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Bacteriologically Safe Oysters? Beware Vibrio!

Vibrio parahaemolyticus (VP) typically cause an inflammatory diarrheal syndrome in people who eat undercooked shellfish. Accordingly, U.S. FDA regulations mandate bacteriological monitoring of shellfish; a concentration of fewer than 10,000 VP organisms per gram of meat is considered nonpathogenic. A widespread outbreak of VP gastroenteritis, caused by ingestion of contaminated raw oysters harvested from Galveston Bay in Texas, now calls these monitoring standards into question.

Gastroenteritis following oyster consumption was reported by 416 people in 13 states from May 31 through July 10, 1998. Epidemiological and environmental traceback studies revealed that approximately 200,000 persons may have become ill during this outbreak, even though oyster harvests met bacteriologic monitoring criteria and fecal coliform levels of water samples were acceptable. Twenty-eight VP isolates were molecularly subtyped as VP serotype O3:K6, a strain previously unreported in the U.S., but recently recognized in Asia. During the outbreak, median water temperature and salinity in the Gulf of Mexico were significantly higher than they had been during the same period during the previous 5 years.

Comment: U.S. food safety standards apparently will not prevent all diarrheal illnesses caused by ingestion of raw or undercooked shellfish contaminated with some Vibrio species (such as VP serotype O3:K6) or enteric viruses (for example, Norwalk-like viruses). Avoiding raw or undercooked shellfish can prevent these illnesses. This prevention strategy is particularly important for immunocompromised patients and those with liver disease, who are vulnerable to severe bacteremic vibrio infections.

— N Blacklow

Published in Journal Watch Infectious Diseases October 19, 2000

Citation(s):

Daniels NA et al. Emergence of a new Vibrio parahaemolyticus serotype in raw oysters: A prevention quandary. JAMA 2000 Sep 27 284 1541 -1545.

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