Food safety officials in New Zealand are urging people to cook raw mussels thoroughly after a recent spate of food poisoning cases caused by Vibrio parahaemolyticus.
According to a spokeswoman from the New Zealand Ministry of Health, one third of those affected have been hospitalised, but no deaths have been reported. “Of the 26 cases acquired in New Zealand, 20 are known to have eaten raw, smoked or partially cooked mussels during the incubation period,” she said.
Additional testing is being done to confirm the type of Vibrio parahaemolyticus that caused the outbreak. “It is possible that the strain of Vibrio parahaemolyticus is unusually aggressive,” said Paul Dansted, New Zealand Food Safety’s Director of Food Regulation. “[This] may mean that even low numbers could cause illness. Additional testing of mussels and the waters that they are being grown in is also underway to help us understand why this has happened.”
Most of the people who fell sick bought commercially grown New Zealand mussels harvested from a growing area in the Coromandel. The growing area has been closed by New Zealand Food Safety while investigations continue.
What is Vibrio parahaemolyticus?
Vibrio parahaemolyticus (also called V. parahaemolyticus) is a bacterium that grows naturally in sea water, and can grow to large numbers when the ocean is warm; 80% of V. parahaemolyticus infections occur between May and October, when coastal water temperatures are highest.
Causes of infection
Bivalve molluscs (e.g. oysters, clams, cockles, scallops, mussels) are filter feeders, which means they feed by straining food particles and small organisms out of water. By filtering great quantities of water, they accumulate a high number of microorganisms, including V. parahaemolyticus, in their tissues, which can cause food-borne diseases in humans.
Infection with V. parahaemolyticus, called vibriosis, can occur if:
- there is a high number of V. parahaemolyticus bacteria in the shellfish when it is harvested; or
- shellfish is not kept cold after harvesting; and
- shellfish is served undercooked or eaten raw.
Symptoms of vibriosis
Diarrhoea is the most common symptom of vibriosis, and is characterised by:
- acute onset of watery stools
- abdominal pain/cramps
Other symptoms may include: nausea, vomiting, fever, headache and/or bloody stools.
The time from infection to the onset of symptoms is typically 12 to 24 hours, but can range from four to 96 hours. Illness usually lasts for three to seven days and generally does not require hospital treatment, except in more severe cases.
Food-borne illness prevention
To avoid infection, or to prevent others from falling ill with vibriosis, it’s extremely important that you purchase bivalve molluscs and other shellfish from reputable suppliers and keep them cold (below 5°C). Only shellfish from approved sources should be prepared or served.
Remember to do the following:
- Refrigerate raw shellfish immediately after purchasing or delivery.
- Keep raw shellfish separate from cooked foods and follow good hygiene practices.
- Cook bivalve molluscs, such as oysters, clams, cockles, scallops and mussels, to 65°C or higher to destroy bacteria and minimise the risk of food-borne illness.
Guidelines for cooking:
- Boil for three to five minutes after the shells are open. (Add shellfish in the shell to water that is already boiling.)
- Steam for four to nine minutes. Throw out any shellfish with unopened shells.
- Fry for at least three minutes at 190°C.
- Bake for at least 10 minutes at 230°C.
Additional warnings:
- Raw shellfish is not recommended for people at increased risk of contracting vibriosis, like children, pregnant women or people with reduced or compromised immune systems.
- Always wash hands with soap and water after handling raw shellfish.
- Avoid contaminating cooked shellfish with raw shellfish and its juices.
Food hygiene and safe food handling
As with any food safety hazard, the only way to prevent vibriosis is to practise safe food handling.
If you are a food business owner or manager, you know that a food-borne illness can ruin your reputation and destroy consumer trust in your brand. To protect your customers, your business and your brand from the consequences of food-borne illness, you should:
- train and certify everyone who works with food in your business
- nominate a Food Safety Supervisor to manage food safety in the workplace
The Australian Institute of Food Safety (AIFS) is a nationally recognised provider of food safety online courses, which can help you to manage the food safety hazards in your business and to comply with federal and state/territorial laws and regulations.
Get more information about the AIFS Food Safety Supervisor online course or the AIFS online food handling course.