Raw Oysters Can Harbor Intestinal ParasiteBy
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
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
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'
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
Scientific contact: Ronald Fayer, ARS
Disease Resistance Laboratory, Beltsville, MD 20705, phone (301) 504-8750,
fax (301) 504-5306, email@example.com.