The Food and Agriculture Organization (FAO) of the United Nations has warned of illness linked to eating raw freshwater fish in Southeast Asia.
Food safety experts said Group B Streptococcus (GBS) has been detected in several countries of the region.
The FAO’s regional office for Asia and the Pacific published a risk profile to raise awareness about the threat.
Masami Takeuchi, FAO food safety officer, said many people aren’t aware of the risks associated with consuming raw freshwater fish, which is very common in Southeast Asia.
“But the illnesses this practice can cause can be serious, though not always obvious, nor immediate, and in some cases that can make it difficult to diagnose and treat in time,” Takeuchi said.
First reported outbreak
The problem was discovered in 2015 when at least 146 people fell ill in Singapore. It remains the only reported foodborne outbreak of invasive GBS disease. Some patients experienced severe consequences, including amputation of limbs caused by blood poisoning linked to the bacterium called Streptococcus agalactiae, also known as GBS. The strain responsible for the outbreak was sequence type 283 (ST283). A resurgence of cases led to legislation in December 2015 banning the sale of raw freshwater fish as a ready-to-eat food.
“Many microbiologists were surprised as invasive GBS disease had not been known to be foodborne previously. Another surprising point was that this foodborne GBS ST283 affected healthy adults. GBS is normally very uncommon in healthy adults,” said Timothy Barkham, associate professor at the Department of Laboratory Medicine, Tan Tock Seng Hospital in Singapore.
Cases of invasive GBS ST283 disease have also been reported in China, Hong Kong, Lao People’s Democratic Republic, Thailand and Vietnam. Small numbers of infections continue to be identified in Singapore, with 18 cases in July 2020 but the source was not found as people said they hadn’t eaten raw freshwater fish during patient interviews.
Risk mitigation steps
The list of uncertainties and identified data gaps is extensive so a full risk-assessment is not yet possible, according to FAO.
Promotion of good aquaculture practices (GAPs) and a food safety campaign, aimed at consumers, local populations or villagers to tell them of the potential risks from raw freshwater fish have been put forward as effective preventative measures.
Takeuchi said it is important to continue strengthening national food control systems but authorities need to be aware of the problem first.
“As the issue is relatively new, having an overview of what is currently known on this foodborne disease is possible by reading the FAO risk profile, discussing among all stakeholders in food safety, public health and fisheries/aquaculture to exchange information and share views — these are good first steps,” she said.
Proper cooking is the only known effective risk mitigation measure. Freezing does not work. Discarding visibly abnormal or diseased fish could reduce risk but visual inspection alone should not be relied upon as healthy-looking fish could also be unsafe. There is no evidence traditional fish preparation methods without heat treatment are sufficient.
GBS in tilapia
Mags Crumlish, senior lecturer in food security and sustainability at the University of Stirling’s Institute of Aquaculture, has identified GBS ST283 in farmed freshwater tilapia, which is produced for the international market as well as consumed locally in Southeast Asia.
“GBS is a known disease in freshwater tilapia, but we are only starting to identify the different strains associated with disease outbreaks in farmed fish species. This hypervirulent GBS strain ST283 is unique and, so far, it has only been confirmed in farmed tilapia in Southeast Asia and Brazil,” she said.
“Our goal in aquaculture is to reduce disease outbreaks in fish and prevent transmission to humans by collaborating with other scientists. This way we will determine the role of fish preparation and consumption with human GBS ST283 outbreaks.”
Fiona Harris, associate professor of health sciences at the University of Stirling, was one of 25 experts who worked on the report, and said it brings together what is known and highlights gaps in knowledge that need further exploration.
“Throughout Southeast Asia traditional recipes using raw or lightly cooked fish are popular foods. This includes fish marinated in lime juice and chili as well as fermented or preserved fish, which are important sources of protein for poor people in the region,” she said.
“We need to work closely with communities to find out more about different preparation methods in local dishes — salting, drying, lime and garlic marinades — as currently the only known effective method is heating or cooking.”
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