Officials in Hong Kong are investigating an outbreak of invasive Group B Streptococcus cases linked to handling freshwater fish.

The Centre for Health Protection (CHP) of the Department of Health asked the public to not consume raw freshwater fish or aquatic products, and to handle such items with caution to avoid contact with wounds, including small cuts and scratches.

This past week, the Hospital Authority, an agency that manages public hospitals, told the CHP that 88 patients had tested positive for invasive Group B Streptococcus (GBS) in September and October, and provided specimens of 68 patients for genetic sequencing analysis.

This has shown that, amongst the 68 patients, 32 of them belonged to a variety of sequence type 283 (ST283), 27 cases are other serotypes or another strain of ST283, while the results for the remaining nine cases are pending.

ongoing investigation
Normal levels have ranged from nine to 26 cases per month in the past three years from January 2019 to August 2021.

The CHP’s epidemiological investigations show the 32 ST283 cases likely have the same source of infection. Patients are 14 males and 18 females with ages ranging from 31 to 87 years old living in various districts.

Half of them reported a history of handling freshwater fish and some handled raw freshwater fish, such as grass carp, with hand wounds. None reported consuming raw freshwater fish. Three of them are cooks in restaurants and one is a part-time fishmonger.

According to information from the Hospital Authority, two people have died, but the cause of death was not clear, and 10 have been discharged from hospitals

CHP has collected fish and environmental samples in markets visited by some cases and confirmed the genetic sequencing of these samples is identical to those of the 32 ST283 patients.

The agency believes that handling raw freshwater fish with hand wounds may be associated with infection but did not exclude the risk from consumption of undercooked freshwater fish. In Hong Kong, the sale of freshwater fish sashimi is prohibited.

The Food and Environmental Hygiene Department has been carrying out cleaning and disinfection of the markets concerned. The Agriculture, Fisheries and Conservation Department is taking more fish samples from the freshwater fish wholesale markets to get an idea of the local situation.

Past outbreak in Singapore
A CHP spokesman said Group B Streptococcus is found in 20 to 40 percent of healthy adults but also causes severe infections among the elderly and vulnerable people.

“The sources of invasive Group B Streptococcal infection are mostly unclear. Literature also reported that it relates to eating or improper handling of contaminated food, and some even reported that invasive Group B Streptococcus of ST283 exists in freshwater fish, especially cultured fish,” he said.

“In 2015, there were a number of cases of Group B Streptococcus of ST283 involving 146 people in Singapore who had consumed raw freshwater fish. Thereafter, presence of Group B Streptococcus of ST283 was also reported in freshwater fish in other Southeast Asia countries including Thailand and Vietnam.”

Epidemiological investigations showed there was a strong link with consuming raw freshwater fish including raw Asian bighead carp and snakehead. This led to a ban on the use of raw freshwater fish in all ready-to-eat raw fish dishes in Singapore since December 2015. Nearly 20 infections were also identified in Singapore in July 2020 but the source was not found.

To minimize infection risks, the CHP advised fish and restaurant workers to wear gloves when handling aquatic products and avoid direct contact with such items or dirty water with bare hands.

When buying fish from markets, the public must not touch the fish. They should wash hands with liquid soap and water as soon as possible if they had contact with raw aquatic products. When handling these products at home, people should wear gloves and wash hands thoroughly afterwards.

Members of the public must not eat any freshwater fish sashimi, raw or undercooked freshwater aquatic products. When consuming items which consist of aquatic products, they should ensure the food is thoroughly cooked.