Pathogenicity of Causing Mass Mortalities of and Its Induced Host Immune Response.

is an opportunistic pathogen of freshwater aquatic animals, which severely restricts the sustainable development of the aquaculture industry. In this study, a dominant strain, named FSNM-1, was isolated from the hepatopancreas of diseased . This strain was identified as based on a comprehensive analysis of its morphological, physiological, and biochemical features and molecular identification. Challenge experiments were conducted to assess the pathogenicity of to . The results showed that the FSNM-1 strain had high virulence to with a median lethal dose (LD) of 1.1 × 10 CFU/mL. Histopathological analysis revealed that infection caused different degrees of inflammation in the hepatopancreas, gills, and intestines of . The detection of virulence-related genes revealed that the FSNM-1 strain carried colonization factor antigen (, ), ureases (, , , ), and outer membrane protein (), and virulence factor detection showed that the FSNM-1 strain had lecithinase, amylase, lipase, gelatinase, and hemolysin activities but did not produce protease and DNase activities. To investigate the immune response of to , the expression levels of , , , , , and immune-related genes were monitored at different points of time in the hepatopancreas, gills, intestines, and hemocytes of after infection. The results demonstrated a significant upregulation in the expression levels of the , , , , , and genes in at the early stage of infection. This study highlights as a major pathogen causing mass mortality in and provides valuable insights into its virulence mechanisms and the host's immune response.