Noroviruses are very contagious and hardy viruses that cause severe stomach upsets (gastroenteritis). You may become infected after eating food that's been exposed to sewage in some way, such as oysters eaten raw which were taken from polluted water.

Note that in New Zealand, commercial shellfish are collected under regulations that prevent harvest from polluted water. As such domestically produced shellfish can be eaten raw. Recreationally gathered shellfish are not produced under this programme, and because it may be contaminated people are advised to take care when preparing it.

What are Noroviruses?

Noroviruses are a group of very contagious and hardy viruses that cause severe stomach upsets (gastroenteritis) in people. Initially someone may become infected with a Norovirus after eating food that’s been exposed to sewage in some way, such as oysters eaten raw that were taken from polluted water.

Norovirus is a relatively recent name given to these viruses - previously they were known as Norwalk-like viruses.

Why are Noroviruses a problem?

Although Noroviruses cannot grow outside the body, or in food itself, an infected person easily contaminates any food they come in contact with - as well as any other surfaces. Contaminated food can then infect several people at once.

Noroviruses are probably the most common cause of stomach bugs in New Zealand and outbreaks (mass illness of many people) are frequent where people live close together eg, rest homes, hostels, restaurants or cruise ships. In New Zealand most outbreaks occur in summer and autumn.

To date, the largest Norovirus outbreak in New Zealand was on 17 June 2006. It was linked to a catering firm who, contrary to cooking instructions, served raw, thawed, imported oysters to fans in a corporate box at a rugby match. An estimated 350 people fell ill with Norovirus infection soon after.

How are they spread?

As few as 10 Norovirus particles may be enough to infect a healthy person. An infected person sheds many Norovirus particles and can easily transmit the virus to other people through sharing their food, utensils, drinks or touching common surfaces.

What are the symptoms?

People get sick suddenly, usually within a day. They have diarrhoea, violent vomiting (often projectile) and stomach cramps. Some sufferers will also have a mild fever, chills, headache, muscle aches, tiredness and nausea. Keeping up fluid intake during this time is important because of the risk of dehydration. The illness only lasts a couple of days and does not usually cause any complications.

What kills Noroviruses?

Prevention is the best cure because there is no effective treatment. There are many different strains of Norovirus and no vaccine to help people develop immunity. Antibiotics do not kill viruses so they won’t help someone who is sick with a Norovirus.

Norovirus particles on food can survive freezing for several months, some even survive the pasteurisation (heat treatment) of milk and the low levels of chlorine used to sanitise drinking water.

Can I lower my risk of contracting a Norovirus?

Yes - you can avoid most routes of Norovirus exposure by following these steps:

  • Although any food can be contaminated by the virus, bivalve shellfish (such as those collected recreationally) are sometimes implicated because they are often eaten uncooked. Seawater contamination is monitored in most recreational shellfish collection areas, and warning signs are put up if the collection area is too polluted. Check for warning signs and don’t collect shellfish near sewage outlets
  • If possible, cook recreationally collected shellfish thoroughly before eating. Boil them for 3-5 minutes (3 minutes if they’re shelled) or steam for 4-9 minutes. Internal temperature must reach 90oC for 90 seconds to kill the virus particles, hence the recommended long cooking times
  • Wash all fruits and vegetables thoroughly, and cook food well, especially if someone in your household is sick.
  • Cook all meat until the juice (blood) runs clear. Make sure your meat is properly defrosted by thawing it covered, in the fridge for 24 hours
  • Keep drips of meat juice (blood) away from raw or already cooked food by covering the meat and storing it on the bottom shelf of the fridge
  • Don’t prepare food if you’re showing symptoms of a Norovirus infection and for at least 3 days afterwards. Wipe all surfaces including handles with bleach-based household cleaner (follow manufacturer’s instructions)
  • Most viruses and other pathogens are washed off your hands with proper washing. Follow the 20-20 rule. Lather your hands with soap then wash for 20 seconds with warm water, then dry your hands for 20 seconds with a dry, clean towel or paper towel
  • Make sure your own hygiene is meticulous if you are caring for someone who is sick. Keep the toilet area and all bathroom surfaces (toilet, taps, handles etc) really clean with household disinfectant. These are significant sources of infection
  • Remove and wash any furnishings - including bedding - that may be contaminated, using soap and hot water (if possible). Viral particles can survive for at least 12 days on some surfaces (workmen at a rest home whose occupants had suffered a bout of Norovirus fell ill with the disease themselves. Viral particles were subsequently discovered in the carpet.)