SINGAPORE - In 2015, a highly infectious bacteria strain caused an outbreak of blood poisoning in more than 160 people in Singapore, who suffered from fever, joint infection and meningitis after eating ready-to-eat dishes containing raw freshwater fish.
One of the victims, a 50-year-old man, lost all of his limbs, and another, a 54-year-old man, fell into a two-week-long coma and eventually lost his hearing in the largest outbreak of its kind in the world.
Now, researchers led by Tan Tock Seng Hospital have discovered that the Group B Streptococcus (GBS) bacteria causing the disease was a hitherto unflagged strain - GBS ST283 - that has caused disease in humans and freshwater fish mainly in South-east Asia for almost three decades.
The ST283 strain is also the only known GBS bacterium to cause foodborne diseases. While most GBS strains do not infect healthy adults, the ST283 turned out to be especially aggressive.
By analysing the DNA of samples collected from local hospitals dating back to 1995 and regional data , the researchers found that more than 350 ST283 infections have occurred in Laos, Thailand, Vietnam, Singapore, Malaysia and Hong Kong.
"Our research reveals a previously unknown disease pattern that has escaped detection," said the principal lead investigator of the research, Dr Timothy Barkham, who is a senior consultant medical microbiologist at TTSH's Department of Laboratory Medicine.
The ST283 could have appeared in Southeast Asia before 1995, but there is no data to prove that, said Dr Barkham.
The strain is almost absent in the rest of the world, except for two cases in France, one in Britain and one in the Netherlands.
Based on data from regional fish farms, the ST283 strain was found mainly in tilapia. Between 2007 and 2016, ST283 was detected in all diseased tilapia in 14 fish farms in Malaysia and Vietnam.
In the 2015 outbreak in Singapore, customers fell ill from eating seven species of freshwater fish, including tilapia, carp and snakehead.
The research team, which comprised 30 international collaborators and the Agency for Science, Technology and Research's Genome Institute of Singapore (GIS), published these findings in the peer-reviewed scientific journal , PLoS Neglected Tropical Diseases, earlier this year.
It is not known why ST283 is localised in Southeast Asia, but the scientists say it could be due to the prevalence of aquaculture and the popularity of raw freshwater fish dishes in the region.
"If people didn't eat raw fish, ST283 would not be found in humans," said Dr Barkham.
The first known cases of ST283 infections in Singapore were in 1998, when five people caught meningitis. But patients' bacteria samples collected between 1995 and 2017 from hospitals such as TTSH, Changi General Hospital and Singapore General Hospital showed that out of 744 GBS infections, 23 per cent were with ST283.
The researchers also suspect that the 2015 outbreak was caused by higher amounts of the bacteria strain in freshwater fish due to an increase in temperature.
"2015 was an El Nino year, so this might support a theory that due to an increase in temperature, the amount of bacteria in fish was more than normal. Human infection depends on the degree of contamination," said Dr Barkham.
Although their analysis showed 1 per cent of imported freshwater fish were infected with ST283 at the ports here, 28 per cent of the fish comprising tilapia, carp and snakehead were infected by the time they reached the markets in 2015.
This suggests that contamination during transport and handling could have amplified the spread of the bacteria into other freshwater fish species, the study said.
The 2015 outbreak prompted the Government to ban hawker stall owners from selling raw freshwater fish. Following the ban, the infection rate dropped. The bacteria has not been found in saltwater fish, such as those in Japanese sashimi.
But the infectious strain still lingers here. Since 2016, TTSH has seen up to 10 patients every year infected with ST283, said Dr Barkham.
ST283 infections can be treated with antibiotics such as penicillin.
After the 2015 outbreak, the GIS designed a test to detect ST283 in patients' bacteria samples, which was then used to identify the strain from the 744 local samples.
The research has not been extended into local fish farms yet, but the GIS is aiming to design a separate test to identify the bacteria in fish, said Dr Swaine Chen, Group Leader of Infectious Diseases at GIS.
Currently, very little is known about GBS ST283, and Dr Barkham said cross-border collaborations in animal and human health are urgently needed to complete the picture so that its transmission can be understood and halted.
The research team is continuing its research to determine the bacterial strain's origin, spread, transmission and extent of harmfulness.
"We need to find out how to curb the strain and explore solutions such as developing a vaccine for freshwater fish or informing people in the region to avoid eating raw fish," said Dr Chen.
The Food and Agriculture Organisation of the United Nations and TTSH are working on an agreement to lead an international risk assessment of ST283. TTSH and six other local hospitals have received a Temasek grant to study whether other GBS strains cause foodborne diseases.
Meanwhile Singaporeans should head the government's advisory not to consume ready-to-eat meals containing raw freshwater fish and avoid consuming raw or undercooked fish when they travel to neighbouring countries, the researchers reminded.
