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United States Department of Agriculture

Agricultural Research Service

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Raw Oysters Can Harbor Intestinal Parasite

By Judy McBride
March 10, 1998

A single-cell parasite joins the ranks of human pathogens harbored by oysters, according to a study reported in the March issue of Applied and Environmental Microbiology.

Oocysts--encased eggs--of the parasite Cryptosporidium parvum were found in oysters from six rivers feeding the Chesapeake Bay. The study--the first to look for C. parvum in shellfish-- was conducted by zoologist Ronald Fayer with USDA's Agricultural Research Service and colleagues with Johns Hopkins University and the National Oceanic and Atmospheric Administration.

The researchers also demonstrated that some of the oocysts would develop in mice, indicating they pose a potential risk to humans who eat raw oysters. There have not been any human outbreaks attributed to oysters. Fayer presented the findings yesterday at the International Conference on Emerging Infectious Diseases in Atlanta, Ga.

C. parvum are protozoan parasites in waterways worldwide. When ingested, they can infect gastrointestinal cells, where they evoke cramping, diarrhea and sometimes nausea and vomiting four to 10 days later. Because of the long incubation period, C. parvum is often not connected with the flu-like symptoms. Symptoms range from mild to severe in healthy people and can lead to chronic diarrhea, dehydration and death in people who have a weak immune system.

The oocysts don't survive temperatures above 164 degrees Fahrenheit, so boiling or frying shellfish would prevent infection. But they do survive chlorine quite well. In 1993, more than 400,000 Milwaukee residents suffered C. parvum infections from contaminated drinking water. Smaller outbreaks have occurred around the country. An expert on the parasite, Fayer developed a video to train water treatment personnel on how to prevent transmission. He conducts studies at ARS' Immunology and Disease Resistance Laboratory, Beltsville, Md.

Suspecting that shellfish might filter the parasite from contaminated waters, Fayer and colleagues sampled oyster beds at the mouth of six Chesapeake tributaries and found oocysts in oysters from each site. Subsequently, they found oocysts in nearly all oysters sampled from commercial beds at five other locations in the Chesapeake--with some oysters having more than 4,000 oocysts.

C. parvum can infect all mammals. Feces from humans or domestic and wild mammals can contaminate waterways. The researchers earlier showed that geese can transport oocysts through feces and contribute to their spread into waterways.

Scientific contact: Ronald Fayer, ARS Immunology and Disease Resistance Laboratory, Beltsville, MD 20705, phone (301) 504-8750, fax (301) 504-5306,

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