Introduction

1. Norovirus is recognised as the most important cause of non-bacterial outbreaks of gastroenteritis. Gastroenteritis caused by norovirus is usually mild and self-limiting. Symptoms may include nausea, vomiting, non-bloody diarrhoea and abdominal cramps.
2. Norovirus can be transmitted by ingestion of contaminated food and water, person-to-person, contact with contaminated objects and by aerosolised vomitus. Overseas research showed that about half of the norovirus infection was foodborne. Worldwide, shellfish especially raw oysters, raw vegetable, salad and ice have been implicated as common sources of foodborne norovirus infection. Locally, raw oyster was identified as the most commonly incriminated food for foodborne norovirus outbreaks.

Public Health Significance

1. An increase in cases of norovirus infection has been reported, both in Hong Kong and many overseas countries, over recent years. There is usually a peak of norovirus infection during winter months and outbreaks at this time of year are more common.
2. Norovirus appears to be infectious in very low dose. Only about 10-100 particles are needed to cause disease and no long term immunity can be developed after infection. Attack rates in outbreaks tend to be high, often exceeding 50% among at-risk groups (i.e. young children, elderly people, pregnant women and others with weakened immune systems) and may reach 90% in seafood-associated outbreaks in these groups.
3. Norovirus is highly infectious and places that have dense population and mass food production, such as nursing homes, hospitals, hotels, as well as cruise ships are places of higher risk of spreading the virus.

Advice to the public

1. Young children, elderly people, pregnant women and others with weakened immune systems or consumers who wish to reduce their risk of norovirus infection should avoid eating foods that are consumed without heat treatment (e.g. shellfish, especially oyster, to be consumed raw) or foods containing ingredients that are not cooked (e.g. sandwiches and salads).
2. All foods, in particular shellfish, should be cooked thoroughly before consumption.
3. Wash hands thoroughly with soap and water after using the toilet, before handling food or before eating.

Advice to the trade

1. All foods, especially shellfish, should be purchased from reliable suppliers.
2. Food handlers with symptoms of vomiting or diarrhoea should not handle food until they are symptom-free for at least two days, and should seek medical advice.

Risk Assessment Section
Centre for Food Safety
June 2007