Infographic: Norovirus

Sometimes referred to as the winter vomiting bug, norovirus, is the most common stomach bug in the United Kingdom (UK) infecting between 600,000 and one million people every year. The virus gets its name from where it was first identified — in Norwalk, Ohio, in 1972.

Since then, several strains of the virus have been identified — all which are able to survive a wide range of temperatures and in many different environments. This makes norovirus more difficult to avoid as proper precautions must be followed in food service environments.  




While it can affect people of all ages it is most commonly contracted by the elderly, those with weakened immune systems, and pregnant women. Known for its symptoms of vomiting and diarrhea, norovirus is extremely contagious and spreads through contaminated food or water, or by touching contaminated surfaces.

Most norovirus outbreaks occur in the food service settings like restaurants when infected food workers touch ready-to-eat foods, such as raw fruits and vegetables, with their bare hands before serving them. However, any food served raw or handled after being cooked can be contaminated with norovirus.

Raw oysters for example are a very common source of norovirus and in a recent study scientists discovered that 76% of oysters tested from UK oyster-growing beds had traces of norovirus. Scientists believe this is because oysters filter large volumes of water to get their food, and any bacteria and viruses in the water can build up inside their shells. While they are subject to purification controls before and after harvesting, oftentimes this process is not completed accurately or is simply just not enough.

This research on raw oysters and norovirus is part of continuing work by the Food Standards Agency (FSA), and will feed into a study being carried out by the European Food Safety Agency in order to advise the European commission on what a legal safe level for norovirus in oysters should be.

As stated in article about the research, scientists from the Centre for Environment, Fisheries and Aquaculture Science (Cefas) took samples from 39 oyster-harvesting areas across the UK over two years. They tested 844 samples in total for norovirus DNA and found that every harvesting area had at least one norovirus-positive oyster. Of the 643 samples that tested positive, 335 had low levels of the virus (less than 100 virus copies per gram). The other 308 samples had medium or high levels of the virus. Nine samples registered virus levels of over 10,000 virus copies per gram.

Based on the study’s findings, Andrew Wadge, chief scientist at the FSA, said “Although oysters are traditionally eaten raw, people should be aware of the risks involved in eating them in this way. 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.”

Despite this, demand for British-grown oysters has soared in the last couple of years, driven by issues around sustainability and ethical sourcing and after serious shortages in France. That being said, approximately 80% of norovirus outbreaks occur between November and April, hence the virus’s nickname. During this time Neogen carries out testing of samples submitted to their laboratory twice weekly and deliver on time results more than 95% of the time.

For more information on how Neogen can assist in your food safety program, click here.

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