Emergence of highly virulent in channel catfish aquaculture: Genomic insights into pathogenicity and antimicrobial resistance.

has emerged as a significant pathogen in aquaculture, causing severe disease outbreaks and resulting in substantial economic losses. However, its pathogenic mechanism and virulence factors remain largely unexplored. In this study, we isolated a highly virulent strain of , CWH5, from a severe disease outbreak in farmed channel catfish (). Through comprehensive whole-genome analysis, we elucidated its pathogenicity and the genetic basis for its high virulence and multi-antimicrobial resistance in channel catfish. Experimental infections showed that CWH5 exhibited exceptional virulence, with an LD of (5.37 ± 0.31) ×10 CFU/fish and causing 100% mortality within 24 h at a concentration of 10 CFU/fish. Histopathological examinations revealed severe multi-organ damage, including extensive hepatocellular necrosis, gill epithelial destruction, and fin tissue deterioration. Whole-genome sequencing revealed a 4.92 Mb circular chromosome encoding sophisticated virulence mechanisms, such as complete Type III, IV, and VI secretion systems, and a vast arsenal of 60 antibiotic resistance genes across 15 drug classes. Comparative genomic analysis positioned CWH5 within the clade, sharing the highest sequence similarity with CIP 107,500. The co-localization of virulence and resistance determinants within mobile genetic elements suggests the potential for horizontal gene transfer. Our work underscored the importance of CWH5 as an emerging pathogen in channel catfish aquaculture, providing crucial insights into the molecular mechanisms of its exceptional virulence and implying significant implications for disease management and antimicrobial resistance surveillance in aquaculture settings.