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Toxoplasma Antibody

Toxoplasma gondii is a small intracellular protozoan parasite that can infect any warm-blooded animal. Three forms of the parasite are recognized. Two of these forms (the trophozoite or proliferative form and the bradyzoite or cyst form) are found in the extra‑intestinal cycle in all hosts, while the third form (the oocyst) is found only in cats.

Transmission may occur after ingestion of raw or undercooked meat containing tissue cysts or by exposure to oocysts in cat feces. After entering the body, Toxoplasma trophozoites invade host cells, either via phagocytosis or direct cell invasion. Parasitemia develops and the organisms spread throughout the body. As the host's immune system becomes active, encystment occurs and the trophozoites are probably cleared. A chronic or latent state ensues with encysted organisms apparently living in symbiosis with the host.

Acquired infections are usually asymptomatic, but there may be fever and lymphadenopathy with an illness resembling infectious mononucleosis. However, on occasion, it can also cause a serious eye infection in adults, a form of chorioretinitis. Acquired toxoplasmosis is almost always self‑ limited in the immunologically normal host and treatment is not required.

Severe toxoplasmosis may develop in immunosuppressed hosts, either as a result of acquiring an acute infection or by reactivation of a latent infection. The disease usually presents with central nervous system manifestations, myocarditis or pneumonitis.

When toxoplasma infection is acquired during pregnancy there is a significant risk of fetal infection. It is generally agreed that congenital infection takes place only when acute infection is acquired during pregnancy. Women, who have serological evidence of toxoplasma infection prior to becoming pregnant rarely, if ever, have infected neonates; it is the seronegative woman who becomes infected and seroconverts during pregnancy who may give birth to an infected fetus.

Congenital toxoplasmosis occurs in 1/500 to 1/1000 of live births. Up to one‑half of the infants are born prematurely. The risk to the fetus appears related to the time of maternal infection. Microcephaly or hydrocephaly with intracranial calcifications may develop if infection is acquired in the first half of pregnancy. Infections in the second half of pregnancy are usually asymptomatic at birth with chorioretinitis or central nervous system manifestations developing months or years later, although there may be fever, hepatosplenomegaly and jaundice at the time of birth. Congenital toxoplasmosis is a significant cause of blindness, psychomotor retardation and convulsive disorders.

The initial serologic response in immunocompetent patients with toxoplasmosis is the simultaneous rise of IgG and IgM antibodies. IgG antibodies persist for years, while IgM antibodies decline to undetectable levels within 6 to 12 months after the initial infection. Therefore, positive IgM antibodies alone, or with IgG antibodies are diagnostic of a recent infection. Positive IgG antibodies alone indicate previous infection.

IgM

IgG

Interpretation

negative

negative

No evidence of infection

positive

negative

Active infection

positive

positive

Recent infection or reactivation

negative

positive

Past infection

Due to FDA recommendations, Toxoplasma IgM antibody testing can no longer be performed as a single test. The FDA and CDC are conducting a pilot study to determine whether Toxoplasma IgM tests produce false positive results. During this evaluation, Toxoplasma IgM testing must be perfomed in conjunction with Toxoplasma IgG antibody

Results are reported as positive or negative. The reference value is seronegative.

Specimen requirement is one SST tube of blood.

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