Norovirus: Not Just a Nursing Home Nemesis

March 22, 2009

We were reminded this past week that Norovirus, which is so often associated with nursing home and hospital outbreaks of gastroenteritis, also can be transmitted in raw seafood.

The Virginia Department of Health issued a recall notice yesterday for raw oysters harvested from the waters along a section of Mississippi's Gulf Coast. Those oysters had been linked to a recent outbreak of Norovirus gastroenteritis among eleven patrons of a Chattanooga, Tennessee restaurant.

FDA has warned consumers, retailers and food service operators to avoid selling or consuming "...oysters harvested between Feb. 24 and March 17, 2009, from Mississippi Area 2C, located in the Mississippi Sound portion of the Gulf of Mexico near Pass Christian, Miss." FDA is working with Mississippi to determine the origin of the contamination. The state has closed the affected area to further harvesting until the source of the Norovirus has been found and the problem corrected.

Last month, Alaska also traced an outbreak of Norovirus to the consumption of raw oysters. The 10 victims had attended a Sitka Superbowl party where raw oysters were served. Norovirus was recovered from the victims and from the same batch of raw oysters that was served at the party.

Norovirus also is suspected of being behind the recent outbreak of gastroenteritis that resulted in the temporary closure of The Fat Duck, a three-star (Michelin Guide) restaurant located in Bray, Berkshire, UK. According to recent news reports, three members of the restaurant's staff have tested positive for Norovirus.

Contracting Norovirus gastroenteritis as a result of consuming raw oysters is not rare. In the 1990s, the virus was recognized as the most common single cause of shellfish-associated gastroenteritis. A 1998 Norovirus outbreak in California that affected 171 individuals was traced to contaminated oysters.

Nor is this source of Norovirus limited to US coastal waters. In recent years, Norovirus-contaminated raw oysters have been the source of gastroenteritis outbreaks in Canada, Australia and France. 

Why are oysters such a common vehicle for transmitting Norovirus? Oyster beds, whether wild or farmed, are located in coastal waters – those same waters that are susceptible to runoff contamination from shore. Oysters filter their food from water, concentrating both their nutrients and any biological or chemical contaminants that may be in the water. Including Norovirus.

It takes no more than a few Norovirus particles to trigger a case of gastroenteritis. The next time you sit down at an oyster bar, think about where those oysters may have been and what they might contain. 

Then think again.