Toxoplasmosis - An update.

Toxplasmosis is an important zoonotic disease caused by protozoan parasite Toxoplasma gondii. The disease affects one-third of the total world population. Transmission of the disease is mainly by ingestion of food or water contaminated with oocysts. Congenital toxoplasmosis occurs from the transplacental passage of the parasite from mother to fetus. In most adults it does not cause serious illness, but it can cause blindness and mental retardation in congenitally infected children, and it is a devastating disease in immunocompromised individuals. Diagnosis of toxoplasmosis can be established by the direct detection of the parasite or by serological methods. The most commonly used and effective therapeutic regimen is the combination of pyrimethamine with sulfadiazine and folinic acid. This article provides an overview and update on transmission, diagnosis, management, and prevention of toxoplasmosis.

INTRODUCTION

Toxoplasmosis, an ubiquitous protozoal disease caused by Toxoplasma gondii, is one of the most common parasitic infections of man and other warm-blooded animals, with definite hosts being felines. It has been found worldwide from Alaska to Australia.[1] Its seroprevalence depends on the location and the age of the population. Generally, hot, arid climatic conditions are associated with a low prevalence of infection. In the United States and the United Kingdom, it is estimated that 16 – 40% of the population are infected, whereas, in Central and South America and Continental Europe, estimates of infection range from 50 – 80%.[2] The seroprevalence average of T. gondii infection in India has been reported to be 24.3%, the lowest being in the northern parts of India and the highest in the south, and a seroconversion rate of 1.5% has been reported.[3] In a study conducted in 2005-06, a seroprevalence of 15.33% has been reported in pregnant women in India.[4] Most infections in humans are asymptomatic, but at times the parasite can produce devastating disease. The socioeconomic impact of toxoplasmosis in human suffering and the cost of care of sick children, especially those with mental retardation and blindness are enormous.[5] The testing of all pregnant women for T. gondii infection is routine in some European countries, including France and Austria. The cost benefit of such screening is being debated in many other countries.[6]

TRANSMISSION

Infection may be congenitally or postnatally acquired. About one-third of all the women who acquire infection with T. gondii during pregnancy transmit the parasite to the fetus, and the rest give birth to normal uninfected babies. The frequency of congenital transmission varies considerably depending on the time, during gestation, when the mother became infected. Congenital infection occurs only when a woman becomes infected during pregnancy. Congenital infections acquired during the first trimester are more severe than those acquired in the second and third trimesters. Only a small proportion (20%) of women infected with T. gondii develop clinical signs of infection. Often the diagnosis is first appreciated when routine postconception serologic tests show evidence of a specific antibody.[7] Early maternal infection (first and second trimesters) may result in severe toxoplasmosis and can result in the death of the fetus in utero and spontaneous abortion. By contrast late maternal infection (third trimester) usually results in a normal appearing newborn. The overall frequency of subclinical infection in the new born with congenital toxoplasmosis is as high as 85%.[8]

Postnatally the infection can be acquired by ingesting tissue cysts in undercooked or uncooked meat or by ingesting food and water contaminated with oocysts from infected cat feces. During acute feline infection, a cat may excrete as many as 100 million parasites per day. These very stable sporozoite-containing oocysts are highly infectious and may remain viable for many years in the soil.[9] Oocysts in the soil do not always stay there, as invertebrates such as flies, cockroaches, dung beetles, and earthworms can mechanically spread these oocysts and even carry them onto food.[10] Currently, there are no tests that can discriminate between oocyst ingestion and tissue cyst ingestion, as an infection route. Available evidence for the oocyst infection route is based upon epidemiological surveys. In certain areas of Brazil, approximately 60% of six-to-eight-year-old children have antibodies to T. gondii linked to the ingestion of oocysts from an environment heavily contaminated with T. gondii oocysts. Several outbreaks of toxoplasmosis in human beings have been linked epidemiologically to drinking unfiltered water.[11] The largest outbreak of clinical toxoplasmosis in humans was epidemiologically linked to drinking water from a municipal water reservoir in British Columbia, Canada. This water reservoir was thought to be contaminated with T. gondii oocysts execrated by cougars (Felis concolor).[12] An outbreak of ocular toxoplasmosis from municipal water contamination was reported from Coimbatore, India.[13] Children and adults also can acquire infection from tissue cysts containing bradyzoites. The ingestion of a single cyst is all that is required for human infection. Undercooking or insufficient freezing of meat is an important source of infection. Cultural habits of a population may affect the acquisition of T. gondii infection from ingested tissue cysts in undercooked meat. In France, a very high prevalence of antibodies to T. gondii (84% in pregnant women) appears to be related to the French habit of eating some meat products raw or undercooked.[14]

Transmission of T. gondii may also occur through blood transfusion and organ transplantation. Viable parasites can be cultured from refrigerated anticoagulated blood, which may be a source of infection in individuals receiving blood transfusion.[15] T. gondii infection has also been reported in kidney and heart transplant recipients who were uninfected before transplantation.[16] Infection in laboratory personnel have arisen due to contact with contaminated needles and glassware, or infected animals.[17]

CLINICAL PRESENTATION

In persons whose immune systems are intact, acute toxoplasmois is usually asymptomatic and self-limited. This condition can go unrecognized in 80 to 90% of adults and children with acquired infection. The most common manifestation of acute toxoplasmosis is cervical lymphadenopathy. The nodes may be single or multiple, are usually non-tender and discrete, and vary in firmness. Lymphadenopathy may also be found in the suboccipital, supraclavicular, inguinal, and mediastinal areas. Generalized lymphadenopathy occurs in 20 – 30% of symptomatic patients. Between 20 and 40% of the patients with lymphadenopathy also have headache, malaise, fatigue, and fever (usually with a temperature of < 40°C). A smaller proportion of symptomatic individuals have myalgia, sore throat, abdominal pain, maculopopular rash, meningoencephalitis, and confusion. Rare complications associated with infection in the normal immune host include pneumonia, myocarditis, encephalopathy, pericarditis, and polymyositis. Symptoms associated with acute infection usually resolve within several weeks, although lymphadenopathy may persist for some months. In one epidemic, toxoplasmosis was diagnosed accurately in only three of the 25 patients who consulted the physician.[17] If toxoplasmosis is considered in the differential diagnosis, routine laboratory and serologic screening should be performed before node biopsy.

Patients with AIDS and those receiving immunosuppressive therapy are at the greatest risk of developing acute toxoplasmosis. This predilection may be due either to the reactivation of latent infection or to the acquisition of parasites from exogenous sources such as blood or transplanted organs. In individuals with AIDS, more than 95% of the cases of Toxoplasmosis Encephalitis are due to recrudescent infection. In immunocompromised individuals, the disease may be rapidly fatal if untreated. Thus, accurate diagnosis and initiation of appropriate therapy are necessary to prevent fulminant infection. In immunosuppressed patients, infection may occur in any organ. Patients may have headache, disorientation, drowsiness, hemiparesis, reflex changes, and convulsions and may become comatose.

Toxoplasmosis is a principal opportunistic infection of the central nervous system (CNS) in persons with AIDS.

Approximately 10% of the AIDS patients in USA and up to 30% in Europe are estimated to die from Toxoplasmosis.[18] Although in AIDS patients any organ may be involved, including testes, dermis, and the spinal cord, infection of the brain is most frequently reported. Most AIDS patients suffering from toxoplasmosis have severe and persistent bilateral headaches, which respond poorly to analgesics. As the disease progresses, the headache may give way to a condition characterized by confusion, lethargy, ataxia, and coma. The predominant lesion in the brain is necrosis, especially of the thalamus.[19]

A wide spectrum of clinical diseases occur in congenitally infected children. Neonatal manifestations of congenital toxoplasmosis vary widely and include hydrocephatus, microcephaly, intracranical calcification, chorioretinits, strabismus, blindness, epilepsy, psychomotor or mental retardation, petechia, and anemia.[2021] The classic triad of chorioretinitis, hydrocephalus, and cerebral calcification is rather rare. None of the signs described in newborns with congenital disease is pathognomic for toxoplasmosis and can be mimicked by congenital infection with other pathogens, including cytomegalovirus, herpes simplex virus, rubella, and syphilis.

Toxoplasmic chorioretinitis can occur as a result of acute infection or reactivation.[2223] Typical findings of toxoplasmic chorioretinitis include noticeably white focal lesions with an overlying and intense vitreous inflammatory reaction. Chorioretinitis, in individuals with acute acquired toxopasmosis, can arise sporadically or in the context of an outbreak of acute disease. Between 1 and 30% of all patients with AIDs develop debilitating chorioretinitis due to T. gondii.

DIAGNOSIS

Diagnosis of toxoplasmosis in humans is made by isolation, serological, histological or molecular methods, or by some combination of these. Clinical signs of toxoplasmosis are nonspecific and are not sufficiently characteristic for a definite diagnosis. Toxoplasmosis in fact mimics several other infectious diseases. Although difficult, T. gondii can be isolated from patients by inoculation of mice and tissue cultures, with blood or other body fluids. Isolation of T. gondii from the patients’ body fluids reflects acute infection, whereas, isolation from biopsied tissue is an indicator only of the presence of tissue cysts and should not be misinterpreted as evidence of acute toxoplasmosis.

Demonstration of tachyzotes in lymph node tissue sections or smears of body fluid establishes the diagnosis of acute toxoplasmosis. Histological demonstration of cysts containing bradyzoites confirms prior infection with T. gondii, but is nondiagnostic for acute infection. The immunoperoxidase technique is found to be both sensitive and specific and is superior to the conventionally stained tissue sections. It has been used successfully to show the presence of parasites in the CNS of AIDS patients.[25]

Detection of T. gondii antibody inpatients may aid diagnosis. Serologic testing has become the routine method of diagnosis. A wide range of serologic tests are available commercially.

Diagnosis of acute infection with T. gondii can be established by the detection of the simultaneous presence of IgM and IgG antibodies to toxoplasma in the serum. The presence of circulating IgA favors the diagnosis of acute infection. The Sabin Feldman dye test, the indirect fluorescent antibody test (IFAT), direct-agglutination test, latex agglutination test, enzyme-linked immunosorbent assay (ELISA), and the immunosorbent agglutination assay test (IAAT), all satisfactorily measure the circulating IgG antibody to Toxoplasma. Tests for the avidity (functional affinity) of IgG antibodies are used to discriminate between recently acquired infection and that obtained in the more distant past.[26] The presence of high avidity antibodies essentially rules out infection acquired in the recent three to four months, and by contrast, low avidity antibodies can persist beyond three months of infection.[26-28]

Positive IgG titers can be detected as early as two to three weeks after infection. These titers peak at six to eight weeks and decline slowly to a new baseline level that persists for life. It is necessary to measure the serum IgM titer in concert with the IgG titer to better establish the time of infection. The methods currently available for this determination are the double sandwich IgM-ELISA and the IgM immunosorbent assay (IgM-ISAGA). Both these assays are sensitive and specific, and their use precludes the false positive results associated with tests for rheumatoid factor and antinuclear antibody. The double-sandwich IgA ELISA is more sensitive than the IgM ELISA for detecting congenital infection in the fetus and newborn.[8] Local antibody production in the eye has been successfully used for diagnosis of occular infection.[29]

Studies have shown that PCR amplification of the 35-fold repetitive B1 gene detection of T. gondii DNA is highly sensitive, specific, and has clinical utility in the diagnosis of congenital, ocular, cerebral, and disseminated toxoplasmosis.[30-33] Real time PCR of the amniotic fluid to detect the B1 gene of the parasite has replaced fetal blood sampling. Peripheral blood, cerebrospinal fluid, and urine should be considered for PCR examination in any newborn, suspected to have congenital disease. PCR of vitreous or aqueous fluid is helpful in establishing diagnosis in patients with atypical retinal lesions.[34]

MANAGEMENT AND TREATMENT

Immunocompetent adults and children who have only lymphadenopathy do not require specific therapy unless they have persistent, severe symptoms. Patients with ocular toxoplasmosis should be treated for one month with pyrimethamine plus either sulfadiazine or clindamycin. Prenatal antibiotic therapy can reduce the number of infants severely affected with toxoplasma infection.[8] Infection acquired by laboratory accident or transfusion of blood products are potentially most severe, and these patients should always be treated.

Treatment with spiramycin should be initiated as fast as possible after diagnosis of recently acquired maternal infection. To date, the studies undertaken have not permitted a definitive conclusion about the use of spiramycin. Spiramycin is recommended for the first and early second trimesters or pyrimethamine / sulfadiazine for the late second and third trimesters, for women with suspected or confirmed acute T. gondii infection acquired during gestation.[3536] As maternal infection does not necessarily result in fetal infection, suspected or established maternal infection acquired during gestation (based on serology) must be confirmed by prenatal diagnosis, by PCR of amniotic fluid. This test has an overall reported sensitivity of 64 – 98.8%.[1037-39] In case of a negative PCR result, pregnant women should receive spiramycin prophylaxis until the seventeenth week of pregnancy and have monthly ultrasound examinations for the entire pregnancy.

In case of a positive PCR result or very highly probably infection of the fetus, treatment consists of pyrimethamine / sulfadiazine with spiramycin. Prenatal treatment with pyrimethamine / sulfadiazine of women suspected or confirmed to have fetal infection reduces the squeal of the disease in the new born.[40]

Anti-toxoplasma treatment should be continued throughout the pregnancy. Folinic acid is added to regimens to reduce bone marrow suppression. In most countries, treatment of the fetus is followed by treatment of the new born throughout the first year of life. Patients with AIDS should be treated for acute toxoplasmosis. In immunocompromised patients toxoplasmosis is rapidly fatal.

Pre-emptive, anti-parasitic treatment should be considered for all symptomatic, seropositive, immunocompromised patients suspected to have toxoplasmosis. When clinical manifestations suggest involvement of the brain, spinal cord, or both, neuroimaging studies such as computed tomography (CT) or magnetic resonance imaging (MRI) are mandatory. These studies should be considered even if neurological examination does not indicate focal deficits. Empiric anti-T. gondii treatment is an accepted practice for patients with multiple ring enhancing brain lesions (usually established by MRI), positive IgG antibody titers against T. gondii, and advanced immunodeficiency; a clinical and radiological response to specific anti-T. gondii treatment is judged supportive of the diagnosis of CNS toxoplasmosis. Patients with cerebral toxoplasmosis usually improve by more than 50% of their baseline neurological examination in seven to ten days.[41]

The most typically used and successful regimen for treatment of toxoplasmosis in immunocompromised patients is the combination of pyrimethamine / sulfadiazine and folinic acid.[42] Clindamycin can be used instead of sulfadiazine in patients intolerant to sulfonamides. Treatment is recommended for four to six weeks after resolution of all signs and symptoms (sometimes for several months or longer). Trimethoprim / sulfamethoxazole appears to be equivalent to pyrimethamine / sulfadiazine in patients with AIDS.[43]

After treatment of the acute phase (primary or induction treatment) in immunocompromised patients, maintenance therapy (secondary prophylaxis) should be started, usually with the same regimen that was used in the acute phase, but at half doses. At present, maintenance treatment is continued for the life of the patient or until the underlying immunosuppression has ceased. In patients with AIDS, primary and secondary prophylaxis are generally discontinued when the patients’ CD4 count has returned to more than 200 cells per μl and the HIV PCR peripheral blood viral load has been reasonably controlled for at least six months.[44]

PREVENTION

Public health measures to prevent T. gondii infection are a possible approach to diminish the burden of the disease in human beings and animals. Wide differences exist in public-health policies to prevent congenital infection; however, data for the efficacy of such policies are scarce. Systematic serological screening of all pregnant women is undertaken only in France and Austria. Uncertainty about the incidence of congenital infection, cost-effectiveness, difficulties with sensitivity and specificity of serological tests, and findings suggesting absence of spiramycin effectiveness, has hampered attempts to implement screening programs in several countries. Neonatal screening has been implemented in several countries (e.g., Denmark) or areas such as Massachusetts, USA, and through these programs, as many as 80% of the infected newborns have been identified.[45]

Vaccine approaches have included the use of purified or recombinant T. gondii surface antigens,[46] live attenuated or mutant strains of the parasite or DNA with plasmids encoding colony-stimulating factors.[47] At present immunization against toxoplasmosis is not available.

All HIV-infected persons, including those who lack IgG antibody to toxoplasma, should be counseled regarding the source of toxoplasma infection. People handling meat should wash their hands thoroughly with soap and water. T. gondii organisms in meat can be killed by exposure to extreme heat or cold. Tissue cysts in meat are killed by heating the meat throughout to 67ºC[48] or by cooling to - 13ºC.[49] Toxoplasma in tissue cysts are also killed by exposure to 0.5 kilorads of gamma radiation.[50] To conclude, infection by the protozoan parasite T. gondii is widely prevalent. Although it causes asymptomatic infection in immunocompetent adults, T. gondii can cause devastating disease in congenitally infected children and those with depressed immunity. Diagnosis of toxoplasmosis can be established by direct detection of the parasite or by serological techniques. The most commonly used therapeutic regimen, and probably the most effective, is the combination of pyrimethamine with sulfadiazine and folinic acid. Despite the significant development, there are many issues regarding epidemiology, diagnosis, treatment, and preventive strategies for toxoplasmosis, which need to be addressed.