A large outbreak of invasive Group B Streptococcus Sequence Type 283 infection linked to physical contact of freshwater fish.

Invasive Group B Streptococcus (GBS) infection caused by the highly virulent Sequence Type 283 (ST283) strain has been linked to consumption of raw freshwater fish. In late summer 2024, enhanced surveillance in Hong Kong detected a surge of invasive ST283 cases.A retrospective case-control study was conducted involving all invasive GBS patients reported during August to September 2024. Data were collected through standardised interviewer-administered questionnaires. Cases were defined as patients infected with the ST283 strain, while controls had non-ST283 strains. A multivariate logistic regression analysis was conducted to determine the risk factors.Among 170 invasive GBS patients, 131 (77%) were identified as cases and 39 (23%) as controls. Physical handling of raw freshwater fish was found to be the strongest risk factor for ST283 infection (adjusted odds ratio: 8.4, 95% confidence interval: 1.4-50.1).This study represents the first epidemiological evidence specifically linking physical contact with raw freshwater fish to an increased risk of invasive GBS ST283 infection. Effective interdepartmental coordination, intensive public health education, active surveillance, and prompt environmental interventions effectively mitigated this large outbreak. The findings underscore the need for sustainable preventive strategies targeting high-risk fish handling practices, particularly during warm periods favouring environmental proliferation of ST283.