Norovirus outbreak tied to Texas oysters

Mar 5, 2007 (CIDRAP News) – Following 25 cases of norovirus infection in people who ate raw oysters from San Antonio Bay in Texas, the US Food and Drug Administration (FDA) has warned against eating raw oysters taken from the bay after Feb 1.

The patients ate raw oysters at a Maryland event the weekend of Feb 9 to 11, the FDA said in a Mar 2 news release. Patients tested by the Maryland Department of Health and Mental Hygiene were found to have the virus.

The implicated oyster beds were closed by the Texas Department of Health Services Feb 24 and remain closed, officials said.

Two seafood distributors—Bayview Seafood, Seadrift, Tex., and Rose Bay Oyster Co., Swanquarter, N.C.—issued recalls in late February as a result of the outbreak, the FDA reported. Rose Bay mislabeled oysters from San Antonio Bay, indicating they were from Galveston Bay instead. Osyters with tags that read "Gal 1, 2/2/07" are subject to recall.

People who ate oyster products served in restaurants after Feb 1 and had symptoms suggesting norovirus are encouraged to contact their healthcare provider and local health department, the FDA said. Symptoms typically include nausea, vomiting, diarrhea, and stomach cramps; other possible symptoms are low-grade fever, chills, headache, muscle aches, and fatigue.

Norovirus contamination in oysters is not unusual, according to Craig Hedberg, PhD, a foodborne disease expert and associate professor of environmental and occupational health at the University of Minnesota School of Public Health in Minneapolis.

"Oysters are filter feeders and do concentrate things in water," which can include noroviruses from sewage that has been discharged into the ocean, Hedberg told CIDRAP News.

"There are a number of people who have tried to decontaminate oysters by flushing them with fresh water, but norovirus will bind to oyster tissues and won't be flushed out," he said.

The FDA said people with weakened immune systems and those who have liver, stomach, or blood disorders; cancer, diabetes, or kidney disease; or a history of chronic alcohol abuse should never eat raw oysters, regardless of where they come from. Thorough cooking destroys noroviruses and other pathogens.

See also:

Mar 2 FDA news release with guidance on safe cooking of oysters
http://www.fda.gov/bbs/topics/NEWS/2007/NEW01579.html

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