Immunogenic response and protective efficacy of feed-based lectin adjuvanted whole cell oral vaccine against Streptococcus agalactiae infection in Nile tilapia (Oreochromis niloticus)

The intensification of aquaculture practices has led to a surge in disease outbreaks, with Streptococcus agalactiae emerging as a significant pathogen in Nile tilapia (Oreochromis niloticus) farming, especially in Asia. Conventional vaccination techniques, including injection, have proven efficacious; however, they are labor-intensive and can be a source of stress for fish. In this study, a feed-based oral vaccine was developed and evaluated. This vaccine incorporated formalin-inactivated S. agalactiae with concanavalin A (ConA) as an immunostimulatory adjuvant. Non-specific immune responses, including lysozyme (LZM), myeloperoxidase (MPO), and superoxide dismutase (SOD) activities, were significantly enhanced in the vaccinated group (ORLV), particularly following booster doses. Specifically, the serum, skin mucus, and gut lavage fluid IgM titers of the ORLV group demonstrated a surge at 42 days post-vaccination (dpv), indicating robust mucosal and systemic immunity. Subsequent gene expression analysis of immune markers (TCRβ, IgM, MHCII, CD4, IL1β, and IL8) in spleen and head kidney tissue further confirmed heightened humoral and T-cell-mediated responses in the ORLV group. Most notably, the relative percent survival (RPS) post-challenge reached 67% in the ORLV group, which is significantly higher than in the control groups. These findings confirm that oral vaccines supplemented with lectin adjuvants can effectively enhance immune protection in tilapia, providing a practical, non-invasive, and sustainable approach to disease management in aquaculture.