregnancy. Although sulpha drugs have been associated with kernicterus when given to premature neonates, this theoretical problem of risk of kernicterus when given in late pregnancy has not been noted in studies using sulphadoxine–pyrimethamine (SP) as intermittent prophylactic treatment (IPT) in pregnancy, where 2 to 3 doses are given, but not intentionally during late pregnancy when the risk of kernicterus is greatest (148). Studies on the safety of sulfadoxine-pyrimethamine in pregnancy have not reported increased risks in spontaneous abortions or congenital defects (119). Sulpha containing drugs are associated with rare but severe cutaneous reactions such as toxic epidermal necrolysis and Stevens–Johnson syndrome, but there is no evidence that this risk is any greater in pregnant women.

Atovaquone-proguanil

This fixed combination of two blood schizonticides either used alone or in combination with artesunate is a highly effective treatment even in areas of multidrug resistant malaria. In a study in a multidrug resistant area on the Thai-Burmese border, the combination of atovaquone/proguanil plus artesunate proved to be well tolerated, highly effective and practical for the treatment of uncomplicated malaria during the second and third trimester of pregnancy. A Thai study in 81 pregnant women comparing atovaquone/proguanil (3 days) to supervised quinine (7 days) showed that fever- and parasite clearance rates, and duration of anaemia were more favorable with atovaquone/proguanil and treatment failure rates were 7 times lower (5% vs. 37%). There were no significant differences in birth weight, duration of gestation, congenital abnormality rates in newborns or in growth and developmental parameters of the infants that were monitored for 1 year (80). So the combination of artesunate with atovaquone/proguanil is safe, efficacious, and practical (3-day course), and is a useful back-up treatment for areas with multidrug resistant malaria (86). However the high cost of this combination limits its global use. Pharmacokinetic studies on atovaquone and proguanil during pregnancy suggest that the recommended dose likely needs to be increased (87).

Treatment of vivax malaria during pregnancy

Vivax malaria during pregnancy can be treated safely with the usual dose of chloroquine in areas where P. vivax is still sensitive to chloroquine. However, primaquine used for the treatment of liver hypnozoites, is contraindicated during pregnancy. To prevent relapse of the infection from liver hypnozoites, it can be recommended that pregnant patients with P. vivax and P. ovale infections should be maintained on chloroquine prophylaxis for the duration of their pregnancy, which completely prevents recurrence of P. vivax episodes (177). Chloroquine prophylaxis during pregnancy is well tolerated and safe. Since chloroquine crosses the placenta and concentrates in the retina, this study specifically examined the impact on neurological development or visual acuity in the infants at one year of age and showed that there were no adverse effects.

The chemoprophylactic dose of chloroquine phosphate is 300 mg base (=500 mg salt) orally once per week. Curative treatment with primaquine can then be safely started a few months after delivery. Areas in New Guinea, Indonesia and South America have reported increasingly resistance of P. vivax resistance to chloroquine. Quinine, artesunate and amodiaquine are used under these circumstances, but no clinical studies on their efficacy and safety are available, yet.

Chloroquine

The safety profile of chloroquine for use in pregnancy is excellent and has been proved over the decades; children born to a cohort of 169 non-immune women who took chloroquine chemoprophylaxis throughout pregnancy had no more birth defects than 454 births to women who had not received chloroquine (209). Among >2500 women who received chloroquine (as IPT or weekly chemoprophylaxis), there was no reported increase in abortions, stillbirths, or congenital abnormalities, although there were frequent non-severe side effects such as itching, dizziness and gastrointestinal complaints (160). There are reports of increased spontaneous abortions, particularly in patients with systemic lupus erythematosus treated with high doses of chloroquine over prolonged periods, and in some settings, chloroquine has gained a reputation as an abortifacient when used in high doses (190), but its abortifacient effects are limited to these very high doses that are life-threatening to the mother.

Treatment of severe malaria in pregnancy

Severe malaria is a medical emergency. Pregnant women are at increased risk of developing severe symptoms. Severe anaemia, hypoglycaemia and pulmonary edema are all more common manifestations of severe malaria during pregnancy. The primary objective of treatment is to save the life of the mother. Although not specifically shown for pregnant women, intravenous artesunate has proven to be the superior antimalarial compared to quinine in the treatment for severe malaria (31), and is safer - as hypoglycaemia is a severe and common adverse effect of quinine in this context. If parenteral artesunate is not available, parenteral artemether remains the best alternative. Treatment doses are the same as in non-pregnant individuals (197) (Table 7). The related compound quinidine, is similar to quinine considered safe in pregnancy and there are no reports of congenital abnormalities associated with its use during pregnancy, although there have been reports of neonatal thrombocytopenia after maternal use (11). Quinidine is only used in the U.S.A., since quinine is not available there.

Breast Feeding

Most antimalarial drugs are detectable in breast milk, but the doses received by the breast feeding infant would be very small. Only primaquine should not be used, and it would be probably be unwise to use sulfonamidesif the newborn baby was markedly jaundiced.

Empiric Standby Treatment

In situations where prophylaxis is not indicated, either because of resistance, potential toxicity, or because the risks of malaria are low or unpredictable, and diagnosis and treatment may be unavailable, then empiric "Standby treatment" is sometimes administered. Patients with fever and a negative blood smear do not usually have malaria, although there are exceptions. Occasionally in a non-immune individual, the fever precedes patent parasitemia. The smear should be repeated in 12-24 hours. Empiric antimalarial therapy administration is common in tropical countries, and the patient may even be unaware of the drugs administered. There are simple dipstick tests for antimalarial drug screening in blood and urine, but these are not generally available (150). If the patient is not severely ill, it is reasonable to withhold treatment and repeat the smears. On the other hand, if the patient cannot be followed reliably, or is seriously ill and no alternative diagnosis is likely, treatment should be continued. In these cases the HRP2 dipstick test is useful as HRP2 is cleared relatively slowly and the test is usually positive well after parasite clearance; it is particularly useful in assessing a severely ill patient who may have been treated. In addition intra-monocytic malaria pigment is often present in peripheral blood or intradermal blood smears (26).

Drug Deployment

In general chloroquine is still used widely despite resistance. In many endemic areas the adverse impact of chloroquine resistance is underestimated because the majority of treatment failures are in young children. In order children and adults, who have significant background immunity, partial or complete responses may be still seen despite high failure rates in younger children (or non-immune travelers). Where high-level resistance precludes its use, pyrimethamine-sulfadoxine has been the usual successor, although in some parts of Africa and Oceania amodiaquine has been used as an alternative. Where pyrimethamine-sulfonamide resistance occurred, the choice of antimalarials was limited. Amodiaquine may still be effective, and chlorproguanil-dapsone is more effective than the pyrimethamine-sulfonamide combinations. A triple combination of pyrimethamine-sulfonamide with amodiaquine was better than either alone. Now ACTs are recommended as first line treatment everywhere although in several areas amodiaquine-SP is a good alternative. Combinations with quinine are widely used, and in children over eight years of age, quinines combined usually with tetracycline. Combinations with clindamycin are equally effective and can be used at any age. Quinine is extremely bitter and reliably produces "cinchonism" and so compliance is very poor with the seven-day quinine treatment regimes required for optimum cure rates. In Thailand, mefloquine (15 mg base/kg) was deployed originally in combination with sulfadoxine and pyrimethaminebut this triple combination failed to delay the development of resistance. From 1991 mefloquine in a higher dose (25 mg base/kg) has been deployed alone, and more recently has been used in combination with the artemisininderivatives. This has improved cure rates and, where the combination has been deployed systematically, levels of mefloquine resistance have fallen by half and the incidence of falciparum malaria has also dropped (114). Interestingly Pfmdr amplification only confers a certain degree of resistance, at a considerable fitness cost, and so is outbred when the selective pressure is reduced. Early diagnosis and treatment with the artemisinin derivatives has also been associated with a marked reduction in the incidence of malaria in Northwest Thailand, Vietnam and in South Africa (in association with DDT spraying). There is now increasing acceptance that antimalarial drugs should be used only in combination to protect them from resistance, and that one of the combination partners should be an artemisinin derivative. The artemisinin derivatives are available in some tropical countries, but have not been registered in most northern countries. Pyronaridine, atovaquone-proguanil, artemether-lumefantrine, and dihydroartemisinin-piperaquine have yet to be deployed widely. Halofantrine is used in some countries although given its toxicity it should probably be withdrawn.

(Printable Version of Antiparasitic Therapy for Pregnant Women)

ADJUNCTIVE THERAPY

When to Admit a Patient to the Hospital

Most patients with malaria in the tropics are treated at home. But any patient about whom the physician is worried should be admitted, particularly if they have falciparum malaria. This is a dangerous disease, and the diagnosis is often delayed outside the endemic areas. Obviously, if there are signs of severity then the patient requires hospitalization, but patients with repeated vomiting, prostration, pregnancy, or extremes of age should all be admitted. P. falciparum parasite counts over 2% in non-immunes are potentially dangerous and need careful monitoring.

Exchange Transfusions

There is no proof that exchange transfusion is an effective treatment of severe malaria. There have been no randomized trials and there is even uncertainty as to how it might work. Nevertheless the large number of anecdotal reports and series do suggest benefit (210). If facilities exist, cross-matched virus free blood is available and there are skilled personnel then it is reasonable to consider exchange transfusion in any seriously ill patient. In the past recommendations have given parasitemia thresholds, but this seems illogical, as the most likely explanation for benefit is removal of rigid erythrocytes (32). This is relatively independent of parasitemia, and more reflective of clinical severity overall - so the decision to exchange should not be based only on the parasite count.

Anti-cytokine drugs

There is no evidence at all which would support anti-cytokine interventions.

ENDPOINTS FOR MONITORING THERAPY

Malaria is commonly associated with nausea and vomiting and children, particularly infants, often regurgitate the initial antimalarial drug doses. This is particularly important for single dose administration (e.g. mefloquine, pyrimethamine-sulphadoxine) where inadequate therapy will result if the drug is vomited. Patients receiving single dose treatment should be observed for at least one hour following drug administration. In the case of mefloquine the full dose should be given if vomiting occurs within half an hour of drug administration, and half the dose should be given if vomiting occurs between 30 and 60 minutes following drug administration. For other antimalarials, the dose should be repeated if vomiting occurs within one hour. In young children, and in patients who have already vomited, it is often better to cool the patient and give antipyretics before administering antimalarial drugs (75). In uncomplicated malaria the objective of treatment is to resolve symptoms as rapidly as possible and to prevent recrudescence of the infection. It is essential that adherence to the prescribed regimen is ensured and a full course of treatment is taken. Without supervision adherence with antimalarial drug treatment is often poor, particularly for drugs with frequent adverse effects such as quinine) and treatment regimens of over three days may not be taken as prescribed. The patient must understand the consequences of not completing the antimalarial treatment course both for themselves, and also, if possible, for the community in terms of increasing antimalarial resistance and transmission. Patients with uncomplicated malaria can be treated on an outpatient basis provided that oral medication can be taken. If the patient is very nauseated, or actually vomiting, but has no other signs of severity, or if the physician or health attendant is in any way worried about the patient, then the first dose of antimalarial drug should be given parenterally, and the patient observed in hospital if possible. Parenteral formulations of sulphadoxine-pyrimethamine and amopyraquine (similar to amodiaquine) are available in some countries (43,208). Neither should be used in severe malaria unless there are no alternatives, but may be used in patients who are vomiting but have no other signs of severity. Blood smears should be checked daily in outpatients, and more frequently in inpatients, until the thick film is clear of malaria parasites. If the patient deteriorates clinically after oral treatment, then parenteral treatment should be substituted, and if deterioration occurs after intramuscular administration of a non-aqueous formulation (e.g. artemether, arteether, pyrimethamine-sulfadoxine), intravenous treatment should be substituted if possible. If the parasite count has not fallen by 75% after 48 hours the count should be re-checked and a change in treatment considered. After parasite and fever clearance the patient should be instructed to return if symptoms recur.

Following short-acting antimalarial drugs such as quinine or an artemisinin derivative, nearly all recrudescences occur within one month of treatment (202). Most recrudescences following chloroquine also occur within one month, although at low levels of resistance recrudescences with chloroquine and mefloquine may occur up to ten weeks following treatment. The species should always be identified on blood smears as co-infection withPlasmodium vivax is very common in some areas (72) and Plasmodium vivax often reappears at three weeks following acute treatment of falciparum malaria. If chloroquine or mefloquine are used the P. vivax infection may occur much later (usually around six weeks post treatment for frequent relapse strains). Persistence of P. falciparum gametocytes (sexual stages) does not imply drug resistance. These forms clear more slowly than the asexual parasite stages.

Uncomplicated Malaria

Following the emergence of chloroquine resistance in P. falciparum in the early 1960s a World Health Organization Advisory Group provided a classification for 4-aminoquinoline resistance, which has provided a useful framework for the evaluation of therapeutic responses to all antimalarial drugs (211). In this classification, resistance in patients was divided into three categories: R1. Recrudescence of the infection between 7 and 28 days after completing antimalarial treatment. R2. Reduction in peripheral parasite count by more than 75% at 48 hours, but failure to clear parasitemia within one week. R3. Parasite count does not fall by more than 75% within 48 hours of starting treatment.

Although nearly all recrudescence following short-acting antimalarial drugs, and most treatment failures following chloroquine and amodiaquine occur within 28 days of treatment, true recrudescence may occur after this time. This is particularly likely with slowly eliminated compounds such as mefloquine, for which true recrudescence has been documented up to 63 days following antimalarial treatment. These late recrudescence occur particularly at low levels of drug resistance as suppressive concentrations of the slowly-eliminated antimalarial drug occur for months after starting treatment (38,106). In this context restriction of the follow-up period in drug assessments to 28 days will under-estimate in vivo mefloquine resistance by at least one third (156). In some infections, again particularly those treated with slowly acting drugs such as mefloquine, parasites may clear slowly from the peripheral blood and parasitemia may still be present 7 days following treatment (i.e. an "R2 response"). Parasitemia in these infections will all have cleared by 9 days in susceptible infections (193). Very occasionally patients are encountered whose parasite count does not fall by 75% at 48 hours but clears well thereafter. On the other hand some patients' parasitaemias will fall by >75% at 48 hours but then rise again with a deterioration in clinical condition in the next one or two days.

Modified criteria for resistance have been suggested for use in high transmission areas with follow-up only to 14 days. These are very insensitive in detecting only high levels of resistance, and they have been applied inappropriately to lower transmission areas. They are no longer recommended; follow up must be for ≥ 28 days everywhere. The latest WHO criteria for in-vivo resistance assessment are shown in Table 10.

As resistance develops in an area, the finding of delayed parasite clearance in a treated patient indicates an increased likelihood of subsequent recrudescence. The PC50, PC90, and PCT can be recorded and the fractional reduction in parasitemia over one asexual cycle, or parasite reduction rate (parasite count before treatment divided by parasite count exactly 48 hours later), all give an in vivo indication of the efficacy of the antimalarial drug on the circulating asexual stages of infection.

Clinical parameters of treatment response include the duration of fever, and the time taken to return to school or work. In drug comparisons parasite count and core temperature should ideally be recorded at least 6-hourly until parasite and fever clearance respectively. In areas of high transmission where reinfection cannot be excluded low-grade resistance (equivalent to R1 or LTF) cannot be assessed. In areas of lower transmission, where patients return home following treatment, recurrence of the infection may be assessed either by a comparison of parasite genotypes in the acute and recurrent infection, or by adjusting apparent treatment failure rates for the background level of infection (i.e. requires concurrent information on the incidence of new infections) (165,166). In P. falciparum infections genotyping of defined polymorphic loci (such as micro satellites or sequences within the genes encoding MSP1, MSP2, TRAP, GLURP, etc) can be used to differentiate reinfection from recrudescence where the prevalent number of genotypes is low (12,178). An alternative approach, which has been used widely, but is expensive, impractical and sometimes inhumane, is to keep people away from malaria transmission during the follow-up period. Patients have been studied in closed wards or in towns outside the transmission areas where reinfection can be excluded confidently.

Severe Malaria

In severe malaria the objective of antimalarial treatment is to save life. Cure of the infection (prevention of recrudescence) is of lesser concern. The overall mortality of severe malaria in experienced tropical centers is approximately 15% in children and 20% in adults (170,173,183,210). The response to antimalarial treatment of severe malaria is assessed overall in terms of mortality (although no trials have had sufficient statistical power to determine anything other than large differences in mortality) and the development of severe manifestations such as acute renal failure or pulmonary edema (in adults), or the number of subsequent convulsions (children). Clinical responses are assessed in terms of the resolution of coma, usually the times for the level of consciousness to rise to Glasgow coma scores of 8, 11 and 15 respectively (170). In children the assessment is usually made with the modified Blantyre coma score and the times taken to reach levels 3, 4 and 5 are recorded (95). The rates of resolution of biochemical abnormalities, particularly hyperlactataemia, acidosis, and in adults normalization of serum creatinine, have also proved useful measures. The time to fever clearance and the times for parasite counts to fall by 50%, 90%, and to become undetectable, are also useful in drug comparisons although they may not reflect accurately resolution of the disease (192). Finally the times taken for the patient to sit, eat, drink, walk, and to leave hospital are also recorded and the incidence of residual neurological sequelae should be documented.

Vivax and Ovale Malaria

These two human malarias are characterized by the persistence of liver stages (hypnozoites), which cause relapses of the infection. Thus in assessing drug efficacy recrudescence must be distinguished from relapse. For those, now rare, strains of P. vivax with long intervals between primary infection and relapse (48) this is not a problem, but in the frequent relapse tropical strains it may be impossible to distinguish the two (55). In general infections, which re-appear within one month following treatment with chloroquine represent resistance. They are either a recrudescence, a relapse, or reinfection, but whichever of these they are, the parasites are still able to grow through blood levels of chloroquine which should suppress them (5,202). The median relapse interval following chloroquine treatment is usually six weeks but following administration of a short acting drug such as quinine or artemisinin it may be as short as three weeks. This is because the more slowly eliminated drugs suppress the first relapse. Thus infections appearing three weeks after a short acting drug could be either relapse or recrudescence (134). In contrast with P. falciparum genotyping of P. vivax cannot distinguish reliably between relapses, reinfections or recrudescences. Until recently the three benign human malarias have all been uniformly susceptible to chloroquine, but there are now well documented foci of chloroquine resistance in P. vivax in South East Asia, Oceania and South America (96,144).

VACCINES

There is no highly effective malaria vaccine. It has proved very difficult to develop malaria vaccines for several reasons. The immune response to malaria infection is complex and inefficient. Malaria parasites have enormous capability for antigenic variation. Most of their life within the human host is spent within a red cell that does not express HLA antigens, and malaria infections induce an intense and non-specific immune response, which may impede the development of specific and effective immune mechanisms. Natural untreated infections persist for up to nine months within the human host. In areas of high transmission true immunity never develops. Parasitemia may be detected in people of all ages. What does develop is a state whereby the individual can control the level of the infection to a level that dose not induce high-level production of pro-inflammatory cytokines, and does not therefore cause symptoms. This state of "antitoxic" immunity is known as premonition. The objective of vaccine development has therefore shifted in recent years from a goal of inducing sterile immunity towards reduction in the risk of the development of severe malaria (i.e. a vaccine which would prevent death and severe disease, but not infection). Vaccine development has concentrated on the three stages of the parasite in the human body. Vaccines against the pre-erythrocytic or liver stage infection are directed against antigens expressed on the sporozoite, or by the intrahepatic stage of development. Initially interest concentrated on sporozoite vaccines because of studies in the early 1970s, which showed that sterile immunity could be induced in volunteers, given a large dose of irradiated sporozoites (17). Since then progress has been slow (214) although there were encouraging results with a recombinant sporozoite vaccine, which contains a T-cell epitope (lacking in earlier versions of this vaccine) and a new and highly active adjuvant (161). The most successful vaccine to date is RTS,S, also directed against the pre-erythrocytic stage of the parasite, where the circumzoite protein is fused with hepatitis S antigen, but with different adjuvants. A phase IIb study in Mozambique showed that a 3-dose vaccination scheme resulted in a 30% risk reduction for clinical malaria and a 58% risk reduction for severe malaria during 6 months follow up, whereas these numbers were 35% and 49% after 12 months follow up (2). Asexual stage vaccines have concentrated on the identification of blood stage antigens, which correlate with protective immunity, and the production of recombinant molecules with these antigenic properties. The vaccine developed initially by Manuel Patarroyo, known as SPf66, has held centre stage for the past decade (121). This vaccine contains several antigenic epitopes from the asexual (blood) stage and also a sequence from the principal sporozoite antigen (the circumsporozoite protein). The SPf66 vaccine initially gave encouraging results (>30% protection) in large uncontrolled trials in South America and more recently in a large double-blind placebo controlled trial in Columbia (172). These were followed by a large double-blind placebo controlled trial from Tanzania (3), which showed borderline protective efficacy. However, two more recent trials from The Gambia (21) and Thailand (105) showed no evidence at all of protective efficacy with this vaccine, and the general consensus is that the vaccine is ineffective. Transmission-blocking vaccines directed against gametocyte antigens are also under development (184). No live attenuated vaccines have been developed. The future will probably lie with the production of multi-stage vaccines either in attenuated viral carriers or as DNA, and these are in early stages of development.

PREVENTION

General Measures

Malaria may be prevented either by reducing contact between man and the vector mosquitoes or by continuously taking antimalarial drugs. For travelers personal protection should be emphasized. This involves avoiding being outdoors at peak biting times, wearing clothes which cover ankles and wrists, and use of effective insect repellents (such as diethyltoluamide 10-20%). Use of insecticide impregnated (usually with permethrin or deltamethrin) mosquito nets is highly effective in preventing night-time biting. They are less effective if the vector is an early evening or early morning biter. Impregnated nets are effective even if there are small holes in the net. Impregnated nets have two anti-mosquito functions, personal protection, and mosquito killing. In endemic areas this latter "mass" effect may be more important, providing a method of control that affects also those who live nearby. In large studies conducted mainly in Africa the use of insecticide-treated bed-nets has been associated with a significant all-cause reduction in childhood mortality (22). In the original studies in The Gambia all-cause mortality in children aged between 1 and 4 was reduced by 60%, indicating the enormous contribution of malaria to childhood death in this area (1). Either nylon or cotton nets may be used although nylon retains more insecticide, and the nets should be washed infrequently and treated at least once yearly. "Permanently" impregnated nets are under development. The use of other anti-mosquito measures such as burning repellent coils and impregnation of curtains and other housing materials also reduces vector contact.

PROPHYLAXIS

Overview

Antimalarial prophylaxis is a most controversial area. The ever changing patterns of antimalarial drug resistance and the great variability in human circumstances and behavior (and thus exposure) make risk-benefit generalizations impossible. Prophylactic efficacy is assessed by the incidence of infections, which break through in subjects taking antimalarial prophylaxis. Obviously poor compliance must be distinguished from drug resistance and either documentation that the subjects have taken the antimalarial prophylaxis, or preferably measurement of a blood level, are required. Prophylactic drugs may work either by preventing pre-erythrocytic development (causal prophylaxis) or by suppressing development of the blood stage infection (suppressive prophylaxis).

Indications

It is generally agreed that antimalarial prophylaxis is indicated if exposure is likely, but it should be emphasized that prophylaxis is never 100% effective, and should be complemented by a strategy of mosquito avoidance (use of impregnated bed nets, screens, insecticides, repellents, avoidance of peak biting times etc). Travelers to South America or East Asia are often at very low risk of malaria as they seldom visit endemic areas. In order to prevent errors of both commission and omission, specific and detailed advice should be sought before travel on the risks of malaria, and the appropriate prophylaxis.

Doses and Schedules

Chloroquine is generally well tolerated. The usual dose is 5 mg base/kg given once weekly, although daily administration (1.5mg base/kg) is often recommended in Francophone countries. Subjects who have received chloroquine continuously for more than 5 years should have regular ophthalmologic examinations to exclude retinopathy. This is relatively unusual with prophylactic use of the drug, particularly if a cumulative total of less than 100g has been consumed. Chloroquine is often combined with proguanil (3 mg/kg/day) to provide additional protection against low-level chloroquine resistant P. falciparum infections. Both drugs are considered safe in young children and pregnant women. Although chloroquine still provides good protection against most P. vivax, and all P. ovale and P. malariae, widespread resistance in P. falciparum now precludes its prophylactic use in most of the tropics.

An alternative is mefloquine (3 mg base/kg once weekly). Mefloquine is effective everywhere, except for focal areas of SE Asia, and it reasonably well tolerated (53,146). Reversible but sometimes debilitating nervous system toxicity is the main adverse effec. Dysphoria, nightmares, giddiness and feelings of dissociation are reported commonly by travelers, although in prospective controlled studies their reported incidence is less. Because of these central nervous system effects some authorities have begun to recommend less effective alternatives for antimalarial prophylaxis. Whether this will result in an increase in malaria morbidity and mortality in travelers remains to be seen (46). Serious neuropsychiatric reactions occur in 1:10,000 prophylactic users (70). This is similar to the rate for chloroquine. These usually occur soon after starting prophylaxis. Mefloquine should not be used by people with epilepsy, psychiatric conditions, or those who have taken halofantrine for treatment, and should not be prescribed to pilots, coach drivers etc. There is increasing evidence that mefloquine prophylaxis is relatively safe in young children, or in patients with cardiovascular disease, although surveillance continues and definitive statements cannot yet be made. There are limited data on long term use of mefloquine prophylaxis.

In areas where mefloquine resistant P. falciparum is prevalent, doxycycline (100mg day) is effective. It should not be given to children ≤ eight years old, or to pregnant women. Monilia (Candida), photosensitivity, and sometimes diarrhea are the main adverse effects reported.

Atovaquone-proguanil (250-100mg adult dose/day) is a well tolerated and effective, although relatively expensive antimalarial prophylactic, which is effective throughout the world including in areas with multi-drug resistant malaria. The majority of experience has been obtained in adults. There is no experience to date in pregnancy. Atovaquone- proguanil is now registered for prophylactic use in several countries.

Recent studies have assessed the prophylactic efficacy of primaquine, and, and have shown that primaquine in an adult dose of 30mg/day is well tolerated and effective against both P. vivax and P. falciparum (147,159). Preliminary studies with the slowly eliminated 8-aminoquinoline tafenoquine also indicate good prophylactic efficacy (69).

Both amodiaquine and pyrimethamine-sulfadoxine are associated with unacceptable risks of serious toxicity when used continuously for prophylaxis (50,93) and should not be prescribed for this indication. In Oceania a once weekly administration of pyrimethamine-dapsone (12.5/100mg; Maloprim™) is effective in some areas. This should not be taken more than once weekly. Dapsone may cause methemoglobinaemia, and haemolysis particularly in subjects who are G6PD deficient.

Antimalarial prophylaxis should start one week before leaving for the endemic area so that adverse effects can be identified, and to ensure therapeutic levels are present when the infection would otherwise develop. Prophylaxis should continue for one month after leaving the transmission area. The great advantage of atovaquone-proguanil and primaquine is that these drugs do not need to be continued after leaving the endemic area, because their effects are principally on the liver stage of malaria parasite development.

CONTROVERSIES, CAVEAT, COMMENTS

Economic Aspects

Relatively few people suffering from malaria have the luxury of optimum treatment. The major factor determining use of antimalarial drugs in most tropical countries is cost, and most tropical countries have a total per-capita health expenditure on all medicines, vaccines, and other health interventions, of less than US$10 per year. Chloroquine and sulfadoxine-pyrimethamine can cost less than 10 cents a treatment but mefloquine, halofantrine, quinine, the artemisinin derivatives, and any new compounds, may cost more than US$1. The ACTs cost between 0.5 and 3$. This puts a tremendous strain on resources; despite policy change to ACTs chloroquine and SP are therefore still widely used in areas where they are ineffective. In many areas of the tropical world drugs are simply unavailable. For patients with severe malaria, facilities for parenteral administration may be hours or days away. Many do not reach these places.

In most countries antimalarial drug availability is limited. In tropical countries the expensive antimalarials are largely confined to the private sector. In temperate countries hospitals and pharmacies often have few or no antimalarials. Poor quality and fake drugs abound. Even in richer countries quinidine has become unavailable in some areas as its use as an anti-arrhythmic declines. There has been a temporary dearth of primaquine worldwide as manufacturers ceased "uneconomic" production of the drug. Availability of the artemisinin derivatives has also recently been limited, although the situation is now improving again. These drugs are usually not registered in temperate countries.

Global strategies for antimalarial drug deployment are needed to reduce malaria morbidity and mortality, halt the spread of resistance, and to provide the maximum benefit from existing antimalarial drugs. A global subsidy is needed so that effective drugs are effectively free.

Management Controversies

Is It Safe to give Treatment Doses of Quinoline Antimalarials in Prophylactic Failures?

Except for halofantrine treatment, where serious cardiotoxicity risks are increased, there is no evidence that this it is dangerous to prescribe quinine, chloroquine, or mefloquine to patients who have already been taking antimalarial prophylaxis. It would not make sense to use the same drugs in treatment that had been used as prophylaxis.

Are the Artemisinin Derivatives Neurotoxic?

The oil based parenteral artemether and arteether are neurotoxic in animal models. The water soluble artesunate is considerably less neurotoxic (102). Oral administration of artemether is significantly less neurotoxic than intramuscular injection of the same drug (103). Neurotoxicity appears to result from sustained exposure of the central nervous system to the artemisinin derivative. Transient intermittent exposure is much less dangerous. There is no evidence to date that neurotoxicity occurs in man (174). As continuous nervous system exposure has been shown to be an important contributory factor in the animal studies, these drugs should not be used in prophylaxis.

When Should a Loading Dose of Quinine be Given?

A loading dose of quinine should be given to any patient with severe malaria who has received less than 40 mg/Kg of quinine over the previous 48 hours. For some reason, physicians treating malaria have been more preoccupied with quinine toxicity (which is very seldom lethal) than with the dangers of under treatment - and consequent fatal outcome. If there is uncertainty, then the loading dose should be given, as the risks of under- treatment in the acute phase of severe malaria, exceed those of over treatment.

What Dose of Quinine Should be Used in African Children?

There is excellent agreement between the published series on quinine pharmacokinetics in African children (120,182). Mean peak plasma or whole blood quinine concentrations following an intramuscular loading dose of 20 mg salt/kg were between 15.3 and 15.7 mg/L with standard deviations between 3.1 and 7.6. The mean peak quinine concentration after the loading dose was 15.6 mg/L (SD 5.2 mg/L). Thus, if an intramuscular quinine loading dose of 20 mg salt/Kg were to be given to a child with severe malaria, there would be an approximately 20% chance that plasma concentrations would exceed 20 mg/L and a 3.5% chance they would exceed 25 mg/L. The use of a lower 15 mg/kg loading dose would reduce these chances to 5.5% and 0.5% respectively. This extra margin of safety is bought at the price of a five-fold increase in the risk (from 2% to 10%) that plasma quinine levels will not exceed 5 mg/L until the second dose is given. This corresponds to a plasma free quinine level of approximately 0.5 mg/L: a plasma level already shown (120) to be associated with a sub-optimal therapeutic response in severe malaria in the first 12 hours of treatment. The corresponding data for intravenous administration in 74 children give a mean peak plasma quinine concentration of 15.2 mg/L (SD 3.6 mg/L). Reduction of the loading dose to 15 mg/Kg would reduce the chances of plasma quinine levels exceeding 20 mg/L from approximately 9% to 0.8%, but would increase the chance that plasma concentrations remained below 5 mg/L in the first dose interval from approximately 0.2% to 3.8% - a sixteen-fold increase. There is no evidence that the standard loading dose produces serious toxicity, yet undertreated severe malaria may be fatal. These data suggest that the initial dose should be 20mg salt/ kg.

Is Parenteral Quinine Safe in the Elderly?

Severe falciparum malaria is associated with an increased mortality in elderly patients. There is no evidence that quinine exhibits greater cardiovascular toxicity in the elderly, but caution dictates that these patients with possible underlying cardiovascular or metabolic diseases should receive intensive monitoring with particular attention to electrolyte and fluid balance. Otherwise patients receiving quinine do not require cardiac monitoring.

But theses questions should become redundant, as parenteral artesunate should now replace quinine.

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Tables

Table 1. Currently Available Antimalarial Drugs

Arylaminoalcohols
4-aminoquinolones:	Chloroquine Amodiaquine Amopyraquine
Structurally Related Compounds	Pyronaridine Piperaquine Quinine Quinidine Mefloquine Halofantrine Lumefrantrine (Benflumetol)
8-aminoquinolones	Primaquine Tafenoquine (Etaquine)
Dihydrofolate reductase inhibitors	Pyrimethamine Proguanil Chlorproguanil Pyrimethamine-sulfadoxine Chlorproguanil-dapsone
Artemisinin and deriatives	Artemisinin Artemether Artesunate Artemotil Dihydroartemisinin
Hydroxynaphthaquinones	Atovaquone Atovaquone-proguanil
Antibiotics with antimalarial activity	Sulfonamides Tetracyclines Chloramphenicol Fluoroquinolones (weak) Rifamycins (weak) Macrolides Clindamycin Lincomycin

Drugs in italics are either still investigational, or are not widely available 

Table 2. Dosing Schedule for Artesunate plus Amodiaquine

Age	Dose in mg (no. of tablets)
	Artesunate			Amodiaquine (base)
	Day 1	Day 2	Day 3	Day 1	Day 2	Day 3
5–11 months	25 (½)	25	25	76 (½)	76	76
1–6 years	50 (1)	50	50	153 (1)	153	153
7–13 years	100 (2)	100	100	306 (2)	306	306
>13 years	200 (4)	200	200	612 (4)	612	612

Table 3. Dosing Schedule for Artesunate plus Sulfadoxine-Pyrimethamine

Age	Dose in mg (no. of tablets)
	Artesunate			Sulfadoxine-pyrimethamine
	Day 1	Day 2	Day 3	Day 1	Day 2	Day 3
5–11 months	25 (½)	25	25	250/12.5 (½)	–	–
1–6 years	50 (1)	50	50	500/25 (1)	–	–
7–13 years	100 (2)	100	100	1000/50 (2)	–	–
>13 years	200 (4)	200	200	1500/75 (3)	–	–

Table 4. Dosing Schedule for Artemether-Lumefantrine

Body weight in kg (age in years)	No. of tablets recommended at approximate timing of dosinga
	0 h	8 h	24 h	36 h	48 h	60 h
5–14 (<3)	1	1	1	1	1	1
15–24 (3–9)	2	2	2	2	2	2
25–34 (9–14)	3	3	3	3	3	3
>34 (>14)	4	4	4	4	4	4

aCo-Artem: 1 Tablet contains 20mg Artemether and 120mg Lumefantrine. The drug should be taken with food or milk as. Bioavailability is significantly enhanced with coadministartion of fat, each dosage should be taken with milk or fatty snack.

Table 5. Dosing Schedule for Artesunate plus Mefloquine*

Age	Dose in mg (no. of tablets)
	Artesunate			Mefloquine (base)
	Day 1	Day 2	Day 3	Day 1	Day 2	Day 3
5–11 months	25 (½)	25	25	–	125 (½)	–
1–6 years	50 (1)	50	50	–	250 (1)	–
7–13 years	100 (2)	100	100	–	500 (2)	250 (1)
>13 years	200 (4)	200	200	–	1000 (4)	500 (2)

*alternatively the total dose of mefloquine may be split into three, with one third of the dose being taken on days 1, 2 and 3.

Table 6. Recommendations on the Treatment of Falciparum Malaria in Non-immune Travellers

For travelers returning to non-endemic countries:1 Artemether-lumefantrine (1.5 mg/12 mg/kg twice daily for 3 days) Atovaquone-proguanil (1 g/400 mg once daily for 3 days) Quinine (10 mg salt/kg 8-hourly) plus Doxycycline2 (3.5 mg/kg once daily) or  Clindamycin (10 mg/kg 12-hourly); all drugs to be given for 7 days.   For severe malaria: Antimalarial treatment of severe malaria in travelers is the same as the general recommendation (table 7) Travellers with severe malaria should be managed in an intensive care unit Haemofiltration or haemodialysis should be started early in acute renal failure or severe metabolic acidosis Positive pressure ventilation should be started in case of respiratory distress and coma with breathing abnormalities

1 Halofantrine is not recommended as first-line treatment for uncomplicated malaria because of cardiotoxicity.

2 Doxycycline should not be used in children under 8 years of age and in pregnancy.

Table 7. Antimalarial Treatment of Severe Malaria

Artesunate1	2.4 mg/kg i.v. or i.m. on admission, at 12, 24 hours, then daily. Artesunic acid (60mg) is dissolved in 1ml of 5% sodium bicarbonate and further diluted into 5ml with 5% dextrose or normal saline for intravenous injection (given as a bolus over 2 min). 1 ampoule=60mg
Artemether	3.2 mg/kg i.m. on admission followed by 1.6 mg/kg daily. NOT for i.v. administration. 1 ampoule = 80mg. Injections to the anterior thigh.
Quinine	20mg /kg of dihydrochloride salt by intravenous infusion over 4 hrs followed by 10 mg/kg infused over 2 - 8 hrs every 8 hours. If intravenous route not possible then give by intramuscular injection to the anterior thigh. The first dose should be divided; 10mg salt/kg to each thigh. Quinine should be diluted, inject no more than 180mg/ml.
Quinidine	10 mg base/kg infused at constant rate over 1-2 hr followed by 0.02 mg/kg/min as constant infusion, with electrocardiographic monitoring.

1 Parenteral artesunate is the drug of choice for the treatment of severe malaria in adults. For children in high transmission areas there is as yet insufficient evidence to recommend any of the above antimalarial medicines over another.

Table 8. Treatment of Uncomplicated Vivax and Ovale Malaria.

Chloroquine 25 mg base/kg bw divided over 3 days, combined with primaquine 0.25 mg base/kg bw, taken with food once daily for 14 days is the treatment of choice for chloroquine-sensitive infections. In Oceania and South-East Asia the dose of primaquineshould be 0.5 mg/kg bw.   Amodiaquine (30 mg base/kg bw divided over 3 days as 10mg/kg bw single daily doses) combined with primaquine should be given for chloroquine-resistant vivax malaria. In moderate G6PD deficiency, primaquine 0.75 mg base/kg bw should be given once a week for 8 weeks. In severe G6PD deficiency, primaquine should not be given.  Where ACT has been adopted as the first-line treatment for P. falciparum malaria, it may also be used for P. vivax malaria in combination with primaquine for radical cure.Artesunate plus sulfadoxine-pyrimethamine is the exception as it will not be effective against P. vivax in many places.

Table 9. Treatment of Malaria in Pregnancy

Falciparum Malaria in Pregnancy
Treatment group	Transmission region	Trimester	Current Treatment regimens
Uncomplicated malaria P. falciparum     Parasitemia <4% and no signs of severity	Chloroquine-sensitive		As this is only valid for a very limited number of regions in the world, we do not advise the use of this drug during pregnancy, unless reliable information on drug resistance is available.   Treat as for Chloroquine-resistant or unknown resistance (see below)
	Chloroquine-resistant or unknown resistance	1st	Supervised Oral Quinine plus Clindamycin Quinine sulfate: 10 mg salt/kg/dose 8 hourly for 7 days plus Clindamycin: 10 mg/kg 12-hourly for 7 days
		2nd & 3rd	1. Supervised Oral Artemisinin Combination Therapy1 ACT should be known to be effective in the region. For dosages see Tables 2, 3, 4 Since mefloquine has been associated with stillbirth, it should only be used if no safer alternative is available.   2. Supervised Oral Artesunate plus Clindamycin1 Artesunate 2 mg/kg once daily for 7 days plus Clindamycin 10 mg/kg 12-hourly p.os  full 7 days   3. Supervised Oral Quinine plus Clindamycin Quinine sulfate: 10 mg salt/kg/dose 8 hourly for 7 days plus Clindamycin: 10 mg/kg 12-hourly for 7 days   4. Supervised Artesunate-Atovaquone-Proguanil (AAP)1,2 can be used in case of recrudescence2 3 day oral regimen:  Artesunate 4 mg/kg/day, Atovaquone 20 mg/kg/day and Proguanil 8 mg/kg/day
Severe Malaria P. falciparum   One or more criteria for severity! (see Table 13)		All	1. Artesunate i.v. initial dose 2.4mg/kg, then 2.4mg/kg at 12h and 24h after admission, then once daily until patient can tolerate oral medication. Total course 7 days.   If artesunate is not available;                              2a. Artemether i.m. initial dose 3.2mg/kg i.m. anterior thigh day 1, then 1.6mg/kg i.m. once daily from day 2-7, or until patient can tolerate oral artesunate3   or 2b. Quinine i.v. loading dose 20mg/kg i.v. over 4h on admission, then 10mg/kg i.v. over 2 to 4h, 8-hourly for a total of 7 days, or until patient can tolerate oral medication.

1 oral Artesunate 1 Tablet contains 50mg, a suspension is made by dissolving 1 tablet in 5ml of water, 1ml equals 10mg, using a syringe

2 Malarone (AQ/PG) - one tablet contains 250 mg Atovaquone and 100 mg Proguanil. Give Atovaquone-Proguanil with food. If patient vomits within 30 minutes of taking a dose, then repeat the whole dose. If vomiting occurs after 30–60mins of taking a dose, repeat half the dosage.

3 to prevent recrudesence follow-on treatment should be given, when the patient is able to take oral medication. Follow-on treatment can be a full course of the ACT of choice in the region (see text).

Malaria in Pregnancy, other than Falciparum
Uncompl. malaria P. vivax	Chloroquine-sensitive (P. vivax and P. ovale)	All	Oral Chloroquine; initial dose 10 mg base/kg then10 mg base/kg after 24 hours, followed by 5 mg base/kg after 48 hours (total dose 25 mg/kg). (for radical treatment see below)
	Chloroquine-resistant or unknown resistance	1st	No studies available but we would consider:   Quinine sulphate:  10 mg salt / kg every 8h for 7 days
		2nd and 3rd	Artesunate –amodiaquine or other ACT available in the region (see treatment of uncomplicated falciparum malaria during pregnancy)
Uncompl. malaria P. ovale	Chloroquine-sensitive		Chloroquine (dosage see above)
Uncompl. malaria P. malariae	All regions		Chloroquine (dosage see above)
Radical treatment of hypnozoites P. vivax & ovale	Primaquine is contraindicated during pregnancy. It is used for eradication of hypnozoites, dormant in the liver to prevent relapses, in P. vivax and P. ovale infections. During pregnancy radical treatment of hypnozoites is not possible. Pregnant patients with P. vivax and P. ovaleinfections should be maintained on chloroquine prophylaxis for the duration of their pregnancy.  The chemoprophylactic dose for chloroquine sulphate is 300 mg base (=500 mg salt) orally once per week.  Radical treatment with primaquine can then be startyed a few months after delivery. After delivery; because primaquine can cause hemolytic anemia in persons with G6PD deficiency, patients must be screened for G6PD deficiency prior to starting treatment with primaquine. For persons with borderline G6PD deficiency or as an alternate to the above regimen, primaquine may be given 45mg orally one time per week for 8 weeks; consultation with an expert in infectious disease and/or tropical medicine is advised if this alternative regimen is considered in G6PD-deficient persons.

 

Table 10. Management of Falciparum Malaria in Neonates

Falciparum Malaria in Neonates1
All cases should initially be considered as severe malaria P. falciparum	All regions	1. Artesunate i.v.  initial dose 2.4mg/kg at T0h and T12h on day 1, then continue with 2.4mg/kg once daily for a full 7-day course..                              2. Artemether i.m. initial dose 3.2mg/kg i.m. in anterior thigh, and then 1.6mg/kg i.m. once daily from day 2-7, or until neonate can tolerate oral artesunate.   3. i.v. Quinine  (in 10% Dextrose); loading dose 20mg (base)/kg i.v. over 4h on day 1, then 10mg/kg i.v. over 2 to 4 hours every 8h, for a full 7 day course.

1a full course of i.v. treatment is recommended.

Table 11. Recent WHO Definitions of Treatment Failure

ETF	Early treatment failure; one or more of the following A) Development of danger signs or severe malaria on Day 1, Day 2 or Day 3, in the presence of parasitaemia; B) Parasite count on day 2 higher than day 0 irrespective of temperature C) Axillary temperature ≥ 37.5oC on Day 3 in the presence of parasitaemia; For areas of low to moderate transmission that there must be a measured increasein axillary temperature on day 3. D) Parasitaemia on Day 3 ≥ 25% of count on Day 0
LTF	Late treatment failure, one of the following A) Development of danger signs or severe malaria in the presence of parasitaemia on any day from Day 4 to Day 14, without previously meeting any of the criteria of early treatment failure; B) In areas of intense transmission; axillary temperature ≥ 37.5oC in the presence of parasitaemia on any day from day 4 to day 14, without previously meeting any of the criteria of early treatment failure. In areas of low to moderate transmission; presence of parasitaemia on any day from day 4 to day 28, and a measured axillary temperature > 37.5oC , without previously meeting any of the criteria of early treatment failure. If history of fever, rather than measured fever was accepted as an entry criterion for the study, then parasitaemia with history of fever suffices for LTF.
LPF	Late Parasitological Failure, in high transmission settings, defined as “presence of parasitaemia on day 14, and a measured axillary temperature <37.5oC without previously meeting any of the criteria of early or late treatment failure”. In low to moderate transmission settings the definition is the same, except that parasitaemia at any time from Day 7 to Day 28 qualifies.
ACR	Adequate clinical response; one of the following during the follow-up period from day 4 to 14 A) Absence of parasitaemia on day 14 irrespective of axillary temperature, without  previously meeting any of the criteria of early or late treatment failure; B) Axillary temperature < 37.5oC irrespective of the presence of parasitaemia, without previously meeting any of the criteria of early or late treatment failure.
ACPR	Adequate clinical and parasitological  response. In a high transmission setting this is defined as above (A), but in low to moderate transmission settings the assessment is made on day 28.

Table 12. Overview of WHO Criteria for Signs of Severity

Signs of severity (WHO Criteria)	Other manifestations
1.       Cerebral malaria 2.       Severe normocytic anaemia 3.       Acute renal failure 4.       Pulmonary edema 5.       Hypoglycemia 6.       Circulatory collapse 7.       Spontaneous bleeding / DIC 8.       Repeated generalized convulsions 9.       Acidaemia / metabolic Acidosis 10.   Malarial Haemoglobinuria	11.   Rousable, but impaired consciousness 12.   Cannot eat & drink unaided 13.   Prostration, extreme weakness 14.   Hyperparasitemia 15.   Jaundice 16.   Hyperpyrexia 17.   Pigments in Neutrophils

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Mege JL, Meghari S, Honstettre A, Capo C, Raoult D.  The Two faces of interleukin 10 in human infectious diseases.  Lancet Infectious Diseases 2006:7;557-569.

Efflux Pumps as a Mechanism of Antimicrobial Resistance. 2008

Ladhani, S., Aibara, R., Riordan, F., and Shingadia, D. Imported Malaria in Children: A Review of Clinical Studies. The LANCET Infectious Diseases 2007; Vol.7, Issue 5, 349-357.

Ward S.A., et al.  Antimalarial drugs and pregnancy: safety, pharmacokinetics, and pharmacovigilance.  Lancet Infectious Diseases 2007;7(2):136-144.

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Packard RM.  The Origins of Antimalarial Drug Resistance.  N Engl J Med 2014;371:397-99.

Benedek TG: History of Malaria Chemotherapy.

White NJ, et al.  Lethal Malaria: Marchiafava and Bignami Were Right.  J Infect Dis 2013;208:192-198.

Grizzard M and Grizzard M. The Source of Quinine. Clin Infect Dis. 2009 Jun 1;48.

Tognotti, E.  Program to Eradicate Malaria in Sardinia, 1946-1950: Communism, DDT toxicity and the economics of malaria. Emerging Infect Dis. 2009;15(9): 140-1466.

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