Authorities may never know exactly what caused an outbreak that has made at least a hundred people ill and shuttered the Coffin Bay oyster industry.
Most states and territories have reported gastro cases from Vibrio parahaemolyticus, an infection that has been pinpointed in Pacific oysters from the famous South Australian area.
A total recall of the oysters has been issued, and Coffin Bay farms will stay closed at least until next week.
Found in raw and undercooked shellfish, Vibrio can cause diarrhoea, vomiting, abdominal pain, fever and headache, and is riskier for the elderly and people with compromised immune systems.
It is common in oysters, which are filter feeders that pick it up from the surrounding water.
Highly pathogenic strains of the bacteria have emerged in the last decade, and they are spreading further around the world because of climate change.
SA’s primary industries and regions department is working with the industry to assess the full extent and impact of the closures and is testing the oysters and environment while the farms are closed.
PIRSA’s executive director of biosecurity, Nathan Rhodes, said the department was also working with a program to require growers to rapidly reduce the temperature of oysters after harvest, to reduce the likelihood of Vibrio bacteria being present.
While investigations are ongoing, he said, PIRSA may not be able to pinpoint the exact cause of the outbreak.
Food Standards Australia New Zealand said the food-borne illness has been detected in SA, New South Wales, the Australian Capital Territory, Queensland, Victoria, the Northern Territory and Western Australia, with people ill from eating raw oysters that were either fresh or had been frozen.
While it had confirmed Coffin Bay as the source, the cause was still being investigated, it said in a statement.
“Consumers are advised not to eat raw Pacific oysters from Coffin Bay, including fresh and frozen products,” it said.
“Any consumers concerned about their health should seek medical advice. Consumers should dispose of the oysters or return the oysters to the place of purchase for a full refund.”
There had been at least 36 cases of Vibrio in SA, 17 in WA, 31 in Victoria, 15 in NSW, and two in the ACT.
SA authorities were investigating whether climate change was a factor in the outbreak. The SA Oyster Growers Association executive officer, Lynlee Lowe, said last week they were examining “highly unusual environmental conditions … not been seen before in SA, which have coincided with this outbreak”.
“[There have been] strange weather patterns, unusual ocean currents, ocean water [temperatures] for this time of year,” she said.
The World Health Organization has already warned that Vibrio has spread in response to climate change.
In August it reported that in the past decade a highly pathogenic strain of Vibrio had “caused infections in new areas and in regions where these diseases have not been observed before”.
“(And), in response to climate change, there has been a significant geographical spread …”
Infections were now being observed in parts of the US, Spain and South America where they had not been before, it reported.
In the US, where there are about a hundred deaths from Vibrio every year (including other strains), the Centres for Disease Control and Prevention warns that neither alcohol, nor hot sauce, not lemon juice will make infected oysters safe. It says cooking them properly kills the bacteria – but the advice in Australia for Coffin Bay oysters, for now, is not to eat them at all.
While the SA oyster market is suffering, Australia’s biggest market says it will have plenty of oysters available for Christmas.
The NSW Farmers oyster committee chair, Todd Graham, said after sales lulls during Covid, NSW growers had plenty of mature stock on hand.
“NSW oyster growers have had a few very challenging years with impacts (from) bushfires, floods and Covid, but our oysters are safe, delicious and readily available, so support your local growers especially this Christmas,” he said.
While most Vibrio cases are from oysters, it can be caught from other shellfish and can even infect open wounds in water where the bacteria is present.