Singapore

Bacteria in raw freshwater fish prevalent in S-E Asia: Scientists

It is more aggressive than previously thought and has caused disease for decades, say researchers

A highly infectious bacterial strain that caused blood poisoning in over 160 people here in 2015 after they ate raw freshwater fish is more entrenched in the region and far more aggressive than previously thought.

Researchers led by Tan Tock Seng Hospital (TTSH) have discovered that the Group B streptococcus (GBS) bacteria - GBS ST283 - has caused disease in humans and freshwater fish in mainly South-east Asia for more than two decades.

The ST283 strain is the only known GBS bacteria to cause food-borne diseases. While most GBS strains do not infect healthy adults, the ST283 strain is especially aggressive.

By analysing the DNA of bacteria samples collected from local hospitals dating back to 1995 and regional data, 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 Timothy Barkham, a senior consultant medical microbiologist at TTSH's department of laboratory medicine.

In the 2015 outbreak, a 50-year-old lost all his limbs, and a 52-year-old fell into a coma for two weeks and lost his hearing.

The infectious disease scare prompted the Government to ban hawkers from selling raw freshwater fish. Infection rates dropped after the ban, which is still in place. The bacteria has not been found in saltwater fish.

The ST283 strain is almost absent in the rest of the world, except for four cases in France, Britain and the Netherlands.

Based on data from regional fish farms, the ST283 strain was found mainly in tilapia. Between 2007 and 2016, it was detected in all diseased tilapia in 14 fish farms in Malaysia and Vietnam.

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 scientific journal PLOS Neglected Tropical Diseases earlier this year.

It is not known why the ST283 strain is localised in South-east 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.

Patients' bacteria samples collected from hospitals from 1995 to 2017 showed that out of 744 patients infected with GBS, 23 per cent were infected with the ST283 strain.

The researchers also suspect the 2015 outbreak was caused by higher amounts of the bacteria strain in freshwater fish due to an increase in temperature.

Dr Barkham said: "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 in food."

The infectious strain is still lingering here. Since 2016, TTSH has seen fewer than 10 patients every year with it, he said.

ST283 infections can be treated with antibiotics such as penicillin.

Currently, little is known about GBS ST283, and Dr Barkham said cross-border collaborations in animal and human health are urgently needed so its transmission can be understood and halted.

Dr Swaine Chen, group leader of infectious diseases at GIS, said: "We need to find out how to curb the strain and explore solutions such as developing a vaccine for freshwater fishes or informing people in the region to avoid eating raw fish."

MEDICAL & HEALTH