- Print Friendly - Help

Handling Potentially Contaminated Oysters

June 2004

Some imported oysters have been contaminated with Noroviruses during growing and harvesting.

If you are using packaged oysters labelled with a phrase such as “Cook before use”, they should be considered to be potentially contaminated.  Please read that the following information and ensure the suggested procedures are followed when using these oysters.

The source

Some imported oysters are grown in waters which can not be guaranteed to be free of noroviruses.  Outbreaks can also result from contamination of foods by infected food handlers.

Recently, in New Zealand several foodborne illness outbreaks have been caused by the consumption of oysters harvested from contaminated waters. 

The Illness

Noroviruses are members of a group of viruses called caliciviruses also known previously as “Norwalk-like viruses.”  Infection with norovirus affects the stomach and intestines, causing an illness called gastroenteritis, or “stomach flu.” The disease is self-limiting, mild, and characterised by nausea, vomiting, diarrhoea, and abdominal pain. Headache and low-grade fever may occur. The infectious dose is unknown but presumed to be low.  Gastroenteritis from a norovirus is contagious.

Norovirus survival

Noroviruses are relatively hardy and can survive from freezing to  relatively high temperatures and high levels of acids. They also can survive chlorination up to 10ppm. 

The following food safety controls should be implemented when preparing the Korean oysters:

Cook

To ensure the virus is inactivated the oysters must be must be cooked in such a way that the temperature at the centre of the oyster reaches 90°C and is held at that temperature for 90 seconds.

Check your cooking process and monitor to establish a procedure that adequately renders the oysters safe to eat.  The overall time may be longer than you expect.

Clean

Preparation surfaces and hands that have come into contact with the suspect oysters should be frequently washed as follows:

Clean surfaces, then sanitise the area with, for example, a dilute bleach solution (1 cup bleach to 9 cups water).

Handwashing

• Wet hands and nailbrush with warm water
• Rub fingertips and nails with soap and brush under running water, for 10 seconds
• Wet hands again using warm water
• Use soap to build up a lather, for 10 seconds
• Rinse hands, dry thoroughly with paper towel for 20 seconds (airtowel, 45 sec)

Cover

Ensure the oysters are stored so that they do not directly contact any other food and so they are covered adequately to prevent leakage to other foods.

Chill

As with any other perishable food – store below 4°C or freeze.

Ensure all other normal food safety controls are in place..