Infection with the intracellular parasite Toxoplasma gondii occurs primarily through ingestion of the infectious tissue cysts in raw or poorly cooked meat or through contact with feces from infected cats, which contain infectious sporu-lated oocytes. The latter may remain infectious in moist soil for more than 1 year. Only cats that hunt and kill their preyare reservoirs for infection; those that eat prepared cat food are not. In immunocompetent adult humans, infection ismost commonly asymptomatic and disease is self-limited. Prior infection confers immunity, unless the individual is immunosuppressed. Approximately 15% of reproductive-age women have antibodies to toxoplasmosis.
Although congenital infection is more common follow-ing maternal infection in the third trimester, the sequelae following 1st-trimester fetal infection are more severe. Over half of infants whose mothers are infected during the last trimester of pregnancy have serologic evidence of infection, but three-fourths of these show no gross evidence of infec-tion at birth. Signs of congenital infection include severe mental retardation, chorioretinitis, blindness, epilepsy, intracranial calcifications, and hydrocephalus.
In some regions with high prevalence of disease (France and Central America), screening is routine in preg-nancy. In the United States, routine screening in pregnancy is not recommended except in the presence of maternal HIV infection. Because identification of the organism in tissue or blood is complex and infection is usually asympto-matic, diagnosis depends on demonstration of seroconver-sion. A positive IgG titer indicates infection at some time. A negative IgM effectively rules out recent infection; how-ever, IgM may persist for long periods and a positive test is not reliable in assessing duration of disease. In addition, false-positive IgM results are common with commercially available assays. Confirmatory testing in pregnancy should be performed in a Toxoplasma reference laboratory prior to initiating any therapy.
Treatment of acutely infected pregnant women with spiramycin may reduce the risk of fetal transmission but does not prevent sequelae in the fetus if infection has oc-curred. This medication is only available through the FDA. If fetal infection has already been noted (through ultra-sound findings or confirmed with testing of fetal blood or amniotic fluid), pyrimethamine and sulfadiazine therapy may decrease the risk of congenital infection and the severity of manifestations.
Prevention of infection should be an important part of prenatal care, including counseling regarding thoroughly cooking all meats, careful hand washing after handling raw meats, washing of fruits and raw vegetables before inges-tion, wearing gloves when working with soil, and keeping cats indoors and feeding them only processed foods. If a cat is kept outside, someone other than a pregnant woman should feed and care for the cat and dispose of its wastes.