Toxoplasma gondii in Transplant Recipients
Authors: Jose Montoya, M.D.
PATHOGEN
Toxoplasma gondii is an obligate intracellular parasite with worldwide distribution. The parasite can be seen in primarily three forms: the tachyzoite, the tissue cyst and the ooccyst (10).
The tachyzoite is responsible for the symptoms and signs observed in patients with active toxoplasmosis, which can be the result of an acute/primary infection or reactivation of a previously acquired infection.
The tissue cyst is the form behind the chronic and latent infection; it is primarily observed in the brain, eye, heart and skeletal muscle of infected humans. The tissue cyst remains in tissues for the life of the host and cannot be eradicated with treatment. During reactivation, tissue cysts rupture and release bradyzoites that are then converted to tachyzoites; the most important factor in the reactivation of the tissue cyst is the integrity of the cell-mediated immune system. The tissue cyst may also be present in contaminated and undercooked meat which is one of the main vehicles of transmission in Europe and the United States.
The oocyst is present in the small intestine and the feces of infected cats; any member of the feline family, small or large, domestic or feral, can be infected with T. gondii and up to 10 million oocysts can be excreted up to 21 days by infected animals. From soil, oocysts can travel long distances by plants, vegetables or other edibles, or reach water reservoirs for animal or human consumption.
Studies on the genetics of the parasite have revealed that T. gondii exists in primarily three main genetic types (Types I, II, and III). Type I strains have been associated with higher virulence in animal models and recent reports suggest the possibility that this Type I and other unusual non-typeable strains may also be associated with more severe clinical manifestations in humans (4, 6). Transplant patients and their physicians must be aware of the possibility of unusual and more aggressive clinical presentations of toxoplasmosis when the parasite has been acquired by the donor or recipient outside the United States or Europe.
EPIDEMIOLOGY
Although the prevalence of T. gondii infection appears to be declining in several geographical areas, toxoplasmosis continues to be a significant clinical challenge for several patient populations (12). The acute infection and reactivation of their chronic infection continues to be, if unrecognized and untreated, a life-threatening and potentially lethal condition for transplant patients. The acute infection appears to be a major problem for patients living in areas of the world where the parasite appears to be more virulent and travelers should be cognizant of this risk (4).
A recent study reported the main risk factors for Toxoplasma gondii infection in the United States: eating raw ground beef; eating rare lamb; eating locally produced cured, dried, or smoked meat; working with meat; drinking unpasteurized goat's milk; having 3 or more kittens (8). In this study eating raw oysters, clams, or mussels was also identified as novel risk factor. Untreated water as a potential vehicle for the transmission of T. gondii has been established in several large epidemiological studies and was found to have trend towards increase risk for acute infection in the United States. As in previous similar studies performed in other geographical areas, approximately 50% of patients do not have identifiable risk factors for toxoplasmosis. Thus, trying to select donor or recipient populations at risk for toxoplasmosis solely on the basis of their epidemiological history is a useless task. Patients may have been infected with T. gondii or may get infected despite that they do not own cats, do not eat undercooked meat or shellfish, or have not apparently drank untreated water.
The overall age-adjusted seroprevalence of T. gondii infection in the United States has been reported at 11%. The seroprevalence for T. gondii infection (a marker for chronic infection) increases with age and varies considerably by locale and socioeconomic strata (12). Therefore, the risk of toxoplasmosis in a given transplant program will vary considerably according to the seropositivity of their recipients and donors as well as the degree of exposure that seronegative individuals have to T. gondii in their own communities. Patients’ toxoplasma serological status should be established prior to the transplant procedure in all patients (independent of their epidemiological history).
For solid organ transplants, the donor/recipient pair at highest risk for toxoplasmosis is the toxoplasma seropositive donor (D+) whose organ is transplanted into a seronegative recipient (R-); in this instance the seronegative recipient is exposed to the parasite for the first time (primary infection) when the organ from the seropositive donor is transplanted at the same time the immunosuppression is being given at maximum doses.
For instance, at Stanford University Medical Center results of serologic testing for Toxoplasma were available prior to transplant for 575 D/R pairs; of these, 454 (79%) were D-/R-, 84 (14.6%) D-/R+, 32 (5.6%) D+/R-, and 5 (0.8%) D+/R+. Of the 32 D+/R- patients, 16 patients were not taking either trimethoprim-sulfamethoxazole or pyrimethamine, 4 (25%) of these 16 patients developed toxoplasmosis, and all died of the infection. Of the 16 patients receiving trimethoprim-sulfamethoxazole and /or pyrimethamine prophylaxis, none of those 16 developed toxoplasmosis (11).
For hematopoetic stem cell transplant (HSCT) patients, the recipient who is toxoplasma seropositive prior to transplantation has the highest risk of reactivation. Martino et al. reported that of 106 T. gondii–seropositive adult recipients of HSCTs, the incidence of toxoplasmosis was 6%. However, in this study patients were routinely followed by toxoplasma peripheral blood PCR every week for the first 100 days. Toxoplasmosis developed in 38% of those patients who had been found to be PCR positive during the surveillance period (9). Thus, pre-transplant serologies are critical to establish whether a HSCT patient is at risk to reactivate their own T. gondii infection. A pre-transplant seronegative patient has a very low risk of toxoplasmosis in the first 100 days of the post-transplant period. Post-transplant serologies in HSCT patients are often not helpful in establishing whether the patient is suffering from acute or reactivated toxoplasmosis.
CLINICAL MANIFESTATIONS
Transplant patients can develop toxoplasmosis as a result of their acute/primary infection or reactivation of their latent infection if they have already been exposed to the parasite. Primary infection can occur by the inadvertent ingestion of food or water contaminated with the parasite or via the donated organ.
Although primary infection tends to be asymptomatic, it may in some patients result in the following clinical manifestations (alone or in combination): lymphadenopathy, chorioretinitis, fever, headache, general malaise, hepatitis, myositis or myocarditis. Recent studies suggest that these manifestations may be more frequent and severe in certain geographical settings (4, 7). Lymphadenopathy is most commonly located in the cervical region but can be observed anywhere and it can be generalized. Toxoplasmic lymphadenitis is usually painless and is non-suppurative. Eye involvement by T. gondii needs to be ruled out by a retina specialist in patients with eye symptoms and, in the absence of eye symptoms, in those with severe systemic manifestations. Ocular symptoms include blurred vision, eye pain, decreased visual acuity, floaters, scotoma, photophobia, or epiphora.
Reactivation of their chronic T. gondii infection in transplant patients may result in brain abscesses, diffuse encephalitis without brain-occupying lesions, pneumonia, fever of unknown origin, myocarditis, hepatospenomegaly, lymphadenopathy and skin rash. Patients with diffuse encephalitis without focal lesions by MRI have a mortality rate close to 100%. Pneumonia with or without fever can be the sole manifestation of toxoplasmosis in transplant patients. Patients with toxoplasmic pneumonia can present with cough, dyspnea, hypoxia, and diffuse bilateral or localized infiltrates. Fever alone has frequently been described in patients with allogeneic HSCT and liver transplant patients.
LABORATORY DIAGNOSIS
Serological tests, the polymerase chain reaction (PCR), histological examination with hematoxylin and eosin (H&E) or Wright Giemsa stains, immunohistochemistry with T. gondii-specific immunoperoxidase, or attempts to isolate the parasite can be used alone or in combination to diagnose toxoplasmosis.
Serological tests are helpful to determine the risk of toxoplasmosis in transplant patients and should be obtained in the pre-transplant period. Pre-transplant serologies in the donor and recipient are critical to establish whether a patient is at risk to acquire T. gondii from the donor or to reactivate their own T. gondii infection in the post-transplant period. A pre-transplant D-/R- patient has a very low risk of toxoplasmosis in the post-transplant period. In the setting of solid organ transplantation D+/R- pairs have the highest risk of developing toxoplasmosis in the post-transplant period (1). In the setting of HSC transplantation previously seropositive recipients (regardless of their donor serological status) have the highest risk of developing toxoplasmosis; this risk is particularly highest in allogeneic transplants and patients who develop graft versus host disease (GVHD). In addition, post-transplant serologies alone are often not helpful in establishing whether the patient is suffering from acute or reactivated toxoplasmosis and may not accurately determine whether the patient has been infected with T. gondii. Post-transplant serologic test results for IgG antibodies may remain positive, may rise, decrease or even become negative. Thus, pre-transplant toxoplasma serologies are critical for interpretation of subsequent testing and clinical evaluation. Pre-transplant serum of the donor and recipient should be routinely tested for toxoplasma serologies.
Initial serological testing can be performed by hospital or commercial (non-reference) laboratories; involvement of a reference laboratory for this first step is not necessary. A negative IgG and negative IgM result in a patient capable of producing IgG antibodies, essentially establishes that the patient has not been infected with T. gondii and should be advised on how to avoid exposure to the parasite (http://www.cdc.gov/toxoplasmosis/prevent.html. ). A positive IgG and negative IgM result establishes that the donor or recipient has been infected with T. gondii in the distant past. Any equivocal or positive IgM test result should be sent for confirmatory testing to a reference laboratory (e.g. Palo Alto Medical Foundation Toxoplasma Serology Laboratory, PAMF-TSL; Palo Alto, CA; http://www.pamf.org/serology/; 1-650-853-4828; e-mail: toxolab@pamf.org ). A positive IgM test result may be diagnostic of a recently acquired primary infection but it may also be observed in chronically infected patients or may be a false positive result. At PAMF-TSL, 40% of the sera that have been found to have positive T. gondii-IgM test results by hospital or commercial laboratories, are confirmed to have a recently acquired acute infection; 40% are found to have a chronic infection by results of additional serological testing despite the fact that the presence of T. gondii-IgM antibodies is confirmed by the PAMF-TSL more specific IgM “double sandwich” ELISA. 20% are found to be false positive IgM test results. Confirmatory serological tests available at PAMF-TSL for positive IgM sera include IgG by the Dye test technique, IgM, IgA and IgE by the “double sandwich” ELISA method, and the AC/HS and avidity tests (10).
The diagnosis of reactivated toxoplasmosis requires the microscopic visualization of the tachyzoite form or tissue cysts surrounded by a strong inflammatory response in biopsy specimens or amplification of the parasite’s DNA in any tissue or body fluid by PCR. The parasite can be visualized by H&E, Wright-Giemsa or T. gondii-specific immunoperoxidase. PCR can performed in any body fluid as clinically indicated including whole blood, cerebrospinal fluid, bronchoalveolar fluid, bone marrow specimen, aqueous humor and vitreous fluid, and in any other tissue. Isolation of T. gondii in the peritoneal cavity of mice or tissue culture can be attempted for genetic studies of the parasite.
PATHOGENESIS
The most common portal of entry of the parasite in humans is the gastrointestinal tract. In transplant patients the donated organ can also be the initial site of infection. T. gondii multiplies intracellularly at the site of invasion, after host-cell disruption, parasites invade adjacent cells from which they spread throughout the body via lymphatics and bloodstream. The combination of innate, humoral, and cellular immune responses are responsible for controlling both primary and latent infections as well as preventing reactivation. Macrophages, dendritic cells, natural killer cells, T cells, TH1 cytokines (e.g., IFN-%, IL-12), tumor necrosis factor-& (TNF-&), costimulatory molecules (e.g., CD28, CD40 ligand), and, to a lesser degree, immunoglobulins are crucial for the effective clearance of tachyzoites from peripheral blood, their conversion to bradyzoites, and subsequent formation of cysts in different tissues. Immunity in the immunocompetent host has been found o be life-long. The greater the defect in T cell–mediated immunity in a transplant patient, the higher the risk for reactivation of their latent T. gondii infection (i.e. an allogeneic HSCT patient with graft-versus-host disease [GVHD] has a greater risk than an autologous HSCT or a heart transplant patient).
Therapy
General
Monotherapy should be avoided in the treatment of toxoplasmosis as it appears to be inferior to combination regimens. The regimen of choice is pyrimethamine/sulfadiazine/folinic acid (10) (Table 1). However, recent studies have provided convincing evidence regarding the similar efficacy rates that can be achieved with IV/PO trimethoprim/sulfamethoxazole in AIDS patients with toxoplasmic encephalitis (2, 14) or immunocompetent patients with toxoplasmic chorioretinitis (13) when compared with pyrimethamine/sulfadiazine/folinic acid (Table 1). Although these studies were not performed in transplant patients, it is probably safe and equally efficacious to offer IV/PO trimethoprim/sulfamethoxazole to this patient population as an alternative regimen for situations in which parenteral therapy is required or when pyrimethamine/sulfadiazine is not available (Table 1). The doses of the anti-toxoplasma drugs used in transplant patients are higher than those used in immunocompetent patients.
Alternative Regimens
The best-studied alternative regimen for the treatment of toxoplasmosis in AIDS patients is the combination of pyrimethamine/clindamycin/folinic acid (Table 1). The combination of pyrimethamine/atovaquone/folinic acid or atovaquone alone can also be tried. These regimens are likely effective in non-AIDS immunocompromised patients and should be attempted in patients who cannot take or tolerate pyrimethamine/sulfadiazine/folinic acid or trimethoprim/sulfamethoxazole. Other combination regimens with much more limited data behind their efficacy include pyrimethamine plus clarithromycin, or dapsone or azithromycin (Table 1).
Adjunctive Therapy
Corticosteroids have been proven of limited value as an adjuvant treatment for toxoplasmosis in the setting of immunosuppression, and in the absence of appropriate anti-Toxoplasma therapy they can be harmful. In patients with severe inflammatory reaction believed to be clinically significant, the addition of corticosteroids should be considered. Corticosteroids are often given to patients with toxoplasmic encephalitis for the reduction of cerebral edema and raised intracranial pressure. Their use thus should be limited to situations where clinically significant edema or a mass-effect are present. Corticosteroids are also often considered when ocular lesions involve the macula or there is significant compromise of the vision.
ENDPOINTS FOR MONITORING THERAPY
Transplant patients with toxoplasmosis should probably be treated for at least 4 to 6 weeks with the acute therapy doses (Table 1). Patients should significantly improve their symptoms and signs before day 10th. Significant worsening during that period of time suggests that the intervention has been instituted too late in the disease course or an alternative diagnosis. The most important endpoints to follow are the original symptoms and signs. Brain MRI studies can still be positive months to years after the acute illness. Thus, if the patient is clearly improving there is no need to request MRI follow up studies within the first year following the diagnosis, unless there is evidence of new onset clinical deterioration.
VACCINES
There are no clinically available vaccines against toxoplasmosis at this time primary prophylaxis should be considered for previously seropositive allogeneic-HSCT, patients particularly if they develop GVHD, and for D+/R- solid organ transplant patients (5). The drug of choice is trimethoprim/sulfamethoxazole and a single-strength tablet per day (this dose needs to be adjusted for patients with abnormal renal function) is recommended. Primary prophylaxis is usually recommended for 100 days following HSCT and one year following solid organ transplantation. An alternative regimen is atovoquone at 1500 mg/day and appears to be a safe and effective alternative to TMP/SMX Routine PCR testing in the peripheral blood of high risk patients (e.g. previously seropositive HSCT or D+/R- solid organ transplant patients) who cannot take anti-toxoplasma prophylaxis can be an effective strategy to institute pre-emptive treatment in those patients who are found to be PCR positive (3, 9).
INFECTION CONTROL MEASURES
Non-applicable.
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Table 1. Drugs Used in Transplant Patients with Toxoplasmosis in the Setting of Acute Infection or Reactivation* (Primary Therapy)
Immunocompromised Patients*** with toxoplasmosis including clinically active, toxoplasmic encephalitis, pneumonia, fever of unknown origin, ocular disease, myocarditis, myositis, hepatitis or in the setting of acute infection | ||
Pyrimethamine (PO): | 200 mg loading dose followed by 50 mg (<60kg) to 75 mg (>60 kg)/day | |
Folinic acid** (PO): | 10 to 20 mg daily (up to 50 mg/day) (during and 1 week after therapy with pyrimethamine) | |
plus | ||
Sulfadiazine (PO): | 1000 (<60 kg) to 1500 mg (> 60 kg) every 6 hour | |
or | ||
Clindamycin (PO or IV) | 600 mg every 6 hours (up to 1200 mg every 6 hours) | |
or | ||
Atovaquone (PO) | 1500 mg orally twice daily | |
Trimethoprim/Sulfamethoxazole (PO or IV) | 10 mg/kg/day (trimethoprim component) divided in two to three doses (doses as high as 15 - 20 mg/kg/day have been used) | |
Pyrimethamine/folinic acid | Same doses as above | |
plus | ||
Clarithromycin (PO) | 500 mg every 12 hours | |
or | ||
Dapsone (PO) | 100 mg/d | |
or | ||
Azithromycin (PO) | 900 to 1200 mg/day | |
Prefered regimens: pyrimethamine/sulfadiazine/folinic acid or trimethoprim/sulfamethoxazole
*Assistance is available for the diagnosis and management of patients with toxoplasmosis at the Palo Alto Medical Foundation Toxoplasma Serology Laboratory, telephone number Palo Alto, CA; http://www.pamf.org/serology/ ; 1-650-853-4828; e-mail: toxolab@pamf.org** Folinic acid = leucovorin; folic acid should not be used as a substittue for folinic acid .***After the successful use of a combination regimen during the acute/primary therapy phase, same agents at half-does are usually used for maintenance or secondary prophylaxis |
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