Emergence and Regional Circulation of Streptococcus iniae Associated With Streptococcosis in Marine Cage-Farmed Golden Rabbitfish (Siganus guttatus).

Streptococcosis caused by Streptococcus iniae is an emerging bacterial disease affecting marine aquaculture worldwide. In recent years, recurrent outbreaks have been observed in golden rabbitfish (Siganus guttatus) cultured in marine cages across the northern and central coastal provinces of Vietnam. This study investigated the epidemiology, pathogenicity, virulence gene profiles and antimicrobial susceptibility of S. iniae associated with disease outbreaks in rabbitfish aquaculture. Between 2023 and 2025, streptococcosis outbreaks were documented across 46 marine cage farms in multiple coastal provinces, with cumulative mortality ranging from 10%-45%, particularly during warm-water periods (28-33°C). Bacteriological examination consistently yielded S. iniae, which was confirmed by biochemical tests and species-specific PCR assays. An experimental challenge demonstrated the high virulence of a representative isolate, with a median lethal dose (LD50) of 6.8 × 103 CFU/fish. Histopathological examination revealed severe multi-organ lesions, including prominent involvement of the brain, kidney, liver, spleen, gills and intestine, consistent with systemic bacterial infection. Molecular screening of virulence-associated genes identified six distinct gene profiles among isolates, indicating substantial genetic heterogeneity within the circulating pathogen population. Several genes, including pdi, simA, cpsB, cpsC, cpsD, pgm, hasA and ydfG were detected in all isolates (100%), whereas others showed marked variability, with prevalence ranging from approximately 17% to 89% among isolates. Antimicrobial susceptibility testing revealed heterogeneous resistance patterns, with high resistance to neomycin (NE) and moderate resistance to trimethoprim-sulfamethoxazole (TRS), while most isolates remained susceptible to tetracyclines (TEs) and β-lactam antibiotics. The widespread occurrence of outbreaks across multiple provinces and over consecutive years suggests sustained circulation of S. iniae within interconnected marine cage aquaculture systems. These findings provide new epidemiological insights into an emerging streptococcal disease affecting rabbitfish aquaculture and highlight the need for improved surveillance and disease management strategies in rapidly expanding marine cage-farming sectors.