On Nov. 4, 2011 the U.S. Food and Drug Administration announced a recall of ASSI branch frozen oysters from South Korea, which has been linked to an outbreak of norovirus in the Pacific Northwest.

Now the Centers for Disease Control and Prevention (CDC) reveals the story behind that recall in the February 17 edition of its Morbidity and Mortality Weekly Report (MMWR). It’s a cautionary tale about a batch of contaminated oysters that could have made many more people sick had public health officials not been able to crack the case quickly.

On Oct. 19, 2011, the CDC relates, a woman in Washington state contacted Public Health – Seattle & King County and said she began suffering from acute gastroenteritis after dining at a local restaurant.

Staff members interviewed the woman and her companions and learned that three of the seven in the dinner party had eaten a raw oyster dish, and from 18 to 36 hours later experienced aches, nausea and diarrhea. One of the three also reported vomiting.

The four diners who did not eat the raw oysters did not become ill.

A stool specimen from one of the ill diners, collected 17 days after onset of symptoms, tested positive for norovirus.

Health department investigators collected eight 3-lb. bags of frozen raw oysters in the restaurant’s walk-in freezer and sent them off to the Food and Drug Administration’s Gulf Coast Seafood Laboratory for testing. They were positive for norovirus.

The frozen oysters, which had been distributed in seven states, had a two-year shelf life and could have made many other people ill — freezing does not kill norovirus — had they not been removed from circulation.

“Such contamination has potential for exposing persons widely dispersed in space and time, making cases difficult to identify or link through traditional complaint-based surveillance,” the CDC authors wrote in the MMWR.

The CDC noted that it has recently launched a national electronic norovirus outbreak surveillance network, called CaliciNet, and recommends the collection of stool specimens to confirm a diagnosis, characterize the norovirus strain and upload the sequence results into CaliciNet.

“Additionally, all suspected and confirmed norovirus outbreaks should be reported to CDC by state and local health departments through the National Outbreak Reporting System,” the authors advise.

Noroviruses are the most common cause of epidemic gastroenteritis, responsible for at least half  of all gastroenteritis outbreaks worldwide, and a major cause of foodborne illness. In the United States, norovirus is estimated to cause about 21 million illnesses each year.