Evaluation of the efficacy of lectin-adjuvanted Bivalent oral vaccine against Aeromonas veronii and Streptococcus iniae in Nile tilapia (Oreochromis niloticus).

Aeromonas veronii and Streptococcus iniae are emerging pathogens that cause co-infections and have been a significant threat to the productivity of Nile tilapia, causing high mortality. Vaccination is a promising strategy to prevent bacterial infectious diseases. Coinfection with these bacterial pathogens is widespread in Nile tilapia culture, highlighting the need to develop a bivalent vaccine. This study explores an innovative oral bivalent vaccination approach using a lectin (Concanavalin A from Canavalia ensiformis) adjuvant to combat A. veronii and S. iniae in Nile tilapia (Oreochromis niloticus). The vaccinated group fed with lectin-adjuvanted bivalent oral vaccine exhibited significantly higher levels of lysozyme, myeloperoxidase, and superoxide dismutase compared to the control group (p < 0.05). Furthermore, increased levels of specific antibody (IgM) in the vaccinated group, with notable differences between the prime and booster vaccinations against both antigens. Upregulation of the adaptive immune genes on the 56 days post vaccination (Dpv) in the spleen was observed, and the vaccinated group fed with the lectin-adjuvanted bivalent oral vaccine demonstrated high survival rates, with relative percent survival of 66.6% against A. veronii and 62.1% against S. iniae at 76 DPV. Furthermore, gut microbiome analysis of post-vaccinated and post-challenged fish revealed improved gut health in the vaccinated group, underscoring the protective and immunomodulatory potential of the vaccine. These findings highlight the potential of a lectin-based oral bivalent vaccine system as a novel and effective approach for aquaculture. This strategy could also be adapted for the development of vaccine against bacterial infections and therapeutics in aquatic species.