Tests have found that three-quarters of British-grown oysters contain norovirus, BBC News has reported. Norovirus, also known as “winter vomiting bug”, is thought to affect up to 1 million people each year in the UK.
The news is based on a two-year examination of UK oyster production sites by the Food Standards Agency (FSA), including the first large-scale analysis of the levels of norovirus they contain. However, although testing showed that most samples contained norovirus, the results have not revealed any new risk from eating oysters, and the FSA’s advice remains unchanged: people should be aware that eating raw oysters carries a risk of food poisoning. The agency also continues to advise that certain groups – such as older people, pregnant women, very young children and people who are in poor health – should avoid eating oysters and other raw shellfish.
The test used to detect the virus was very sensitive and most of the positive samples contained very low levels of the virus. The FSA says that its test cannot distinguish between infectious and non-infectious types of norovirus, and it is not known how much norovirus a person would need to consume before it made them ill.
What is norovirus?
Norovirus is the most common non-bacterial cause of diarrhoea and vomiting in the UK. It is usually caught through person-to-person transmission, but some cases are caused by eating contaminated food. Norovirus is highly contagious. It is estimated that between 600,000 and 1 million people in the UK catch norovirus each year. It is sometimes known as the winter vomiting bug because the illness is more common in the winter months. However, it is possible to catch it throughout the year.
Should I avoid eating oysters?
This news is not based on any outbreak of norovirus related to shellfish consumption or any increase in the levels of norovirus historically found in oysters. Instead, it is based on the first systematic examination of norovirus levels in UK oyster production areas. There have been no events that suggest the norovirus risk from oysters has increased.
Indeed, the FSA’s guidance on eating oysters remains unchanged in light of the results. It recommends that people should be aware of the risks of eating raw oysters. The agency advises that certain people should avoid eating oysters or other raw or lightly cooked shellfish. These include older people, pregnant women, very young children and people who are in poor health.
The current report will contribute to ongoing research that will need to determine a safe level of norovirus in oysters.
Why might oysters contain norovirus?
Oysters feed by filtering large volumes of water to get their food. This means that any bacteria and viruses in the water can build up within the oyster. However, methods are employed to lower the chance of oysters being contaminated. If oysters are known to be taken from an area at risk of pollution, they are purified by a process called “re-laying”, in which they are moved to clean areas of the ocean or into tanks. Over a couple of months, the oysters will clear harmful bacteria through their natural feeding process, although this process is not so good for protecting against viruses.
Commercial oyster and shellfish harvesters also use a process called depuration. The shellfish are placed in tanks of clean recirculating seawater and are treated with UV irradiation. The oysters purge their contaminants over several days. This process is regulated by the FSA, and a purification time of at least 42 hours is required in the UK.
Are there any other health risks from eating oysters?
As described above, because of the way oysters feed, they are a potential source of various bacteria and viruses, some of which may be harmful and lead to food poisoning. Shellfish may also cause an allergic reaction in some people, and certain groups of people are advised not to eat raw shellfish. For most people, however, eating oysters may be a rare indulgence, with no major health risks.
How can I avoid catching norovirus?
Most norovirus cases are caught through person-to-person transmission, although food poisoning can be a source of infection.
To lower the risk of both catching norovirus and spreading it to other people, wash your hands frequently and thoroughly with soap and water, especially after going to the toilet and before making food. If toilet areas or surfaces have been contaminated, it is best to use a bleach-based household cleaner. This is because, unlike some other viruses or bacteria, norovirus is not as susceptible to alcohols and detergents. Because of this, hand washing is better than using alcohol hand gels to avoid acquiring or spreading an infection.
It is also important to make sure that raw food is washed. Before the current report, oysters were already known to be a source of norovirus, so people are advised to only eat oysters from a reliable source.
What should I do if I think I have norovirus?
The main symptoms of norovirus usually appear one to two days after you become infected. These symptoms may include suddenly feeling sick, followed by vomiting and diarrhoea. Some people may also have a raised temperature, headache, stomach cramps and aching limbs. The illness is not generally dangerous but there is a risk of dehydration. Therefore, it is important to drink lots of fluid. Children and elderly people have a greater risk of becoming dehydrated. There are no known long-term effects of norovirus.
There are no specific treatments for norovirus and it is best to let the virus run its course (which takes a couple of days). Paracetamol may help with aches, pains and fever. You are advised to drink plenty of fluids and try to eat easily digestible food. Babies should continue their normal feeds.
If you have norovirus, you are advised to avoid direct contact with other people and not to prepare food for other people for at least two days after the vomiting has stopped, as you may still be contagious in this period. This means that it is best to stay at home for two days after the symptoms have gone. It is not usually necessary to see a doctor. As people who are in poorer health may be at greater risk, you are advised to avoid visiting hospitals while in this contagious period.
You are also advised not to share towels and flannels as this is a possible route of transmission. Contaminated bedding should be washed separately on a hot wash and contaminated surfaces should be cleaned with a bleach-based cleaner.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
BBC News, 29 November 2011
The Daily Telegraph, 29 November 2011
Links to the science
(PDF, 1.7MB). Centre for Environment, Fisheries & Aquaculture Science. 2011.
Food Standards Agency. November 29 2011.