Raw oyster risk: norovirus infections associated with frozen raw oysters
Posted: March 31st, 2012 - 8:17am by Doug Powell
Oysters from Australian waters are a delight on the grill, although I’ve graduated to scallops on the half-shell, also grilled.
But whenever I go see Paul the fish monger, he’s offering me a sample of his wares – raw – and I politely decline.
Or, as Dr. Ken Buckle, professor emeritus at the University of New South Wales commented when our hosts took us to a seafood buffet in Abu Dhabi, I spent too much time researching pathogens in raw fish.
He chose the cooked kind.
The U.S. Centers for Disease Control recently described a norovirus outbreak in frozen raw oysters in Seattle imported from South Korea.
On October 19, 2011, Public Health – Seattle & King County was contacted regarding a woman who had experienced acute gastroenteritis after dining at a local restaurant with friends. Staff members interviewed the diners and confirmed that three of the seven in the party had consumed a raw oyster dish.
Within 18–36 hours after consumption, the three had onsets of aches, nausea, and nonbloody diarrhea lasting 24–48 hours. One ill diner also reported vomiting. The four diners who had not eaten the raw oysters did not become ill.
An inspection of a walk-in freezer at the restaurant revealed eight 3-pound bags of frozen raw oysters, which the restaurant indicated had been an ingredient of the dish consumed by the ill diners. The oysters had been imported from South Korea by company A and shipped to a local vendor, which sold them to the restaurant. All eight bags were sent to the Food and Drug Administration's Gulf Coast Seafood Laboratory for norovirus testing and characterization by real-time reverse transcription–polymerase chain reaction (rRT-PCR).
A stool specimen from one of two ill diners collected 17 days after symptom onset tested positive for norovirus; sequence analysis identified GI.1 and GII.17 strains. Sequence analysis of the oysters identified a GII.3 strain. Because oysters can harbor multiple norovirus strains that are unequally amplified by rRT-PCR, discordance between stool specimens and food samples in shellfish-associated norovirus outbreaks is common and does not rule out an association. On November 4, 2011, company A recalled its frozen raw oysters.
The frozen oysters implicated in this outbreak were distributed internationally and had a 2-year shelf-life. Contamination of similar products has been implicated previously in international norovirus transmissions. 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.