HERD HEALTH
PIH-130
pork industry handbook
PURDUE UNIVERSITY • COOPERATIVE EXTENSION SERVICE • WEST LAFAYETTE, INDIANA
Toxoplasmosis in Pigs
Authors
J. P. Dubey, USDA, ARS, Beltsville, Maryland
Reviewers
R. M. Weigel, University of Illinois
D. S. Lindsay, Auburn University
B. E. Stromberg, University of Minnesota
Toxoplasmosis was first reported in pigs in 1951 on a farm in Ohio. The etiologic agent is a protozoan parasite, Toxoplasma gondii. It is microscopic, measuring 1/200th of a millimeter in length. Infection by this parasite is widespread in pigs worldwide. The primary concern regarding Toxoplasma infection in pigs is that edible tissues become infected with T. gondii and the ingestion of undercooked pork can become a source of infection for humans.

Etiology and Transmission
Toxoplasma gondii causes infection in most warm-blooded animals including livestock and humans. Approximately 40% of adult humans in the U.S. are infected with the parasite and T. gondii infections are more prevalent in Continental Europe. The three major sources of infection with T. gondii are: 1) ingestion of food and water contaminated with Toxoplasma oocysts from cat feces, 2) ingestion of infected meat containing tissue cysts, and 3) congenitally from an infected mother to her fetus (Figure 1).

Cats (both wild and domestic) are the definitive (reservoir) hosts for the parasite because they are the only hosts known that excrete in their feces the resistant stage of Toxoplasma, the oocysts. Cats become infected by eating tissues of infected animals (rodents, birds, rabbits). After the ingestion of infected meat, Toxoplasma multiplies in the intestine of cats and eventually produces oocysts. A cat may excrete several hundred million oocysts in a period of one to two weeks. The oocysts are resistant to environmental influences including disinfectants, and can survive for months. Oocysts are infectious to most warm-blooded animals, including pigs and humans. After the ingestion of the oocysts, Toxoplasma multiplies in many host animal tissues eventually becoming encysted in the muscles, liver, brain, and other tissues. Tissue cysts (resting stage of T. gondii) are microscopic and they can remain dormant in tissues for the life of the animal. Humans and other carnivores can become infected with T. gondii in a variety of ways including ingestion of pork from pigs and other tissues from animals.
Toxoplasma infection during pregnancy can lead to infection of the fetus and the birth of an infected child.

Infection in Pigs
About 15% to 25% of apparently healthy pigs in the U.S. are estimated to be exposed to T. gondii, as determined by antibody tests. Toxoplasma generally does not make pigs ill. The parasite can persist in the edible tissues of pigs and other food animals for years, perhaps for life. The parasite has been found in virtually all body muscles of pigs.

Clinical Toxoplasmosis in Pigs
Most T. gondii infections in swine are subclinical, but toxoplasmosis can cause clinical signs in pigs of all ages. Clinical toxoplasmosis has been reported most often in nursing pigs. Infected piglets are bom dead, sick, or become sick within 3 weeks after birth; some remain clinically normal. Labored respiration is the most common clinical sign of toxoplasmosis. Other clinical signs include fever, general weakness, diarrhea, nervous signs and rarely, loss of vision. Detection of antibodies to T. gondii by a blood test can aid diagnosis. Dead piglets should be submitted to diagnostic laboratories for necropsy and histologic evaluation. Toxoplasma can cause mummified fetuses and stillborn piglets. A blood test for T. gondii antibodies in body fluid of the fetus can detect toxoplasmosis.

Public Health Concerns
As mentioned earlier, about 40% of adult humans in the U.S. are infected with the parasite Toxoplasma. In most humans, like pigs, Toxoplasma does not cause clinical signs of illness. The primary human population at risk are pregnant women and individuals with depressed immunity, including cancer patients, organ transplant recipients receiving immunosuppressive therapy, and people with AIDS.

Toxoplasmosis may cause mental retardation, loss of vision, and birth defects in children whose mothers acquired
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