Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense (African Trypanosomiasis)

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Epidemiology

Human African Trypanosomiasis, also known as sleeping sickness, is caused by two subspecies of trypanosomes transmitted to man by various subspecies of Glossina (tsetse flies): Trypanosoma brucei gambiense is the etiological agent of Gambian trypanosomiasis, prevalent in West and Central Africa while Trypanosoma brucei rhodesiense is the agent of Rhodesian trypanosomiasis, seen in East and Southern Africa.

Sleeping sickness was the utmost public health problem of equatorial Africa in the first half of the century. Its incidence decreased considerably following massive efforts for case-finding and chemoprophylaxis. Resurgence of African trypanosomiasis was documented in the 1990s in several countries where war or civil strife hampered control programs, but renewed investment in its control eventually proved successful. For the whole continent, 11382 cases of Gambian trypanosomiasis were reported in 2006, compared to only 486 cases of Rhodesian trypanosomiasis (34). The Democratic Republic of Congo (DRC) remains the country with the highest incidence of Gambian trypanosomiasis, with 8023 cases in 2006 (from a peak of 26318 cases in 1998) (27, 34). Angola reported 1105 cases in 2006 (compared to 8275 cases in 1997), and Sudan 809 cases. Uganda has the highest reported incidence of Rhodesian trypanosomiasis (245 cases in 2006).

In industrialized countries, rare cases of Gambian trypanosomiasis are seen in migrants from Central Africa, while Rhodesian trypanosomiasis may be diagnosed among tourists returning from the game parks of East Africa (15).

T. b. rhodesiense trypanosomiasis is a zoonosis, and cattle the main animal reservoir. In contrast, T. b. gambiense trypanosomiasis results essentially from man-to-man transmission, and the animal reservoir plays a role in sustaining the disease only in low-incidence areas. The very long duration of infection (months to years) is the major factor behind epidemics of Gambian trypanosomiasis, as the tsetse fly is a relatively inefficient vector. Man-fly contact and its many determinants as well as the effectiveness of control programs (based on the identification and treatment of the human reservoir) are also key factors in the dynamics of this infection (27).

Parasitology and Pathogenesis

Life cycle

When a tsetse fly bites an infected person (or animal), bloodstream trypanosomes are ingested along with the blood meal. In the fly, trypanosomes move to the lumen of the midgut, where they transform into the procyclic stage. After two or three weeks, procyclic trypanosomes migrate to the salivary gland where, after several developmental changes, they become mature metacyclic trypanosomes (the infectious form), which are injected into the skin of a human (or animal) during a blood meal. The parasites eventually enter the bloodstream and the lymphatic system, becoming trypomastigotes.

Trypanosomes escape the immune response through a unique process of antigenic variation. The genome contains as many as 1000 different variant surface glycoprotein (VSG) genes, but at a given time, each trypanosome expresses only one VSG. The VSG serves as a protective barrier for the other invariant constituents of the parasite’s outer membrane. As antibodies are developed against the VSG, the trypanosome switches from the expression of one VSG to another. New antibodies are then developed against the new VSG, eventually prompting a further switch to another VSG and so on. This process leads to the intermittent parasitemia characteristic of T.b. gambiense infection: as a novel VSG is expressed, the parasitemia increases, only to be reduced when antibodies against the currently expressed VSG are developed. There is thus a massive polyclonal activation of B cells, and high total IgM level is a classical feature of this infection. There is also some suppression of B-cell and T-cell immune functions, but without clinical consequences. The elevated IgM levels and resultant antigen-antibodies complex as well as the lymphocytic proliferation lead to hyperplasia of the lymph nodes and spleen. At some point in this long process, trypanosomes manage to cross the blood-brain barrier and infect the central nervous system, causing a chronic lymphocytic meningo-encephalitis.

Clinical Manifestations

Gambian trypanosomiasis is characterized by a very long (months to years) incubation period during which infected persons are asymptomatic but may have enlarged cervical lymph nodes, followed by the appearance of non-specific symptoms such as intermittent fever corresponding to fluctuations in the degree of parasitaemia as antigenic variation of their variant surface glycoproteins allows trypanosomes to escape successive waves of antibodies produced by the host.

After a few months, the infection reaches the central nervous system resulting in a chronic lymphocytic meningo-encephalitis. Neurological symptoms such as daytime somnolence and persistent headaches become evident, and weight loss can be profound. Many patients complain of pruritus. Neurological focal signs are rare and there is generally no neck stiffness. If untreated, the patient becomes comatose and dies of a superinfection.

T.b. rhodesiense is more virulent and causes a much more aggressive febrile disease with the incubation period, non-specific symptoms (high fever, myalgia, rigors, sweating) and neurological involvement progressing over a matter of weeks. Some patients develop septic shock, acute renal failure, disseminated intravascular coagulation and multiorgan failure within a few days of the first episode of fever (15).

Inoculation chancre, at the site of the fly bite, and other cutaneous manifestations (called trypanids) are seen only in Caucasians.

Laboratory Diagnosis

Clinicians treating cases of trypanosomiasis in rural hospitals throughout Africa have access only to basic laboratory back-up. The diagnosis is based on the visualization of trypanosomes (or rarely on a positive serological assay), the choice of treatment depends essentially on the CSF white blood cell count performed with simple counting chambers, drug toxicity is assessed clinically, and long-term response to treatment is evaluated with the same basic tests. To make the diagnosis, trypanosomes need to be documented in blood, lymph node aspirate or cerebrospinal fluid (CSF). A field-adapted serological assay, the Card Agglutination Test for Trypanosomiasis (CATT), is available for Gambian trypanosomiasis to identify suspects on whom parasitological assays are carried out. In Gambian sleeping sickness, trypanosomes are often found in unstained aspirates of enlarged cervical lymph nodes. If this is negative, trypanosomes can be looked for in the blood by several techniques, in increasing order of sensitivity: wet (unstained) smear, Giemsa-stained thick smear, haematocrit centrifugation technique, miniature anion-exchange centrifugation technique and quantitative buffy coat (9).

A lumbar puncture must always be carried out in patients in whom trypanosomes have already been documented (for disease staging) or in suspects with neurological symptoms. Trypanosomes are seen in the CSF more often as disease progresses, and there is an increase in the CSF white blood cell count (essentially mononuclear cells) and proteins. A modified method for single centrifugation of CSF, in which the sediment is examined with a miniature anion-exchange centrifugation technique viewing chamber, seems to be the most sensitive method to detect CSF trypanosomes (17). Presumptive diagnoses are sometimes made in patients residing in an endemic area, having typical symptoms, a positive CATT, an elevated CSF white blood cell count but in whom trypanosomes can not be seen. In Rhodesian trypanosomiasis, lymph node aspirates are rarely possible, but the degree of parasitaemia is higher and trypanosomes are easily found in wet or thick smears.

Once trypanosomes have been documented, two questions must be answered before selecting the best treatment. The first is whether the infection is caused by T. b. gambiense or T. b. rhodesiense since some drugs (pentamidine, eflornithine) are effective in Gambian trypanosomiasis but less so in Rhodesian sleeping sickness. This is easily addressed on geographical grounds as Uganda is the only country where both subspecies are found (but without overlap: T.b. gambiense in the northwest of the country, T.b. rhodesiense in the southeast) (31). For the rare patient potentially exposed to both subspecies, it is prudent to assume that the infection is caused by T. b. rhodesiense. If available, detection of the serum resistance-associated gene can confirm that the infection is caused by T. b. rhodesiense, and not T.b. gambiense (31, 36). The second question is whether the patient is in early or late stage, since pentamidine and suramin, because of inadequate CSF penetration, are not effective in late stage. Many patients without neurological symptoms nevertheless have biological evidence of late-stage trypanosomiasis (CSF white blood cell count >5/mm3) and a lumbar puncture must always be carried out. Any patient with a CSF white blood cell count >5/mm3 should be treated with late-stage drugs (eflornithine or melarsoprol).

SUSCEPTIBILITY IN VITRO AND IN VIVO

In research laboratories, in vitro cultivation of trypanosomes can be carried out to measure sensitivity to trypanocidal drugs. This is very useful to screen new drugs for activity against African trypanosomes, but the results can not easily be extrapolated to clinical efficacy due to the paucity of data on plasma and CSF levels obtained with currently available trypanocidal drugs and on levels and durations needed for a cure. Animal models, mostly murine, are also extensively used, but with Trypanosoma brucei brucei (T. b. gambiense does not infect mice) which does not necessarily have the same sensitivity to trypanocidal drugs as the two human-infecting subspecies. None of these assays can be used for clinical decision-making.

ANTIPARASITIC THERAPY

T.b. gambiense: Early Stage

Drug of Choice

Pentamidine: The drug of choice for early-stage Gambian trypanosomiasis is pentamidine isethionate It cures 93% of patients, has adverse effects which are substantial but acceptable, and is currently available for free from Sanofi-Aventis through the World Health Organization (WHO). The WHO-recommended regimen is 4 mg/kg IM daily for 7 injections. Pentamidine salts have been marketed since the late 1940s but the methanesulfonate is no longer available. Both salts had the same efficacy and toxicity. Pentamidine is used in the prophylaxis and treatment of Pneumocystis jiroveci pneumonia and will undoubtedly remain available in coming years. Pentamidine is an aromatic diamidine which reversibly inhibits trypanosomal S-adenosyl-L-methionine decarboxylase, thereby reducing polyamines synthesis (3).

Pentamidine can be given IM or IV. The latter route avoids pain at injection sites, but is cumbersome for rural hospitals and causes hypotension if the drug is given too fast. A study in patients given 10 injections of the methanesulfonate salt on alternate days showed a median half-life of 22 h after the first injection, 47h after the last one and pronounced drug accumulation (6). CSF levels are 0.5-0.8% of plasma levels. Renal clearance corresponds to only 2.5-5% of plasma clearance, most of it being presumably excreted unchanged in the bile so that dosage needs not to be adjusted in the presence of renal failure.

Compared to melarsoprol, pentamidine acts slowly as trypanosomes are found in the blood or lymph nodes aspirates for up to 48 h after the first injection. Cure rates with pentamidine have been remarkably uniform throughout endemic areas and over time despite 60 years of use, including a long period during which single injections of pentamidine were given to millions of individuals as chemoprophylaxis. Pentamidine penetrates to some extent into the CSF and is curative for patients with normal CSF white blood cell count but CSF trypanosomes. For patients with an abnormal CSF white blood cell count, who have a higher parasite load, the failure rate is unacceptable (2).

Adverse effects are substantial. The injections are exquisitely painful and may result in sterile abscesses (7%). Asymptomatic early-stage patients are often reluctant about receiving such painful injections. Hypotension, induced by histamine release, can be seen even after IM injections but syncope is rare (0.02%); if it occurs, patients usually respond to IV fluids and SC adrenaline. Pentamidine can induce insulin release and hypoglycemia shortly after injections (8-33%), pancreatitis days later and glucose intolerance or frank diabetes (5%) weeks later. Other adverse effects, difficult to document in rural hospitals with little laboratory back-up but well described in patients with P. jiroveci pneumonia treated in industrialized countries, are mild and reversible renal failure (25%), hyperkalemia, neutropenia (14%), liver function tests anomalies (10%), thrombocytopenia (4%), hypocalcemia (1%) and ventricular arrhythmias. Some of these undoubtedly occur in trypanosomiasis patients, about 1.0-1.5% of whom die during or shortly after pentamidine treatment (25).

Alternative Therapy

Suramin is thought to be less effective than pentamidine in Gambian early-stage trypanosomiasis. Diminazene is another diamidine, widely used for animal trypanosomiases but never licensed for human use because of its neurological toxicity in some animals and the lack of commercial potential. It has been used in cases of African trypanosomiasis but not in a carefully monitored situation where toxicity, or lack thereof, could be documented. It is less effective than pentamidine with a 15% failure rate (25), and there is no reason to use this drug. Melarsoprol would be effective in early-stage Gambian trypanosomiasis but is not recommended because of its potential toxicity. Although much less frequent than in patients with high CSF white blood cell count, melarsoprol-induced encephalopathy can occur in patients with normal CSF. Fatalities during treatment of asymptomatic patients do not increase the popularity of the control programme nor compliance with case-finding activities.

T. b. gambiense: Late Stage

Drug Of Choice

Eflornithine: Eflornithine is now the drug of choice for late-stage Gambian sleeping sickness (33). For several years now, Sanofi-Aventis has made it available free of charge through the WHO. It cures ≈95% of new cases of Gambian trypanosomiasis and is much less toxic than melarsoprol. The recommended dosage for new cases is 100 mg/kg IV q6h for 14 d. Eflornithine, formerly known as DL-alpha-difluoromethylornithine (DFMO), is an irreversible inhibitor of ornithine decarboxylase, blocking the conversion of ornithine to putrescine, the first and rate-limiting step in the synthesis of putrescine, spermidine and spermine (1). Spermidine is the precursor of trypanothione, a glutathione-spermidine conjugate that is one of the targets of melarsoprol which binds to it irreversibly. Eflornithine can be given orally or IV. The bioavailability of orally administered eflornithine is 54% but it is difficult to increase dosage over 300 mg/kg/day without inducing a severe osmotic diarrhea. The elimination half-life is 3.3 h and renal clearance corresponds to 83% of elimination, mostly as unchanged drug (13). Eflornithine has excellent CSF penetration which contributes strongly to its efficacy. Mean CSF/plasma ratio at the end of a 14-day IV course is 0.91 in adults and 0.58 in children (19). At the same 400 mg/kg/d dosage, children (<12 y) have a mean steady state serum level half that of adults, and a mean CSF level only one third of what is seen in adults.

A multicentre study showed that with the standard 14-day course, the cure rate with eflornithine was 97% for new cases in the DRC, Congo-Brazzaville and Cote d’Ivoire, in line with previous trials (18, 26). However, a much lower cure rate (73%) was seen in patients from northwest Uganda, the cause of which is unclear. Treatment failures were more common in patients with trypanosomes in the pre-treatment lymph node aspirate or in the CSF and in patients with a CSF white blood cell count >100/mm3. Giving only 7 days of IV eflornithine to new cases is unfortunately inadequate: cure rates went down to 87% in the first three countries, and to 62% in Uganda (26). Adding a few weeks of oral eflornithine after 14 d of IV administration has no effect on the cure rate. Administering the drug as 200 mg/kg IV q12h rather than 100 mg/kg q6h seems to be less effective. Treatment with only oral eflornithine is also avoided as its cure rate is ≈80%; it can be considered when venous access is problematic, for instance in infants. Even when treated IV, children have a lower cure rate than adults.

Eflornithine is much better tolerated than melarsoprol. Only 1-2% of patients die during treatment, mostly of advanced disease rather than from drug toxicity (33). Convulsions are seen in 6% of patients, more frequently in relapsing than in new cases, and are correlated with high CSF eflornithine levels (18). Convulsions are usually seen after the first or second dose of eflornithine, stop when the drug is withheld and do not recur when it is resumed after a day or two. Convulsions were associated with mortality but the causes of death were unclear (26). Bone marrow suppression is frequent but without clinical consequences: one half of patients develop anemia, one-third leukopenia and one half thrombocytopenia but bleeding is rare and opportunistic infections are not seen. About 15% of IV-treated patients complain of diarrhea, sometimes with abdominal pains, but this is not severe and oral hydration suffices. The numerous IV injections which need to be given after proper dilution in normal saline are not an easy task in understaffed rural hospitals.

Melarsoprol: Melarsoprol, a trivalent arsenical drug, remains used in the treatment of late-stage Gambian trypanosomiasis in hospitals where eflornithine is unavailable. Melarsoprol cures 94-97% of late-stage patients but is very toxic. Also known as Mel B, melarsoprol is the addition of dimercaprol, a heavy metal chelator, to melarsen oxyde. It is insoluble in water and dissolved in propylene glycol to be sold as 5 mL (180 mg) ampoules of a 3.6% solution. It has to be injected IV in dry syringes. One of the targets of melarsoprol is trypanothione, a cellular protectant against free radicals, to which it binds irreversibly, resulting in a compound called Mel T (12). The traditional, intermittent, treatment schemes were developed empirically, decades before the pharmacokinetics of melarsoprol were investigated. Using a bioassay and atomic absorption spectrometry, it has been found that serum levels are between 2 and 4 ug/mL, while CSF levels are between 0 and 0.1 ug/mL (7). The terminal elimination half-life is 35 h. During the drug-free intervals, serum melarsoprol levels drop to almost zero as do CSF levels 5 d after the last injection of a series.

Despite having been used in millions of patients over 60 years, melarsoprol remains remarkably effective in most foci (28). There are however foci in northwest Uganda and northern Angola where the failure rate is superior to 25%; these seem to be geographically limited but are a cause of much concern (16, 35). Treatment failures are more frequent in patients with trypanosomes found in the CSF or in lymph node aspirates. The traditional regimen for the treatment of Gambian trypanosomiasis had been 2 or 3 series of 3 daily injections separated by one-week drug-free intervals and there was no consensus about whether graded dosing should be used or if the full dose should be given from the first injection. The latter approach was used by most treatment centers with the same 3.6 mg/kg dosage (maximum: 180 mg) administered from the first to the last injection. Traditionally, the number of injections had been proportional to the CSF white blood cell count (2 series of 3 injections for patients with a CSF white blood cell count of 6-19/mm, and 3 series of 3 injections for those with a CSF white blood cell count >20/mm3).

However, a randomized controlled trial showed that a regimen of 10 consecutive daily injections of 2.2 mg/kg of IV melarsoprol had the same efficacy and the same toxicity as the intermittent dosing (8). The advantages of this new regimen are a much shorter duration of hospitalization, and a reduction in the quantity (and cost) of melarsoprol required. This is now considered as the standard therapeutic regimen.

Melarsoprol toxicity is a major problem, around 5% of patients dying during treatment. The main toxicity is reactive encephalopathy. Indirect arguments suggest that it is an immune-mediated reaction rather than a direct toxic effect of arsenic. The risk of reactive encephalopathy increases in parallel with the CSF white blood cell count (6-99/mm3: 8%; 100/mm3: 16%) but, within a given stratum of CSF white blood cell count, its occurrence is unpredictable. With the intermittent regimen, most encephalopathies occurred between 3 and 15 days after the first injection of melarsoprol. With the new concise regimen, the frequency and timing of encephalopathies were similar (8). Patients who looked well an hour before suddenly develop grand mal seizures and coma, sometimes with pulmonary oedema. Milder forms are occasionally seen in which behavior change is the predominant symptom. The case-fatality rate is over 50% in the first 48 hours. Prednisolone reduces by two-thirds the risk of encephalopathy, without decreasing the likelihood of achieving a cure (22-24), and should be given to all Gambian trypanosomiasis patients treated with melarsoprol. With the 10-day regimen, the following doses of prednisolone have been used: 1 mg/kg (days 1-7), 0.75 mg/kg (day 8), 0.5 mg/kg (day 9), and 0.25 mg/kg (day 10). One wonders if prednisolone is effective through its immunosuppressive properties or by decreasing CSF penetration of melarsoprol. Patients who survive an encephalopathy should be treated with eflornithine if the drug is available; if not, although this option makes both patient and doctor uncomfortable, there is little, if any, risk of a second bout of encephalopathy upon resuming melarsoprol.

The other frequent (10%) adverse effect of melarsoprol is polyneuropathy which, if untreated, can progress to paraplegia or quadriplegia. Patients first complain of paresthesias in the feet or hands; melarsoprol should then be withheld for a few days while thiamine (100 mg TID) is given. Melarsoprol can be resumed once paresthesias have regressed. Melarsoprol should be definitively stopped if a motor deficit is present. Less common adverse effects are rash and tremors (not predictive of encephalopathy and usually responsive to beta-blockers). Fever can be caused by the lysis of trypanosomes, but superinfections should always be looked for carefully, especially aspiration pneumonia in debilitated patients. Phlebitis at injection sites is frequent and induced by the propylene glycol solvent; if extravasation occurs, a chemical cellulitis develops.

Alternative Therapy

Nifurtimox: Nifurtimox is a nitrofuran used in the treatment of American trypanosomiasis (Chagas disease). It inhibits trypanothione reductase, resulting in the production of superoxide and peroxide. After oral administration, peak serum levels are seen after 3.5 h and the elimination half-life is 3h (21). It is extensively metabolized by the liver, but the antiparasitic activity of its metabolites has not been studied. For African trypanosomiasis, conflicting results have been reported, with cure rates between 30 and 80%. The poor efficacy of nifurtimox monotherapy was confirmed in a more recent RCT carried out in the DRC (5), where one third of patients in that arm eventually relapsed. However, nifurtimox may find an interesting niche in combination therapy. In relatively small cases series, promising results were documented when nifurtimox (15 mg/kg per day for 10 days) was combined with low-dose melarsoprol (5) or with a 7-day course of eflornithine (10, 32).

T. b. rhodesiense: Early Stage

Drug of Choice

Suramin: Suramin is thought to be the best treatment of early-stage Rhodesian trypanosomiasis. There has been no recent publication, but suramin is said to be curative in more than 90% of such cases. It is more effective than pentamidine, which should not be used. A sulfated naphtlylamine developed in the 1920s, suramin binds to and inhibits numerous enzymes but it is unclear from the inhibition of which one comes its trypanocidal effect. It acts relatively slowly, trypanosomes disappearing 12-36 h after it is injected. Suramin is 99.7% protein-bound, which explains its very poor CSF penetration and its extraordinary half-life of 44-54 d (11). After a test dose of 5 mg/kg (up to 200 mg) (although anaphylaxis is rare), suramin should be administered IV as 20 mg/kg (up to 1.0 g) on days 1, 3, 6, 14 and 21. Various other regimens have been advocated, for instance 5 mg/kg on day 1, and 20 mg/kg (up to 1.0 g) on days 3, 10, 17, 24 and 31 (37): given the drug's half-life, these variations in schedules are unlikely to impact on results. Adverse effects are febrile reactions (10%), urticaria and other types of cutaneous reactions, proteinuria, more rarely polyneuropathy, keratopathy and stomatitis. Concomitant onchocerciasis increases the risk of hypersensitivity reactions.

Alternative Therapy

Melarsoprol would also be effective but is avoided because of its toxicity.

T. b. rhodesiense: Late Stage

Drug of Choice

Melarsoprol: Eflornithine is ineffective against T. b. rhodesiense and melarsoprol is the only option. Melarsoprol is as effective (95%) as in Gambian trypanosomiasis but even more toxic. Clinicians from endemic areas prefer to start with small doses in the hope of reducing the risk of encephalopathy. Several regimens are recommended (37), the following seems more widely used: 0.36 mg/kg (day 1), 0.72 (day 2), 1.1 (day 3), 1.8 (days 10, 11, 12), 2.2 (day 19), 2.9 (day 20) and 3.6 mg/kg (up to 180 mg, on days 21, 28, 29 and 30). However, in Gambian trypanosomiasis, reactive encephalopathy is more common when graded dosing is used (29) and a clinical trial is underway to evaluate the efficacy and toxicity of the concise regimen used against T.b. gambiense. Prednisolone should probably be given to all T. b. rhodesiense patients treated with melarsoprol even though there are no data specific to that subspecies as encephalopathy, whose underlying mechanism must be the same, is more frequent (5-18%) than in Gambian trypanosomiasis resulting in a higher mortality (3-18%) during treatment.

Treatment of Relapses

T. b. gambiense

Patients who relapse after pentamidine therapy of early stage Gambian trypanosomiasis respond well to eflornithine or melarsoprol (same regimen as for new cases). There is no cross-resistance between melarsoprol and eflornithine, in contrast to data from animal studies. Patients who relapse after having received eflornithine as the first treatment of late-stage Gambian trypanosomiasis should be treated with melarsoprol (same regimen as for a new case). Patients who relapse after melarsoprol treatment of late-stage T. b. gambiense sleeping sickness should be treated with eflornithine, which is indeed even more effective in relapsing cases than in new cases (probably due to alterations in the blood-brain barrier). For such patients, there is now reasonable evidence that 7 d at the same daily dosage (100 mg/kg IV q6h) may be sufficient (cure rate: 94%) (26). Giving additional courses of melarsoprol (12 injections of 3.6 mg/kg) to patients relapsing after a first course of melarsoprol is not successful in more than one third of cases.

What should be given to the rare patient who relapses after having received eflornithine once and melarsoprol once? Certainly a combination of two trypanocidal drugs; whether this should be nifurtimox+melarsoprol or nifurtimox+eflornithine remains to be determined.

T. b. rhodesiense

Patients who relapse after having received suramin for early-stage T. b. rhodesiense trypanosomiasis should be given melarsoprol (same dosage as new cases). Patients relapsing after melarsoprol treatment of late-stage Rhodesian trypanosomiasis often respond to a second course of melarsoprol (12 injections of 3.6 mg/kg), in contrast to Gambian sleeping sickness. For those relapsing after a second course of melarsoprol, the prognosis is guarded. A small trial showed that high-dose eflornithine (200 mg/kg q6h for 14 d) cures only 25% of such cases. Nifurtimox, preferably in combination with melarsoprol, can be considered for these patients, although there is little if any clinical experience.

Special Situations

Children

Higher treatment failure rates in children compared to adults are seen both with eflornithine and with the pentamidine/suramin combination but not in children treated with melarsoprol (19, 23, 25). Children usually get the same dosage in mg/kg as adults, which is irrational since children’s pharmacokinetic parameters differ from those of adults. Pending specific pharmacokinetic studies, children should be given dosages of eflornithine and pentamidine roughly 25% higher, in mg/kg, than those of adults.

Pregnant Women

In pregnant women, adequate treatment is probably more important to the foetus (and the mother) than avoidance of adverse effects of trypanocidal drugs. Pentamidine has no mutagenic effect and was found in a murine model to have no teratogenicity but to be embryocidal (14). If possible, it should be avoided during the first trimester. Melarsoprol is thought to have no adverse effects on pregnancy. There is little data on the risk of suramin in pregnancy. In rodents and rabbits, eflornithine is embryotoxic and abortive in early pregnancy but not teratogenic (20). It should be avoided if possible because clinical experience with the drug in pregnant women is limited.

HIV-Infected Patients

The only drug for which information is available concerning its efficacy in HIV-infected patients is eflornithine, which should be considered inadequate in that context. Among four eflornithine-treated patients known to be HIV infected, two subsequently relapsed, one died shortly after treatment and one died at home two years after treatment, probably of AIDS (18). These unsatisfactory results are not surprising considering that eflornithine is trypanostatic rather than trypanocidal, and that animal studies have shown that a normal immune system is necessary to achieve a cure (4). Patients known to be co-infected with HIV should be treated with melarsoprol.

Combination Therapy

Combination therapy might potentially result in higher efficacy, shorter duration of treatment, lower costs and may also prevent the emergence of resistance. Animal models with a large number of drug combinations have shown that synergism can be achieved, although most studies were done with small numbers of infected mice and included drugs not licensed for human use. However, until recently, combinations had been little used in human African trypanosomiasis. A combination of pentamidine (6 IM injections) and suramin (2 IV injections) was used in DRC for many years for the treatment of early-stage Gambian trypanosomiasis and was thought to be superior to pentamidine or suramin monotherapy. This was poorly documented and in theory the combination could indeed be inferior: suramin may bind a high proportion of pentamidine and reduce free pentamidine levels in blood and CSF. In a retrospective study, the failure rate with the combination was 7.5%, identical to what has been reported from other countries with pentamidine monotherapy (25). The combination is no longer used.

More recently, as mentioned above, clinical trials have examined various combinations of trypanocidal drugs: nifurtimox/melarsoprol, nifurtimox/eflornithine, eflornithine/melarsoprol, with encouraging results (5, 10, 32). Larger trials will be necessary to identify the combination presenting the best balance between efficacy and toxicity.

ADJUNCTIVE THERAPY

Classical recommendations about adjunctive therapy in trypanosomiasis patients are more tradition than science. Antimalarials and anthelminthics are routinely given at the beginning of treatment. Albendazole or ivermectin must be administered to patients found to have Strongyloides stercoralis in their stools, especially in melarsoprol-treated patients to avoid steroid-induced dissemination. Preventive anticonvulsant therapy has never been evaluated. Pretreatment with pentamidine (1-2 doses of 4 mg/kg 1-3 d before the first injection of melarsoprol) was advocated for decades in the hope of reducing the frequency of melarsoprol-induced encephalopathy by decreasing the parasite load before melarsoprol is given. Its efficacy was never proven. With the new regimen of 10 daily injections, no pentamidine pretreatment is given (8). In patients with Rhodesian trypanosomiasis, pretreatment with suramin is usually given as 2-3 injections (5, 10, 20 mg/kg) over 3-5 d before the first injection of melarsoprol; again, whether this impacts on the risk of encephalopathy is unknown.

Patients who develop a melarsoprol-induced encephalopathy should be treated with IV dexamethasone to reduce cerebral oedema and with anticonvulsants (phenytoin, phenobarbital, etc). Dimercaprol, a heavy metal chelator used for decades in the treatment of encephalopathy, is probably deleterious and should be abandoned.

ENDPOINTS FOR MONITORING THERAPY

After treatment of early or late-stage Gambian trypanosomiasis, patients should be followed with a lumbar puncture every 6 months for 2 years. Patients whose symptoms have not improved 3 months after treatment should have a lumbar puncture then, without waiting any longer. Similarly, patients who develop symptoms compatible with a relapse (headaches for more than 2 weeks, somnolence) between the routine interval should have a lumbar puncture sooner. Two years after treatment, the incidence of relapses is not higher than that of new cases among other inhabitants of the same foci and it is not useful to perform systematic lumbar punctures any more. In T. b. rhodesiense trypanosomiasis, relapsing cases progress more rapidly so that lumbar puncture should be performed every 3 months during the first year, and every 6 months during the second year.

A relapse is obvious when trypanosomes are found in the CSF but three-fourths of relapses are characterized only by an elevated CSF white blood cell count. Even in patients who are cured, the CSF white blood cell count can be elevated 6 months post-treatment, and the trend is more important than the absolute value. The presence of symptoms must also be taken into consideration. A CSF white blood cell count of 20/mm3 in an asymptomatic patient whose previous lumbar puncture showed a white blood cell count of 50/mm3 is less a matter for concern than the same value in a slightly somnolent patient whose previous CSF examination was normal. There are no perfect criteria but the following seem reasonable: in patients previously treated for late-stage trypanosomiasis, a relapse is diagnosed (and a second treatment given) if trypanosomes are found in the CSF (or blood or lymph node aspiration) or if the CSF white blood cell count is > 50mm3 and higher than the previous determination or if the CSF white blood cell count is 20-49/mm3 , higher than the previous one and the patient has symptoms compatible with a relapse. In doubtful cases, for instance a white blood cell count of 10-19/mm3 after a normal one in a patient with unconvincing symptoms, it is better to repeat the lumbar puncture after 1-2 months before taking a decision. In patients who have been treated for early-stage trypanosomiasis, the criteria for diagnosing a relapse must be different since the CSF was normal to start with. Such patients who, during any follow-up lumbar puncture, have a CSF white blood cell count of 10/mm3 or higher should be considered as relapsing and given a new treatment. Those with mild CSF anomalies (white blood cell count 6-9/mm3) should be observed, with a lumbar puncture carried out 2-3 months later.

VACCINES

No vaccine is currently available nor under development.

PREVENTION

General

Vector control is difficult and costly. It is however the only preventive method available for residents of endemic countries (27). Vector control would also provide risk reduction for visitors to T.b. rhodesienseendemic game parks such as the Serengeti.

Antiparasitic Agent Prophylaxis

Chemoprophylaxis with pentamidine was implemented in endemic regions in the 1940s and 1950s (30). Millions of individuals received an IM injection of pentamidine every 6 months for several years which, in retrospect, corresponded probably more to early treatment than to prophylaxis, given the drug's pharmacokinetics. This effective intervention was abandoned in the 1950s when the incidence decreased to the point where such efforts could not be justified. Furthermore, epidemics of gas gangrene were reported in several locations. Chemoprophylaxis is not used any more, and is not indicated either for tourists and other visitors to endemic areas, whose risk is very low.

COMMENTS AND CONTROVERSIES

The market for trypanocidal drugs is unattractive because of the limited number of patients and their lack of purchasing power and industry can not be expected to invest in the development of new drugs. Fortunately, through a private-public partnership between Sanofis-Aventis and the WHO, and more recently between Bayer and WHO, existing trypanocidal drugs will remain available in the foreseeable future.

Much could be accomplished, however, in improving the way currently available drugs are used. Treatment protocols for pentamidine, suramin and melarsoprol were developed empirically, without pharmacokinetic data, and before modern concepts of clinical epidemiology existed. For instance, in vitro studies combined with pharmacokinetic determinations suggest that the current treatment of early-stage trypanosomiasis with 7 daily injections of 4 mg/kg of pentamidine is exaggerated and that it may be possible to lower the daily dosage or the number of injections without reducing efficacy (6). This can only be addressed in a properly designed clinical trial. For melarsoprol, the demonstration that the drug could be given as 10 consecutive daily injections of 2.2 mg/kg in late-stage cases of T. b. gambiense trypanosomiasis has been a major progress. There is now a clear need to test this regimen in patients infected with T. b. rhodesiense.

Following renewed interest and funding for the control of African trypanosomiasis, the incidence has been decreasing in all endemic countries (Simarro). The challenge now will be to sustain control efforts in the context of lower incidence, so as to eventually eliminate African trypanosomiasis as a public health threat in sub-Saharan Africa. Eradication is probably beyond reach, given the broad animal reservoir of the parasite.

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Tables

Table 1. Comparison of T. B. Gambiense and T.B. Rhodesiense Trypanosomiasis: Epidemiology, Clinical Course And Diagnosis.

	T.b. gambiense	T.b. rhodesiense
Distribution	Central and West Africa	East and Southern Africa
Countries with highest incidence	Democratic Republic of Congo,Angola, Sudan, Central African Republic, Congo-Brazzaville. Tchad, Uganda	Uganda, Tanzania, Malawi, Zambia
Main reservoir	Man	Cattle, antelope and other animals
Vector	Glossina palpalis group	Glossina morsitans group
Incubation period	Months to years	Weeks
Interval between first symptoms and central nervous system involvement	Months	Weeks
Parasitaemia	Low and intermittent	High and more constant
Diagnosis	Lymph node aspirate Wet and thick smears Miniature anion exchange centrifugation technique Quantitative buffy coat CSF examination	Wet and thick smears CSF examination
Serology	Card Agglutination Test for Trypanosomiasis	None
Inoculation chancre	Very rare	Unusual
Cervical lymphadenopathy	Frequent	Unusual
Neck stiffness	Rare	Rare

Table 2. Comparison of T.b. gambiense and T.b. rhodesiense Trypanosomiasis: Treatment of Early-Stage and Late-Stage Patients.

	T.b. gambiense	T.b. rhodesiense
Early-stage	Pentamidine	Suramin
Late-stage	Eflornithine or melarsoprol	Melarsoprol
Melarsoprol-induced encephalopathy: Frequency in late-stage Prevention Treatment Case-fatality rate	5-10% Prednisolone (well studied) Dexamethasone Anticonvulsants >50%	Up to 18% Prednisolone (not studied) Dexamethasone Anticonvulsants >50%
Pre-treatment before melarsoprol Drug Value	Pentamidine Doubtful	Suramin Doubtful
Adjuvant drugs	Albendazole Antimalarials	Albendazole Antimalarials

Table 3. Comparison of T.b. gambiense and T.b. rhodesiense Trypanosomiasis: Treatment of Relapses.

	T.b. gambiense	T.b. rhodesiense
Post-pentamidine or Post-suramin	Eflornithine or melarsoprol	Melarsoprol
Post-eflornithine	Melarsoprol
Post-melarsoprol	Eflornithine	Melarsoprol (high-dose)
Additional relapses	after melarsoprol once and eflornithine once: nifurtimox + melarsoprol or nifurimox + eflornithine	after 2 courses of melarsoprol: consider nifurtimox + melarsoprol