The consumption of raw bivalve molluscan shellfish (e.g. oysters, clams, and mussels) is a risk factor for contracting acute gastroenteritis particularly caused by norovirus.
Norovirus is highly infectious and, although mainly spread from person to person, can be caused by the consumption of contaminated food. Food can be contaminated at source; sewage entering rivers and estuaries can cause norovirus contamination of shellfish beds. Molluscan shellfish feed by filtering water through their gills and as a result may concentrate norovirus to much higher levels than the surrounding water. Norovirus is extremely difficult to remove from contaminated shellfish*. Food handlers may also contaminate food if they are working while they are still infectious±. Molluscs, such as oysters, are often eaten raw creating the potential for norovirus infection because any contamination, either from source or through poor food handling, is not removed by cooking.
Norovirus infections are seasonal; the number of laboratory confirmed isolates and reported outbreaks of norovirus infection increases markedly during the winter months. Studies of the detection of norovirus in oysters also show higher rates of detection during the months of October to March.
Between January and March 2010, several outbreaks linked to the consumption of raw oysters were reported across Europe (including the UK), which affected over 300 people (120 of which were in the U.K). Between 1992 and 2009, 94 outbreaks of food poisoning associated with the consumption of oysters were reported to the HPA. Forty-three of these reported outbreaks were laboratory confirmed as due to norovirus and a further 29 were suspected to be due to norovirus – but not laboratory proven. The number of outbreaks reported is likely to be an underestimate of the true number that occurred during this period.
People should be aware of the potential risks of norovirus associated with the consumption of raw shellfish such as oysters in order to make an informed choice about eating them.
*Current purification standard procedures (depuration) is insufficient to remove norovirus from contaminated shellfish. The Food Standards Agency and the UK shellfish industry are presently looking at finding ways of improving the depuration process to minimise the risk of contaminated shellfish reaching consumers.
±Food handlers should remain away from work until at least 48 hours after the last episode of diarrhoea or vomiting. The Food Standards Agency provides information and guidance for the food industry this can be found at:
Last reviewed: 27 June 2011