Toxoplasma gondii is an obligate intracellular parasite capable of infecting most mammals, including humans. Toxoplasmosis usually is asymptomatic, but primary infection during pregnancy can result in congenital disease. The domestic cat is the only definitive host for T. gondii and is the reservoir of the infective oocysts that are passed in the feces. Human infection may be acquired by consuming cysts in uncooked or undercooked meat of infected animals or by contact with oocysts from the feces of an infected cat.
Acute Toxoplasma infection can pose a serious threat to immunocompromised individuals and newborns who acquire the infection in utero. Immunosuppressed patients may develop encephalitis, myocarditis, or pneumonitis. Congenital infections usually result as a consequence of asymptomatic acute maternal infection. This infection can cause premature delivery, spontaneous abortion, or stillbirth.
Management of toxoplasmosis requires serologic monitoring of infected individuals, as the organism is not readily available for culture.
Normal range: Negative.
Use
Aids in the diagnosis of toxoplasmosis.
First-line test in endemic areas for identifying T. gondii infection in pregnant women.
Testing for the presence of Toxoplasma IgG can be useful to determine prior infection and indicate reactivation of the infection.
Testing for the presence of Toxoplasma IgM is useful to determine acute infection.
Interpretation
Positive in Toxoplasma infection.
Individuals infected with the Toxoplasma organism typically exhibit detectable levels of IgM antibody immediately before or soon after the onset of symptoms. IgM titers normally decline within 4 " “6 months but may persist at low levels up to a year. Patients with active Toxoplasma chorioretinitis usually have undetectable levels of IgM.
Limitations
IgG is not useful for diagnosing infection in infants <6 months of age, because they are usually the result of passive transfer from the mother.
Low levels of IgM antibodies may occasionally persist for >12 months postinfection. For the determination of seroconversion from nonreactive to reactive, two serum samples should be drawn 3 " “4 weeks apart, during the acute and convalescent stages of the infection. The acute-phase sample should be stored and tested in parallel with the convalescent sample.
CDC suggests equivocal or positive results should be retested using a different assay from another reference laboratory specializing in toxoplasmosis testing (IgG dye test, IgM ELISA, reflex to avidity, and/or other tests).
In a pregnant patient, if both IgG/IgM positive, an IgG avidity test should be performed. A high avidity result in 12 " “16 weeks of pregnancy essentially rules out an infection acquired during gestation.
A low IgG avidity result should not be interpreted as recent infection, because some individuals have persistent low IgG avidity for many months after infection.
Newborn infants suspected of congenital toxoplasmosis should be tested by both an IgM- and an IgA-capture EIA (CDC recommendation). Detection of Toxoplasma-specific IgA antibodies is more sensitive than IgM detection in congenitally infected babies.