Abstract
Mucosal vaccination, which has the potential to induce both mucosal and systemic immune responses, is considered the most suitable method of preventing infectious diseases in farmed fish. Aeromonas veronii and Edwardsiella ictaluri are two pathogenic bacteria found in yellow catfish and often infect the fish through mucosal surfaces. Delivery of a bivalent inactivated vaccine by injection has been shown to induce a strong systemic immune response against both bacterial infections. However, mucosal immune responses and protective efficiency induced by this inactivated vaccine administrated via immersion are yet to be investigated. We developed a bivalent vaccine containing formalin-inactivated A. veronii and E. ictaluri and evaluated the immune response in yellow catfish after immersion vaccination using body fluids biochemistry indices, agglutinating antibody titers, and the expression level of immune-related genes in the skin, gills, spleen, and head kidney. The activities of innate immune-related enzymes and specific agglutination antibody titers in body fluids, as well as the expression of innate and adaptive immune-related genes in both the mucosal and systemic tissues of vaccinated fish, were significantly higher than that in control fish. Next, we assessed the protective efficacy by a challenge model of virulent strains of E. ictaluri and A. veronii. The relative survival percent of vaccinated fish was 80% and 87% after challenging fish with E. ictaluri and A. veronii, respectively, which was higher than unvaccinated control fish (43% and 57%). These results confirm that the bivalent inactivated vaccine administered via immersion induces a strong mucosal immune response and confers good protection against both E. ictaluri and A. veronii. Our results also reinforce the notion that immersion vaccination could stimulate both mucosal and systemic immunity contributing to protection against pathogens.
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