Abstract
Aeromonas hydrophila and Aeromonas caviae are two economically significant pathogenic bacteria in aquaculture and are the main causes of bacterial septicemia in fish. Controlling disease caused by these two bacterial pathogens is becoming a greater challenge due to increases in the antibiotic resistance of bacteria. Here, we developed and applied novel DNA vaccines carrying antigen genes for ovalbumin (OVA) and the outer membrane protein (OMP) of A. hydrophila to evaluate their immunoprotective effects against these two Aeromonas species and promote the green development of the aquaculture industry. Sinocyclocheilus grahami ( S. grahami ) and Oreochromis niloticus ( O. niloticus ) were subjected to oral vaccination treatment for a total of 21 days. Seven days after being injected with 100 μL of 1 × 10 9 CFU/mL of A. hydrophila or A. caviae , significant decreases were observed in the following non‐specific immune indicators: superoxide dismutase (SOD), complement C3 (C3), lysozyme (LZM) and immunoglobulin M (IgM). However, levels of SOD, C3, LZM and IgM remained stable in fish serum following administration of the OVA, OMP and OVA‐OMP vaccines. Histopathological analysis indicated that oral vaccination with the OVA vaccine could alleviate the splenic lesions caused by A. hydrophila and A. caviae in S. grahami . After challenge with A. hydrophila , the immune protection rates of the OVA, OMP and OVA‐OMP vaccines in S. grahami were 92.56%, 88.89% and 73.33%, respectively, and the relative percent survival (RPS) of O. niloticus was 75%, 83.38% and 50%, respectively. Following challenge with A. caviae , the RPS values for S. grahami vaccinated with the OVA, OMP and OVA‐OMP vaccines were 100%, 86.96% and 95.70%, respectively, and the RPS values for O. niloticus were 92.27%, 96.19% and 92.27%, respectively. In conclusion, oral administration of the OVA, OMP and OVA‐OMP DNA vaccines significantly enhanced both non‐specific and specific immune responses, providing strong protection against A. hydrophila and A. caviae infections in S. grahami and O. niloticus . Future work should focus on optimizing vaccine stability (e.g., improving the OVA‐OMP fusion construct), expanding coverage to other economically important and endangered fish species and clarifying long‐term immune memory mechanisms to support sustainable aquaculture development.