New research identifies norovirus levels in oysters

Last updated:
29 November 2011
Research published today by the Food Standards Agency shows that a significant proportion (76%) of oysters tested from UK oyster growing beds contained norovirus. The virus was detected at low levels in more than half of the positive samples (52%).

It is difficult to assess the potential health impact of these findings, as the available research techniques are not able to differentiate between infectious and non-infectious norovirus material within the oysters. Furthermore, a safe limit for norovirus has not been established.

Between 2009 and 2011, scientists from the Centre for Environment, Fisheries and Aquaculture Science (Cefas), on behalf of the Food Standards Agency, took samples from 39 oyster harvesting areas across the UK. More than 800 samples of 10 oysters each were tested.

The research will contribute to a European Food Safety Authority review of norovirus levels in oysters, which will advise the European Commission on setting a specific legal safe level for norovirus in oysters placed on sale in the EU. The Agency and the shellfish industry are also continuing to work together to develop controls for norovirus.

Andrew Wadge, Chief Scientist at the Food Standards Agency, said: ‘This research is the first of its kind in the UK. It will be important to help improve the knowledge of the levels of norovirus found in shellfish at production sites. The results, along with data from other research, will help us work with producers to find ways to reduce the levels of norovirus in shellfish, and work within Europe to establish safe levels.

‘Though oysters are traditionally eaten raw, people should be aware of the risks involved in eating them in this way,’ he explains. ‘The Agency advises that older people, pregnant women, very young children and people who are unwell should avoid eating raw or lightly cooked shellfish to reduce their risk of getting food poisoning.’

David Lees, the lead investigator at Cefas, said: ‘We were fortunate to have excellent cooperation from the oyster producers and from local authority officers in conducting this study. Norovirus is a recognised problem for the sector, and this study provides important baseline data to help the industry and regulators to focus on the key risks.’

Science behind the story

Oysters filter large volumes of water to get their food, and any bacteria and viruses in the water can build up within the oyster. Controls before and after commercial harvesting of oysters, such as re-laying and depuration, provide good protection against harmful bacteria, but are less effective at removing viruses from live shellfish.

Re-laying is a purification process used to treat bivalve shellfish. Shellfish are harvested from a contaminated area and moved to clean areas, where they are placed on the ocean floor or into containers laid on the ocean floor, or suspended in racks. Re-laying will generally be for at least two months.

Depuration is a purification process used commercially and regulated by the Food Standards Agency. It is commonly used by producers to reduce or eliminate microbiological contamination in oysters and other shellfish. Shellfish are placed in tanks of clean re-circulating seawater, treated by UV irradiation, and allowed to purge their contaminants over several days. In the UK a minimum purification time of 42 hours is required.

Most norovirus infections are thought to spread from person-to-person, although contaminated food is still thought to account for a proportion of cases. It can be carried on different types of food, not just shellfish.

Norovirus is the most common viral cause of diarrhoea and vomiting in the UK, according to recent Agency research (the IID2 study). It is highly infectious, but the illness is generally mild and people usually recover fully within two to three days. There are no known long term effects.

Developing ways of reducing foodborne norovirus infection is a key priority in the Food Standards Agency’s Foodborne Disease Strategy for 2010-15. Further information on the IID2 study and the Agency’s Foodborne Disease Strategy are available at the links below.

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