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==Description==

==Description==

T  gondii  is an important zoonotic agent. In some areas of the world, up to 60% of the human population have serum IgG titers to T  gondii  and are likely to be persistently infected.

Human exposure to toxoplasmosis is common wherever cats are found; 20 to 40% of healthy adults in the US are seropositive. The risk of developing disease is very low except for a fetus infected in utero and people who are or become immunocompromised

Toxoplasma gondii is present worldwide wherever there are cats. The parasite infects a large number of animals as well as people. Many people in the United States have been infected, although few ever develop symptoms. Severe infection usually develops only in fetuses and people with an immune system weakened by AIDS, cancer, or drugs used to suppress rejection of an organ transplant (immunosuppressants).

T. gondii is ubiquitous in birds and mammals. This obligate intracellular parasite invades and multiplies asexually as tachyzoites within the cytoplasm of any nucleated cell (see Fig. 2: Extraintestinal Protozoa: Toxoplasma gondii life cycle.). When host immunity develops, multiplication of tachyzoites ceases and tissue cysts form; cysts persist in a dormant state for years, especially in brain and muscle. The dormant Toxoplasma forms within the cysts are called bradyzoites. Sexual reproduction of T. gondii occurs only in the intestinal tract of cats; the resultant oocysts passed in the feces remain infectious in moist soil for months.

Eggs of the toxoplasmosis parasite can grow only in the intestine of cats.

Ingestion of oocysts in food or water contaminated with cat feces is the most common mode of oral infection. Infection can also occur by eating raw or undercooked meat containing tissue cysts, most commonly lamb, pork, or rarely beef. After ingestion of oocysts or tissue cysts, tachyzoites are released and spread throughout the body. This acute infection is followed by the development of protective immune responses and the formation of tissue cysts in many organs. These cysts can reactivate, primarily in immunocompromised patients. Toxoplasmosis reactivates in 30 to 40% of AIDS patients who are not taking antibiotic prophylaxis, but the widespread use of trimethoprim/sulfamethoxazole Some Trade Names

Although the parasite can grow in the tissues of many animals, it produces eggs (oocysts) only in cells lining the intestine of cats. Eggs are shed in a cat's stool and can survive for up to 18 months in the soil.

for Pneumocystis prophylaxis has dramatically reduced the incidence.

People may acquire the infection by transferring Toxoplasma eggs from contaminated soil or other objects to their mouth or by eating contaminated food. Occasionally, animals such as pigs acquire toxoplasmosis from contact with contaminated soil by eating eggs. People can become infected by eating raw or undercooked meat from infected animals. The parasite is rarely transmitted through blood transfusions or by an organ transplanted from an infected person.

Toxoplasmosis can be transmitted transplacentally if the mother becomes infected during pregnancy or if immunosuppression reactivates a prior infection. Transmission of Toxoplasma to a fetus is extraordinarily rare in immunocompetent mothers who have had toxoplasmosis earlier in life. Transmission may occur via transfusion of whole blood or WBCs or via transplantation of an organ from a seropositive donor. In otherwise healthy people, congenital or acquired infection can reactivate in the retina. Past infection confers resistance to reinfection.

 

A woman who acquires the infection during pregnancy can transfer Toxoplasma gondii to her fetus through the placenta. The result may be a miscarriage, stillbirth, or a baby born with congenital toxoplasmosis (see Problems in Newborns: Some Infections of Newborns). A woman who was infected before the pregnancy does not pass the parasite on to the fetus.

 

People with a weakened immune system, primarily those who have AIDS or cancer or who take drugs to suppress rejection of an organ transplant, are especially at risk of toxoplasmosis. Symptoms usually develop in these people when a previously acquired Toxoplasma infection is reactivated but can develop when an organ is transplanted from an infected person. The infection usually affects the brain, but it may affect the eye or spread throughout the body (disseminate). In people with a weakened immune system due to AIDS, cancer, or immunosuppressants taken after organ transplantation or for other reasons, toxoplasmosis is very serious and may be fatal if untreated.

==Signalment==

==Signalment==