Streptococcus iniae infection in cultured pompano (Trachinotus spp.) in Vietnam: Field-based characterisation, pathogenicity, and antimicrobial susceptibility

Streptococcus iniae is an important bacterial pathogen of marine aquaculture. However, field-based epidemiological data and population-level characterisation of S. iniae in Vietnamese pompano aquaculture remain limited. This study investigated streptococcosis outbreaks in cultured pompano ( Trachinotus spp.) in Vietnam through a multi-farm, field-based approach that integrated epidemiological surveys, bacteriological and molecular identification, virulence gene profiling, experimental challenge, and antimicrobial susceptibility testing. A total of 54 S. iniae isolates were recovered from diseased fish across multiple farms and provinces, indicating a widespread occurrence of the pathogen in both cage and pond systems, as determined by phenotypic characteristics, biochemical profiling, and species-specific PCR assays. Screening of 14 virulence-associated genes revealed substantial genetic heterogeneity, with multiple virulence phenotypes circulating in the farming population. The experimental challenge reproduced the clinical signs and histopathological lesions observed in natural cases, and the estimated LD₅₀ of 2.2 × 10⁴ CFU/fish indicated high pathogenicity of a representative field isolate under experimental conditions. Antimicrobial susceptibility testing demonstrated full susceptibility to doxycycline and amoxicillin-clavulanic acid but high resistance to trimethoprim-sulfamethoxazole and neomycin, highlighting the need for evidence-based antibiotic use. Overall, these findings provide field-based evidence of disease occurrence and offer comprehensive insights into the pathogenicity, genetic diversity, and antimicrobial resistance of S. iniae in cultured pompano aquaculture.