The genus Aeromonas has been recognised as an important pathogenic species in aquaculture that causes motile Aeromonas septicaemia (MAS) or less severe, chronic infections. This study compares the pathogenicity of the different Aeromonas spp. that were previously isolated from freshwater fish with signs of MAS. A total of 124 isolates of Aeromonas spp. were initially screened for the ability to grow on M9 agar with myo-inositol as a sole carbon source, which is a discriminatory phenotype for the hypervirulent A. hydrophila (vAh) pathotype. Subsequently, LD50 of six selected Aeromonas spp. were determined by intraperitoneal injection of bacterial suspension containing 103, 105, and 107 CFU/mL of the respective Aeromonas sp. to red hybrid tilapias. The kidneys, livers and spleens of infected moribund fish were examined for histopathological changes. The screening revealed that only A. dhakensis 1P11S3 was able to grow using myo-inositol as a sole carbon source, and no vAh strains were identified. The LD50–240h of A. dhakensis 1P11S3 was 107 CFU/mL, while the non-myo-inositol utilizing A. dhakensis 4PS2 and A. hydrophila 8TK3 was lower at 105 CFU/mL. Similarly, tilapia challenged with the myo-inositol A. dhakensis 1P11S3 showed significantly (p < 0.05) less severe signs, gross and histopathological lesions, and a lower mortality rate than the non-myo-inositol A. dhakensis 4PS2 and A. hydrophila 8TK3. These findings suggested that myo-inositol utilizing A. dhakensis 1P11S3 was not a hypervirulent Aeromonas sp. under current experimental disease challenge conditions, and that diverse Aeromonas spp. are of concern in aquaculture farmed freshwater fish. Therefore, future study is warranted on genomic level to further elucidate the influence of myo-inositol utilizing ability on the pathogenesis of Aeromonas spp., since this ability correlates with hypervirulence in A. hydrophila strains.

Vibrio spp. are important aquaculture pathogens that cause vibriosis, affecting large numbers of marine fish species. This study determines the field efficacy of a feed-based inactivated vaccine against vibriosis in cage-cultured Asian seabass. A total of 4800 Asian seabass, kept in a field environment, were separated equally into two groups (vaccinated and non-vaccinated) in duplicate. Fish of Group 1 were orally administered the feed-based vaccine on weeks 0 (prime vaccination), 2 (booster), and 6 (second booster) at 4% body weight, while the non-vaccinated fish of Group 2 were fed with a commercial formulated pellet without the vaccine. Fish gut, mucus, and serum were collected, the length and weight of the fish were noted, while the mortality was recorded at 2-week intervals for a period of 16 weeks. The non-specific lysozyme activities were significantly (p < 0.05) higher in the fish of Group 1 than the non-vaccinated fish of Group 2. Similarly, the specific IgM antibody levels in serum and mucus were significantly (p < 0.05) higher in Group 1 than in Group 2, as seen in the second week, with the highest level 8 weeks after primary immunization. At week 16, the growth performance was significantly (p < 0.05) better in Group 1 and showed lower bacterial isolation in the gut than Group 2. Despite the statistical insignificance (p > 0.05), the survival rate was slightly higher in Group 1 (71.3%) than Group 2 (67.7%). This study revealed that feed-based vaccination improves growth performance, stimulates innate and adaptive immune responses, and increases protection of cultured Asian seabass, L. calcarifer, against vibriosis.

Pangasius catfish, Pangasius nasutus, is a promising candidate for aquaculture due to its high market value. However, the presence of pathogenic bacteria in Aeromonas hydrophila is a major concern in P. nasutus farming in this country. This study determines the pathogenicity of A. hydrophila in P. nasutus. A total of 80 P. nasutus juveniles were intraperitoneally injected with 0, 103, 105, and 107 CFU mL-1 of A. hydrophila and monitored until 240 hr. The infected moribund fish’s kidneys, livers, and spleens were collected for histopathological analysis. The LD50-240hr value was found at 0.8 × 104 CFU/ml of A. hydrophila. The percentage of mortality in 0, 103, 105, and 107 CFU/ml infected groups were found to be at 0, 40, 60, and 90%, respectively. The infected fish showed congestion at the base of the fin, ascites, enlarged gall bladder, and swollen spleen. It is the earliest report on A. hydrophila’s pathogenicity in high-value native fish, P. nasutus.

Pangasius catfish, Pangasius nasutus, is a promising candidate for aquaculture due to its high market value. However, the presence of pathogenic bacteria in Aeromonas hydrophila is a major concern in P. nasutus farming in this country. This study determines the pathogenicity of A. hydrophila in P. nasutus. A total of 80 P. nasutus juveniles were intraperitoneally injected with 0, 103, 105, and 107 CFU mL-1 of A. hydrophila and monitored until 240 hr. The infected moribund fish’s kidneys, livers, and spleens were collected for histopathological analysis. The LD50-240hr value was found at 0.8 × 104 CFU/ml of A. hydrophila. The percentage of mortality in 0, 103, 105, and 107 CFU/ml infected groups were found to be at 0, 40, 60, and 90%, respectively. The infected fish showed congestion at the base of the fin, ascites, enlarged gall bladder, and swollen spleen. It is the earliest report on A. hydrophila’s pathogenicity in high-value native fish, P. nasutus.