Norwalk-like Viruses

Norwalk-like viruses are recognized as the most important cause of non-bacterial outbreaks of gastroenteritis worldwide. Although it is a serious problem in many countries, this food borne hazard could be eliminated and prevented.

What are Norwalk-like viruses?
Norwalk-like viruses are known as Small Round Structured Viruses (SRSVs). Gastroenteritis caused by Norwalk-like viruses is usually mild and self-limiting. Symptoms may include nausea, vomiting, non-bloody diarrhea, and abdominal cramps. According to the literature, Norwalk-like viruses are common pathogens of viral gastroenteritis, and Norwalk-like viral gastroenteritis are more common in winter.

How do Norwalk-like viruses spread?
The only known host for Norwalk-like viruses is infected human. It can be spread by fecal-oral route via contaminated food and water. Other major modes of transmission include person-to-person spread, contact with contaminated object and aerosol spread.

What foods are commonly identified as source of outbreaks?
As Norwalk-like viruses are commonly found in sewage-contaminated water, the shellfish harvested from polluted water or vegetables irrigated with polluted water are highly likely to be contaminated. Therefore, ingestion of raw or inadequately cooked foods, including shellfish, salad and raw vegetables, poses a high risk of Norwalk-like viruses infection. In Hong Kong, raw oysters have been implicated in food poisoning outbreaks associated with Norwalk-like viruses. In other countries, ice, salad, raw vegetables and raw shellfish have also been identified as the vehicles of food poisoning outbreaks caused by Norwalk-like viruses. Anyone who consumes food contaminated with Norwalk-like viruses is susceptible to the infection.

How can Norwalk-like viruses be prevented?
The Key to prevention of Norwalk-like viruses infection is strict observance of food, personal and environmental hygiene.

Advice to the public

• The public should patronize licensed and reputable food premises, especially when taking high-risk food like raw oysters.
  
• All food, in particular shellfish, should be cooked thoroughly before consumption.
  
• If the vegetables are taken fresh of for salad preparation, they must be thoroughly washed, and kept well-covered in refrigerators at 4°C.
  
• Hands should be washed thoroughly with plenty of soap and hot water after using toilet and before handling food or eating.
  
• Travellers visiting places appeared to have polluted water are advised to drink pasteurized milk or bottled beverages without ice, and take thoroughly cooked foods while serving hot.
  
• Food handlers with symptoms of vomiting or diarrhea should not handle food until they are symptom-free for at least two days.
  

Source of references: Food and Environmental Hygiene Department
Enquiry Hotline: 2868 0000
Email: enquiries@fehd.gov.hk
Website: www.fehd.gov.hk
CT/No. 35 (Dec, 2003)