So far, norovirus (formerly known as Norwalk-like virus) is the primary pathogen responsible for non-bacterial acute gastroenteritis and a common cause of food poisoning. Norovirus infection can happen all year around but tends to be more common in winter. The virus is highly contagious, of high survivability and able to survive between 0^oC and 60 ^oC. It may cause outbreak of acute gastroenteritis in crowded and closed places, like in schools and elderly homes.

Modes of transmission

Norovirus is usually transmitted by consumption of food or drinks contaminated with the virus, like contaminated shellfish (e.g. raw oysters), undercooked vegetables or salads. It can also be spread through direct person-to-person contact, direct contact with contaminated substances (e.g. vomit and faeces of infected persons) or viral aerosol spread with contaminated droplets during vomiting.

Health impacts

The incubation period of norovirus infection is usually 24 to 48 hours, after which symptoms begin to appear, like vomiting, non-bloody diarrhoea, abdominal pain, nausea and, occasionally, low-grade fever. The symptoms are mostly mild, self-limiting and the infected victims generally recover within 1 to 3 days, provided that they are given adequate fluids to prevent dehydration and with supportive treatment.

Advice to the trade－Cautious in procurement, reliable supply

Members of the trade should order food products from reliable sources and keep the receipts of their purchases and the sales records. For seafood (e.g. shellfish) which is intended to be eaten raw, make sure that it is suitable for human consumption. Food has to be kept under appropriate temperature and environment, properly handled and thoroughly cooked to reduce risk of contamination.

Advice to the public－How to prevent norovirus infection

Food and environmental hygiene

• Patronise reputable food establishments in good hygiene condition, especially when eating high risk food, like raw oysters;

• The shell of shellfish must be properly washed, cleaned and scrubbed. Discard immediately those with damaged shell;
• Raw and cooked food must be handled and stored separately to avoid cross-contamination;
• For vegetables to be eaten raw or used for salad preparation, they must be thoroughly washed and kept under 5^oC;
• Water must be boiled before drinking;
• All food, shellfish in particular, must be thoroughly cooked;
• Shellfish must be cooked until their shells open wide. Keep cooking them for another 5 to 10 minutes till they are thoroughly cooked;
• High risk individuals (e.g. young children, the elderly, pregnant women and people with weakened immunity) should avoid consumption of raw or undercooked shellfish and vegetables.

Personal hygiene

• Wash hands thoroughly with liquid soap and water after using the toilet, before handling or eating food;
• Never handle food when suffering from sore throat and gastrointestinal discomfort like diarrhoea and vomiting;
• Wear gloves and a mask while cleaning up the vomit of infected persons and use disinfectant (e.g. 1:49 diluted household bleach) to disinfect the contaminated areas and objects. Wash hands thoroughly with liquid soap and water once the cleaning is done.