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Removing the Pork Connection For a People Parasite

By Judy McBride
January 19, 1998

Human infections with Toxoplasma gondii have dropped in recent years, thanks to changes in personal hygiene, swine production and household cooking and freezing of meat. But Agricultural Research Service scientists won't be happy until the potential for swine-transmitted toxoplasmosis is zero.

ARS researchers recently cloned pig genes they suspect play a major role in enabling the animals to resist T. gondii, according to ARS immunogeneticist Joan Lunney. She described their progress Sunday, Jan. 18, at the Plant and Animal Genome VI International Conference in San Diego, Calif.

Overall, the studies will help identify the genes that endow natural resistance against specific parasitic infections or help newborn pigs more rapidly resist many infectious diseases. Lunney is based at the Immunology and Disease Resistance Laboratory in Beltsville, Md.

T. gondii usually doesn't make pigs or people sick, but it can cause birth defects when a woman becomes infected during pregnancy. It can also be a serious problem in AIDS patients and others with a suppressed immune system. In the U.S., about 10 percent of young adults have T. gondii cysts in their tissues, increasing to 40 percent of older people. Improperly cooked meat appears to be an important source of infection as are cat feces. Although only about 2 percent of market pigs carry the parasite, the lifetime chance of ingesting a contaminated serving is high. But the parasite can be easily killed by freezing meat or by cooking it thoroughly.

Lunney and colleagues have found genetic differences among pigs in the ability to prevent T. gondii from getting a toehold. With further research, they expect to identify pig genes responsible for T. gondii resistance. This information will help swine breeders select more disease-resistant stock.

They have so far cloned pig genes for seven cytokines--substances that enable immune cells to talk to one another. They suspect cytokines play a big role in resistance to T. gondii and many other diseases because they coordinate the body's immune response to infection.

Scientific contact: Joan K. Lunney, Immunology and Disease Resistance Laboratory, Beltsville, MD 20705, phone (301) 504-8201, fax (301) 504-5306,