Dozens of illnesses have been reported in two Australian states linked to the consumption of oysters in the past couple of months.

In total, 36 Vibrio parahaemolyticus infections from eating raw oysters have been notified since September 2021 in South Australia, compared to none in 2020 and eight cases in 2019.

In Western Australia, there have been 17 cases reported since late September. This compares to an annual average of six Vibrio parahaemolyticus infections acquired in the state in the previous four years.

Joanne Cammans, South Australia Health’s acting director of food and controlled drugs branch, said people with liver disease are particularly vulnerable to Vibrio and should avoid eating raw oysters.

“This increase in cases reported to us in such a short period of time is very concerning, as foodborne illnesses can be quite serious for more vulnerable people in our community, such as older South Australians, pregnant people and people with compromised immune systems. The infection can be acquired by eating undercooked shellfish and fish, however raw oysters are often the most common cause.”

No violations found during inspections

Seafood eaten raw or ready-to-eat cold cooked prawns or cold smoked salmon, are not recommended for pregnant women, those with reduced immune systems or older people because of the risk of bacterial infections such as Vibrio parahaemolyticus. If people have what they suspect might be vibriosis they should see their doctor.

Nathan Rhodes, executive director of biosecurity at the Department of Primary Industries and Regions (PIRSA), said the department was working with industry to manage the issue.

“The number of cases can vary substantially from year to year, and it is difficult to pinpoint the potential cause. Raw unshucked oysters, should be stored at less than 10 degrees C (50 degrees F) and shucked oysters at less than 5 degrees C (41 degrees F) to minimize the risk of Vibrio parahaemolyticus infection,” he said. 

“We are looking at potential factors that may have an influence on the recent cases, but the best international science has yet to determine a likely cause. Inspections by PIRSA officers have found that food safety practices are being appropriately applied on farm.”

Illnesses in Western Australia

Paul Armstrong, the Western Australia Department of Health’s director of the communicable disease control directorate, advised people to avoid eating raw or partially cooked oysters that originate in South Australia.

Health officials are currently recommending that oysters from South Australia should be cooked prior to consumption.

In Western Australia, all 17 sick people ate raw oysters apart from one who reported eating partially cooked oysters. Eleven patients have been linked to oysters originating in South Australia with work ongoing to determine the source of the others. There is as yet no evidence linking Western Australia oysters, or those from states except South Australia, to any of the sick people.

Vibrio infection, which usually begins 24 hours after exposure, includes watery diarrhea, abdominal cramping, nausea, vomiting, fever and chills. The illness lasts about three days. 

When cooking oysters, they need to reach an internal temperature of 145 degrees F for at least 15 seconds. Clams, mussels and oysters in the shell will open when cooked. People are also encouraged to wash their hands with soap and water after handling raw shellfish.

(To sign up for a free subscription to Food Safety News, click here.)