Researchers have identified handling of freshwater fish as the key risk factor in an outbreak in Hong Kong.
Invasive Group B Streptococcus (GBS) infection caused by Sequence Type 283 (ST283) has also been linked to consumption of raw freshwater fish. In late summer 2024, surveillance in Hong Kong detected a surge in cases.
A case–control study was conducted involving all invasive GBS patients reported during August to September 2024. Cases were defined as patients infected with the ST283 strain, while controls had non-ST283 strains. Findings were published in the journal Epidemiology and Infection.
Among 170 invasive GBS patients, 131 were identified as cases and 39 as controls. Physical handling of raw freshwater fish was the strongest risk factor for ST283 infection.
Past incidents highlight risk
Scientists carried out environmental sampling at four of the wet retail markets visited by the ST283 cases, a wholesaler and three local fish farms.
In 2015, Singapore recorded a major foodborne outbreak involving more than 100 cases of invasive ST283 infection. Epidemiological investigations indicated an association with the consumption of raw freshwater fish. Cases of invasive GBS ST283 have continuously been reported in and around Southeast Asia.
In September 2021, Hong Kong experienced a rise in invasive GBS cases attributed to the ST283 strain. Epidemiological investigations conducted by the Centre for Health Protection (CHP) suggested handling raw freshwater fish and eating undercooked freshwater fish as possible risk factors.
In the 2024 outbreak, 61 of the 131 cases were males and 70 females, with ages from 29 to 97. Onset dates ranged from Aug. 8 to Sept. 30, 2024. Clinical presentations primarily involved sepsis, joint abscesses and meningitis. Four people died.
Role of public awareness
The freshwater fish involved included a variety of species, with grass carp being the most common. These fish were purchased from more than 50 markets throughout all districts, ruling out a single common source.
Only four cases and one control reported consuming undercooked freshwater fish. No cases involved professional fish handlers, such as fishmongers, who are equipped with protective equipment like gloves and possess relevant knowledge and skills.
Two freshwater fish samples and three environmental swabs from a freshwater retail stall in a wet market tested positive for ST283.
Government departments conducted cleaning and disinfection in the relevant markets and inspected all fish stalls, licensed fresh provision shops, and permitted premises selling freshwater fish in Hong Kong. They offered hygiene education to operators, advising them to perform deep cleaning and disinfection after business hours.
Public awareness about preventive measures should include not eating raw or undercooked freshwater fish and also wearing appropriate protective gear and practicing good hand hygiene when coming into contact with such fish.
Scientists said the study represents the first epidemiological evidence specifically linking physical contact with raw freshwater fish to an increased risk of invasive GBS ST283 infection.
“The findings underscore the need for sustainable preventive strategies targeting high-risk fish handling practices, particularly during warm periods favoring environmental proliferation of ST283. This knowledge will enable the development of more targeted preventive and control strategies, including precise health education messages,” according to the researchers.
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