Proving again that risk comparisons are risky and that maybe raw oysters are as dangerous as Cheetos, Eurosurveillance today reports on the fourth norovirus-related outbreak linked to raw oysters in recent memory – and there’s a bunch of them.

Since January 2010, 334 cases in 65 clusters were reported from five European countries: the United Kingdom, Norway, France, Sweden and Denmark. The article describes the available epidemiological and microbiological evidence of these outbreaks.

?Oysters are grown in coastal waters of several countries and are considered a delicacy in most parts of the world. Like all bivalve molluscs, they feed by filtering large amounts of water through their gills. In situ studies with bioaccumulation of a virus indicator in oysters have shown that oysters can concentrate viruses up to 99 times compared to the surrounding water [1]. In water contaminated with norovirus, this leads to the accumulation of the virus within the flesh and gut of the oyster.

Norovirus has been detected in 5 to 55% of oysters from Europe and the United States (US) by random sampling at market places and oyster farms [2-4]. The detection of norovirus in oysters follows the same seasonal trend as the norovirus epidemiology in the general population, i.e. norovirus in oysters is generally detected between October and February [1, 12].

Seventy-eight percent of shellfish-related illness from noroviruses in the US between 1991 and 1998 were associated with the consumption of oysters harvested between the months of November and January [1]. Contamination of oyster beds with noroviruses can occur after heavy rains cause flooding, which results in combined sewer overflow or hydraulic overload in sewage treatment plants [5, 13]. There are also examples of oyster harvesters disposing sewage into oyster-bed waters causing multi-state outbreaks of norovirus in the US [6]. Noroviruses are difficult to remove from oysters through cleansing and also stay infectious [7]. Oysters are often eaten raw, creating the potential for foodborne enteric virus infections.

From January to March 2010, the European Centre for Disease Prevention and Control (ECDC) was informed through its Food- and Waterborne Diseases and Zoonoses (FWD) surveillance network about norovirus outbreaks linked to consumption of oysters in five EU/EEA countries: the United Kingdom (UK), Norway, France, Sweden and Denmark. In total 65 small clusters involving 334 cases were reported. Most cases had eaten oysters in restaurants. …

??In conclusion, an increased number of norovirus outbreaks related to the consumption of oysters have been observed at EU level in the last three months. … consuming raw oysters involves potential exposure to norovirus and is particularly hazardous for immunocompromised or chronically ill persons. Therefore, countries might consider informing the public about the risks linked with consuming raw oysters.

Or Cheetos.

As James Wesson, oyster scientist with the Virginia Marine Resource Commission, told the Daily Press the other day,

"More people die each year from eating Cheetos than from eating oysters.”

No data was provided.