Acute septicemia and diagnostic evaluation of Aeromonas veronii infection in American bullfrogs (Aquarana catesbeiana)

Abstract

In this epidemiological investigation of farmed bullfrogs spanning over 2 years, multiple batches of symptomatic American bullfrogs exhibiting red-leg syndrome and abdominal distension were identified across several farms. Subsequently, 12 type strains of Aeromonas veronii were isolated and identified from naturally infected bullfrogs. To assess the pathogenicity of these strains, 10 known virulence factors associated with high pathogenicity in A. veronii were examined. Among them, strain V10 possessed all 10 virulence genes and was selected as the key strain for subsequent artificial infection experiments. Infection assays on 30 bullfrogs, segmented by varying V10 strain doses (10 frogs/dose), indicated a survival of only 4, with 100% mortality at 1 × 108 CFU/mL. Additionally, all strains displayed resistance to vancomycin, sulfamethoxazole-trimethoprim (SMZ-TPM), lincomycin, penicillin, and ampicillin. Bacterial load assessment of blood and tissue samples indicated that the bacteria rapidly disseminated to the liver, spleen, and kidneys following host infection. Histopathological examinations indicated damage and inflammatory responses in the liver, spleen, kidney, and intestines. Furthermore, Hematological tests showed declines in white cells, hemoglobin, and platelets. Within 48 h post-infection, there was an upregulation of certain pro-inflammatory genes(C3, CXCL10, IL-1β, IL-8) and downregulation of anti-inflammatory(IL-4, IL-10) and antioxidant gene(GSH-px). These rerults indicate that infected bullfrogs can be diagnosed with acute septicemia. This is the first report on pathogenicity of A. veronii in bullfrogs, and it highlights its widespread prevalence and high resistance in farmed bullfrogs. These results have substantial implications for the prevention and treatment of A. veronii infections in aquaculture practices.