Pathogenicity and inactivated vaccine treatment of Aeromonas veronii JW-4 on crucian carp

Aeromonas veronii is a common bacterium found in a variety of aquatic environments, capable of causing a diverse array of diseases in both aquatic animals and humans. Therefore, evaluating the pathogenicity of A. veronii and implementing measures to control its spread are essential. In this study, a strain JW-4, identified as A. veronii, was isolated from diseased Scaphesthes macrolepis, a grade II protected animal in China. To investigate the pathogenicity of the strain, fish were fed with serial levels JW-4 supplemented diet or basal diet (control group 1, CG1) for 28 days (d). Results showed that JW-4 stimulated an immune response, evidenced by an increase in immune-related enzyme activities (GOT and GPT) of serum and liver and an upregulation of genes expression levels (TNF-alpha and IFN-gamma) of liver and spleen, and these effects gradually decreased over time. Histopathological examination revealed that JW-4 could alter the tissue structure of immune organs, such as liver and kidney. These changes were accompanied by vacuolar degeneration, nuclear dissolution, and an increased lymphocyte count. To assess protective effects of a vaccine against this strain, fish were injected with an inactivated vaccine (immunization group, IG) or 0.85% sterile saline (control group 2, CG2) for 28-day observation period, then challenged with JW-4 on the 28th day. The inactivated vaccine enhanced total and specific IgM to A. veronii levels of the fish, resulting in a relative percentage survival of 75% in IG. These findings provide a foundation for identifying pathogenic bacteria and developing more effective prophylactic strategies in aquaculture.