Yesterday evening, in a joint statement issued by the Ministry of Health (MOH), Agri-Food &Veterinary Authority of Singapore (AVA) and the National Environment Agency (NEA), the authorities revealed that there had been 238 cases of Streptococcus agalactiae (GBS) bacteraemia between 1st January and 30th June this year, compared to 150 cases per year on average for the past 4 years. More importantly, the AVA had also found evidence of GBS in bighead carp (although it was not explicitly stated whether these fish samples were from the porridge stalls or from fishmongers and suppliers), and more surprisingly, also in the snakehead or toman fish. I had not known that this fish could be eaten raw or was sold at porridge stalls as “yu sheng”.
The NEA also advised (I am not certain if this is the equivalent of “demanded”, just couched more politely) licensed stall holders to withhold selling raw fish dishes using the above fish. This decisive action in the face of incomplete proof is certainly courageous and welcome. I am fairly certain that the case count will fall once people stop eating this type of raw fish (sashimi and sushi lovers are still free to indulge).
The AVA findings are intriguing because they imply that GBS may be colonisers of fish rather than just pathogens, which is logical but had previously not been highlighted from virtually all reports of this organism in fish. GBS had also previously never been reported in bighead carp, either because it never killed the fish, or because this is a relatively recent phenomenon. In any case, because this outbreak appears to be relatively new (perhaps starting from late last year), and Singaporeans have been enjoying raw “yu sheng” from porridge stalls for decades, there is an underlying story that has yet to come to light. Did the supply of bighead carp (and toman fish) change recently to other newer fish farms? Or was there a change in farming practice at AVA-licensed farms? It will be interesting to know if and when AVA sees fit to reveal such information.
Because the majority, but by no means all, of the patients with GBS infection had reported raw fish consumption, it is plausible that this fish link is only part (albeit a major part) of the whole outbreak story, and there are other potential sources that need to be evaluated with an open mind. There is indeed a lot of investigating and research to be done to understand this outbreak and the unusual clinical manifestations (severe GBS disease in mostly healthy non-pregnant adults).
One other question is what about the other parts of the world that enjoy raw freshwater fish? Hong Kong had seen a rise in ST283 serotype III GBS in the late 1990s, but the author of that paper, when contacted, mentioned that only sporadic cases are now seen. Part of the answer for Hong Kong (besides the obvious point that this particular GBS strain may only affect the fish from a few farms and not all bighead carp farmed worldwide) may be that it had banned the sale of “yu sheng” for the past 30 years. Not because of bacterial disease, but because of the liver fluke Clonorchis sinensis, whose larva may be found in the meat of freshwater fish such as carp, from which infections in humans may result in liver or bile duct cancer. How about southern China? In this excellent and detailed post, Dr. Leslie Tay mentioned that the origins of raw freshwater fish consumption was from Canton. I do not know the answer, but there have been no reports of increased GBS disease from that part of China.