Outbreak of 32 Mycobacterium marinum infections traced from a single outpatient case: genomic and epidemiological evidence.

Outbreaks of Mycobacterium marinum (M. marinum) infections associated with recreational water exposure, aquarium maintenance, and fish-related injuries have been increasingly reported, raising concerns as an emerging public health issue in certain regions. The outbreak investigation began with the identification of an index case presenting with papules and nodules on the hand following a sea bass handling injury. Epidemiological tracing focused on seafood exposure, and laboratory confirmation of M. marinum infection was achieved through PCR, culture, and histopathology. Whole-genome sequencing was performed on representative isolates to conduct comparative genomic analysis and establish clonal relationships. Between October and December 2020, 32 cases of M. marinum infection were identified, after detecting the initial index case. All cases were epidemiologically linked to a single seafood store. Genomic analysis revealed that the outbreak isolates shared >99.5% average nucleotide identity (ANI) with strains from the 2019 Weifang outbreak and exhibited fewer than 20 single-nucleotide polymorphisms (SNPs), confirming their clonal lineage. All patients received appropriate therapy and achieved complete clinical remission within 3-6 months, with no recurrences observed during follow-up (median duration: 6 months). This study underscores the importance of integrating clinical vigilance, epidemiological tracing, and genomic tools to uncover hidden transmission events and guide timely public health interventions. Strengthening hygiene practices, enhancing seafood supply chain monitoring, and promoting protective measures during fish handling are crucial to preventing future outbreak.