It has been quite a long wait, but finally, investigators from the Environmental Health Institute (EHI – a part of the National Environment Agency) and the Agri-Food & Veterinary Authority of Singapore (AVA) have published their work on tracing the source of the unusual ST283 Group B Streptococcus (GBS) that caused the huge raw freshwater fish-associated outbreak in 2015.

Screen capture from the PDF version of the article published in Emerging Infectious Diseases.

In the December issue of U.S. CDC’s Emerging Infectious Diseases journal, the local investigators sampled a large number of fish (and water from the tanks where the fish were held) along the supply chain of fresh and saltwater fish in Singapore – from the fishery port in Jurong to the stalls and restaurants.

Screen capture of the Technical Appendix Table from EID. Sampling plan.

GBS could be found in fish from the fishing port to the food stalls and restaurants, except for the saltwater fish from dedicated sashimi suppliers. What was interesting – but perhaps unsurprising – was that the proportion of fish from which GBS could be detected increased throughout the supply chain: from the fishery ports (4.6% of 586 fish tested) to fresh produce markets including supermarkets (76.9% of 39 freshwater fish tested, and 21.7% of 23 saltwater fish tested) to the food stalls.

Screen capture from the EID article. Percentage of fish with Group B Streptococcus detected.

The outbreak strain – ST283 GBS – could similarly be detected in a greater percentage of fish as they moved along the supply chain. At the fishery port, ST283 GBS were only isolated from Asian bighead carp farmed in Malaysia, which suggested that the source of the Singapore outbreak was from Malaysia, although the authors were quick to point out that similar ST283 GBS had also been isolated from patients in Thailand and Hong Kong, implying a more regional presence of this particular virulent strain of GBS.

There is a wealth of other data and analyses in the paper, including results from culture of other potentially pathogenic bacteria from fish, which those interested should read. Some of the other key takeaway points include:

• There are actually two different subtypes (or clades) of ST283 GBS isolated from fish, one of which did not have representative isolates from infected humans in Singapore, Thailand or Hong Kong.
• It would seem extremely unwise to consume raw (and perhaps even undercooked) freshwater fish bought from the supermarkets or wet markets in Singapore.
• Saltwater fish brought in via sashimi suppliers appear to be generally safe for raw consumption with regards to bacterial contamination, although 1 (of 21) samples was positive for Listeria monocytogenes (a bacterium that can cause sepsis and meningoencephalitis in the immunocompromised, as well as a variety of ill effects on fetuses in pregnant women).