Abstract
In March 2024, a severe mass mortality event occurred at a grass carp ( Ctenopharyngodon idella ) farm in Zhejiang, China, resulting in a cumulative mortality rate of 70%. To identify the causative agent, 12 bacterial isolates that exhibited the same phenotypic characteristics were isolated from diseased fish. A further phylogenetic (16S rDNA) characterization identified the bacterium as Flavobacterium psychrophilum . Genetic characterization revealed that all 12 bacterial isolates belonged to serotype Type‐1 and sequence type ST418, a newly documented ST closely associated with cyprinid fish hosts. In infection trials, the bacterium exhibited clear virulence in grass carp, producing clinical signs consistent with those observed during the outbreak. Furthermore, histopathology of diseased fish with F. psychrophilum revealed ulcerative dermatitis with associated myositis, proliferative branchitis, vascular congestion and focal necrosis in the liver, multifocal necrosis in the spleen, and tubular dilation with renal corpuscle necrosis in the kidney. Given the potential risk of cross‐species transmission among cyprinid fish, we also evaluated the pathogenicity of this bacterium in Prussian carp ( Carassius gibelio ), a widely cultured species in China. To our knowledge, this study represents the first report of F. psychrophilum causing mass mortality in farmed grass carp in China and provides essential information to support improved prevention and risk mitigation strategies for this emerging disease.