First a few facts about norovirus and oysters

  • Recent Food Standards Agency research has shown that 75% of oysters carry Norovirus. See this item from the Food Standards Agency.
  • Food poisoning acquired from eating oysters infected with norovirus is still common.
  • In a recent Food Standards Agency funded study, more than 10 percent of all batches of oysters served in restaurants were associated with symptoms of norovirus food poisoning in the diners that consumed them.
  • There have been several high profile outbreaks of food poisoning recently where oysters were identified to be the primary source.
  • Not only can diners become infected by eating contaminated oysters, but norovirus infected juices dripping from unopened oysters can contaminate other food stuffs and staff handling the oysters.

How oysters become contaminated with norovirus.

Oysters feed by filtering phytoplankton (natural microscopic plant cells) from the seawater they live in. They also filter bacteria and viruses if they are present in the seawater (which they usually are).
Norovirus particles are found in sewage and oysters acquire norovirus contamination by filter feeding in seawater contaminated by sewage.

Food safety controls on oyster production.

Because food poisoning acquired from eating oysters contaminated with sewage has been a problem for many years, National and EU legislation has developed to attempt to control it.

Two methods are used to control food safety in oysters:

1.  Oyster harvesting areas are classified by the Food Standards Agency according to their recent historical exposure to sewage contamination based on measuring E. coli in samples of oyster flesh.

Only areas with relatively low exposure to sewage contamination (Class A) can be sold directly for human consumption. These are Class A oyster beds. Oysters for sale from these beds must not contain more than 230 E. coli per 100g of oyster flesh.

Oysters from areas where sampling reveals slightly more sewage contamination (Class B beds) have to be purified (depurated) in an approved system to remove the bacterial contamination (or be re-laid in a Class A area for 2 months). Oysters from areas with even more sewage contamination cannot be sold, except for cooking.

2.  Oysters from class B waters are put through a standardized purification process called depuration.  They are placed in clean recycled seawater for a minimum of 42 hours. These conditions have been shown to reliably reduce E. coli contamination to below 230 per 100g.

Why oysters are still a significant food poisoning risk.

Current legislation and thus Food Standards Agency effort is directed at reducing the level of bacterial contamination in oysters to relatively low levels.

But the problem is, E. coli is a poor indicator of norovirus contamination because there is very little relationship between norovirus and E. coli.

E. coli can be shed from oysters in a matter of hours whilst norovirus can take from a few days to several weeks or months to be shed if a heavy load is acquired in the cold winter months.

Because of this larger difference between bacteria and viruses, E. coli is not a good predictor of norovirus contamination.

As the recent Food Standards Agency research has shown, even oysters harvested from A grade beds can be contaminated with norovirus.

Oysters can pass all the tests required of them by current legislation but still be heavily contaminated with norovirus. This is why food poisoning acquired from eating oysters is still so common.


Norovirus (sometimes called Winter Vomiting Disease and formerly called Norwalk virus or Small Round Structured Virus SRSV) causes gastroenteritis (food poisoning) in humans.

When a person becomes infected with norovirus, symptoms, characteristically nausea, vomiting, abdominal cramps and diarrhea appear after 12 to 48 hours and can last a further 12 to 48 hours. Unlike gastroenteritis caused by bacteria (such as salmonella food poisoning caught from undercooked chicken), in the case of norovirus, severe illness and hospitalization is rare.

Transmission of norovirus is predominantly by the so-called fecal-oral route, from person to person and via contaminated water and food. It is highly contagious.

Many norovirus outbreaks have been traced to food that was handled by one infected person.

Eating uncooked food particularly salad, cold chicken and raw shellfish are the foods most often implicated in norovirus outbreaks.

Norovirus is thought to be responsible for more than 50% of all food borne outbreaks of gastroenteritis in the UK.

Large outbreaks of norovirus infection can occur in confined communities, such as hospitals and cruise ships, where the infection spreads rapidly by either person-to-person transmission or through contaminated food.

Some people are genetically less susceptible to norovirus than others.

Immunity to norovirus is short term and people can become reinfected only 6 weeks after a previous bout.

Transmission is prevented or limited by robust personal hygiene including thorough hand washing (not alcohol rubs as they are not very effective at dealing with Norovirus) and sanitizing of surfaces where the norovirus may be present.

Cooking rapidly inactivates Norovirus.  This is the reason hot cooked food does not present a significant risk of norovirus infection.  However, cooked food that has been allowed to become cold, and is then handled can become recontaminated with norovirus if the food handler is infected.