Toxoplasmosis is a disease caused by the parasite Toxoplasma gondii. These protozoa belong to intestinal coccidium that parasitizes members of the cat family as definitive hosts and has a wide range of intermediate hosts. T. gondii infection in humans is widespread throughout the world. Approximately half a billion humans have antibodies to T. gondii. Environmental conditions, cultural habits, and animal species are among factors that may determine the degree of natural spread of T. gondii.
The life cycle of T. gondii involves members of the family Felidae, including domestic cats as the definitive hosts, and various warm-blooded animals and humans that serve as intermediate hosts. T. gondii is transmitted by either congenital transmission, consumption of uncooked infected meat, or fecal-oral transmission. Cats acquire T. gondii by ingesting any of three infectious stages of the organism: tachyzoites, bradyzoites in infected tissue, and the oocysts shed in feces. Most cases of toxoplasmosis in humans are probably acquired by the ingestion of either tissue cysts in infected meat or oocysts in food contaminated with cat feces. T. gondii usually parasitizes both definitive and intermediate hosts without producing clinical signs. Lymphadenitis is the most common manifestation in humans. Lymphadenopathy may be accompanied by fever, malaise, fatigue, muscle pains, sore throat, and headache. Severe disease is usually observed only in congenitally infected children and in immunosuppressed individuals, including patients with HIV/AIDS. Opportunistic toxoplasmosis in immunosuppressed patients usually represents reactivation of chronic infection, with encephalitis as the most common manifestation. Prenatally acquired T. gondii often infects the brain and retina of the fetus. Hydrocephalus is the least common but most dramatic lesion of congenital toxoplasmosis.
Diagnosis of toxoplasmosis must be aided by serologic or histocytologic examination. This includes indirect hemagglutination test, the latex agglutination test, modified agglutination test, and the enzyme-linked immunoabsorbent assay (ELISA) to detect presence of, or to quantify, IgM and IgG against T. gondii. Diagnosis can also be made by finding T. gondii in host tissue removed by biopsy or at necropsy. Sulfonamides and pyrimethamine are two drugs widely used to treat toxoplasmosis in humans. These two drugs usually are well tolerated by the patient, but sometimes thrombocytopenia, leukopenia, or both may develop. These effects can be overcome without interrupting treatment by administering folic acid. Although these drugs are helpful when given in the acute stage of the disease, usually they will not eradicate infection when active multiplication of the parasite occurs. Spiramycin is a drug used to treat pregnant women to minimize the effects of congenital toxoplasmosis.
To prevent T. gondii infection, several precautions should be taken. Meat should be cooked thoroughly before eating and hands should be washed with soap and water after handling meat. Cats should be kept indoors and litter boxes should be changed daily. Cat feces should be flushed down the toilet or burned. Gloves should be used while working in the garden. Currently, no killed vaccine is available to reduce or prevent congenital infections in humans.
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