I am in my 36th week of pregnancy. I have two outdoor cats that are successful hunters and eaters of their kills. One of the cats had a blood test this week and was found to be positive for toxoplasmosis. I have not cleaned the litter box since becoming pregnant, but I did spend a lot of time gardening where the cats frequented during my first trimester. I never came in direct contact with feces. What is the chance that my baby is infected at this point? I am scheduled for a blood test soon.

Judy

Toxoplasma gondii is a parasite that infects humans and animals. Millions of people in the United States, and the majority of people in some areas, are infected with this organism but have no symptoms of illness. When adults with an intact immune system are newly infected, they are usually asymptomatic. They may also have a feverish disease or simply a swollen lymph node. Toxoplasmosis can reactivate later in life, resulting in infection of the retina of the eye and even blindness. In a patient with a weakened immune system such as in AIDS, toxoplasmosis often results in a serious brain infection.

Toxoplasmosis infection in a fetus can result in a disease ranging in severity from a completely silent infection to serious organ involvement and stillbirth. If a pregnant woman develops toxoplasmosis in the first trimester, chances are between 10 and 25 percent that the fetus will be infected. If the mother becomes infected in the third trimester, the rate of fetal infection approaches 65 percent. However, the likelihood that the infection in the fetus will cause symptoms is far higher if the infection occurs in the first trimester than in the third trimester.

Cats become infected with the parasite after eating rodents and other animals that carry forms of the parasite in their muscle. The cats become infected and pass the parasite's eggs in the stool. They shed the organism for only a few weeks, but the eggs can persist in the right environment for longer than a year. People become infected when they consume food or water contaminated with the egg. People may also become infected when they consume improperly cooked meats, usually pork, containing the organism.

The diagnosis of a symptomatic case of toxoplasmosis is usually not very difficult. However, diagnosis is very tricky in pregnant women, in whom the infection is often asymptomatic. It is extremely important to determine when the infection occurred. If it happened long before pregnancy, then the fetus is at essentially no risk. If infection occurred early in pregnancy, there is a substantial chance that serious infection has developed in the fetus. Several different antibody tests can help determine the timing of infection. They are not foolproof, however, and mistakes sometimes occur.

If tests indicate that infection occurred during pregnancy, then the mother is immediately started on a medication to try to minimize the risk to the fetus. However, transmission to the fetus may have already occurred, so further testing of the fetus is usually performed. After 18 weeks of pregnancy, an amniocentesis is performed, and the amniotic fluid is tested for toxoplasmosis by PCR (a way of finding minute amounts of genetic material, such as that of the toxoplasma organism). If the test is positive, then the fetus is very likely already infected, and the woman has to decide what to do. If it is early in pregnancy, she may choose to have an induced abortion. A sonogram (ultrasound exam) can sometimes pick up serious malformations even at this early stage and aid in this decision. If abortion is not performed, medications are given to the mother to treat her and the fetus.

At your stage, antibody testing can be a little tricky, but I suggest you get tested. If you test completely negative, then you have nothing to worry about. If testing suggests that your infection was not very recent, then you are probably okay, but it is still possible that you were infected early in pregnancy. Your baby should then be tested at birth and treated if infected.