Toxoplasmosis, a zoonotic disease, has the potential for severe manifestations, most importantly in fetuses of humans and several domestic animal species. Because this is an emotionally charged issue, and physicians often are not well informed about the disease, it is important that veterinarians understand the life cycle of Toxoplasma gondii (T. gondii) and how it relates to cats and the transmission to other species.

The public health significance of toxoplasmosis is most critical to women of child bearing age. Transplacental transmission is the most important and common route of fetal infection is species other than cats. Approximately 25-45% of women in the U.S. aged 20 to 39 have chronic asymptomatic toxoplasmosis and are immune to re-infection. If a woman in this group becomes pregnant, the fetus will not become infected. To endanger the fetus, the initial infection must occur after conception. Acute maternal infections are responsible for the transplacental infection of more than 3,000 infants each year in the U.S. In the human fetus, as in other species, T. gondii has a predilection for the brain, often producing chorioretinitis, microcephaly, mental retardation, or other abnormalities. If a woman who plans to conceive is already seropositive, there is no danger of infecting the fetus. Routine serologic testing of pregnant women is done in several countries, but not in the U.S.

The role of cats in transmission of the disease was not discovered until 1970. Only members of the Felidae family have been shown to shed Toxoplasma oocysts. Cats shed this infective stage in their feces usually only once in their lives, for approximately 14 days after original infection. Transmission to humans can occur not only-by-ingestion of infective oocysts, but also by ingesting raw or rare pork or lamb containing tissue cysts (bradysoites). Eating raw beef has not been shown to produce infection, since cattle have not been found to experience clinical infection from T. gondii.

Herbivores and carnivores become infected by ingesting oocysts directly in contaminated feed. Carnivores also become infected by ingesting bradyzoites in raw or poorly cooked meat from herbivores with tissue cysts. After ingestion, the organism can encyst in many tissues, including the fetus. In sheep, goats and swine, this can lead to abortions, stillbirths and fetal resorption.

Because of the role of the domestic cat veterinarians should be able to answer their clients' questions about this disease. Some recommendations are:

-- serologic testing of prospective mothers before they become pregnant

-- eat only cooked meat

-- wash hands after handling raw meat and before eating, especially if you own a cat

--feed cats only cooked or commercially prepared foods

--cover children's sand boxes when not in use

--dispose of cat feces on a 24 hour basis

--wear gloves when emptying the cat litter box

--pregnant women should not clean the cat litter box