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Researchers Track Parasite Contamination

By Rosalie Marion Bliss
March 4, 2002

Scientists with the Agricultural Research Service and the Centers for Disease Control and Prevention (CDC) have been awarded a grant to study sources of exposure to an insidious parasite. The study will provide the first risk assessment of the likelihood of exposure to the parasite Toxoplasma gondii by ingesting raw or under-cooked meat.

T. gondii is a single-cell parasite that infects about 23 percent of the U.S. population. The annual cost from both acute illnesses and complications due to toxoplasmosis is between $3.3 billion and $7.8 billion, according to the latest estimates from the U.S. Department of Agriculture’s Economic Research Service.

Parasitologist Jitender P. Dubey and epidemiologist Delores E. Hill of ARS’ Parasite Biology, Epidemiology and Systematics Laboratory, Beltsville, Md., and parasitologist H. Ray Gamble with the National Academy of Sciences are preparing to test 6,000 meat samples. The three-year study will look at beef, chicken and pork.

Collaborators at the CDC now are in the process of selecting 28 major U.S. metropolitan areas from which to take meat samples. According to the CDC’s Jeffrey L. Jones, the tests will provide a sense of how much T. gondii there may be in meat and how that level differs throughout various regions in the country. The $853,000 grant is from USDA’s National Research Initiative.

Cats are the only animal in which T. gondii completes its sexual phase. A rodent- or bird- eating cat that becomes host to T. gondii transmits millions of infectious-stage oocysts in its droppings for a week. These hardy oocysts survive freezing temperatures in soil and sand. If the oocysts come into contact with farm feed, they can infect farm animals during feeding. When deposited in litter and sand boxes, the oocysts can infect humans.

T. gondii’s favorite hiding place in humans is inside brain and muscle tissue. Although healthy people other than pregnant women can weather T. gondii with few symptoms, it poses a risk to developing fetuses and to persons with depressed immune systems.

ARS is the USDA’s principal scientific research agency.

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