In 2015, around 146 people became ill after eating a traditional raw freshwater fish dish in Singapore, with several people eventually having to have limbs amputated. It turns out that they had developed blood poisoning linked to a bacterium called Streptococcus agalactiae, also called Group B Streptococcus (SBS). The specific strain responsible for the outbreak was a unique sequence type 283 (ST283). A four-page factsheet, Invasive disease linked to raw freshwater fish, has just been made available, as has a more comprehensive Risk profile – Group B Streptococcus (GBS) – Streptococcus agalactiae sequence type (ST) 283 in freshwater fish.
Dr Tim Barkham, an associate professor at Tan Tock Seng Hospital in Singapore, first identified the human health risk. “Many microbiologists were surprised, as invasive GBS disease in people has not been known to be foodborne previously,” he said.
“Another surprising point was that this foodborne GBS ST283 affected healthy adults. GBS is normally very uncommon in healthy adults.”
Dr Mags Crumlish, senior lecturer in food security and sustainability at the University of Stirling’s Institute of Aquaculture, specialises in aquatic microbial diseases. She was one of the founding members of the GBS network and has identified GBS ST283 in farmed freshwater tilapia.
Dr Crumlish said: “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. 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.”
GBS disease due to ST283 has also been confirmed in China, Hong Kong, Laos, Thailand and Vietnam.
Dr Fiona Harris, associate professor of health sciences at the University of Stirling, wh also worked on the report, said: “Throughout Southeast Asia traditional recipes using raw or lightly cooked fish are popular foods. This includes fish marinaded in lime juice and chilli as well as fermented or preserved fish, which are important sources of protein for poor people in the region.
“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. We may eventually need a public health campaign, so talking to people about these foods and what they mean to them is really important.”
“Many people aren’t aware of the risks associated with consuming raw freshwater fish, and it is a very common practice in Southeast Asia,” said Dr Masami Takeuchi, FAO food safety officer. “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.”
While more research is undertaken, the following advice is being shared with stakeholders in the region to reduce risk:
- Visual inspection: discarding visibly abnormal/diseased fish is expected to reduce risk, but they should not rely on visual inspections alone, as healthy-looking fish are no guarantee of safety.
- Heat-treatment: proper heating /cooking is the only known effective risk mitigation measure.
- Non-heat-treatments: there is no evidence that traditional fish preparation methods without heat treatment are effective. Freezing is not an effective control measure.