Immunogenicity and protective efficacy of chitosan/β-glucan nanoparticle encapsulated inactivated Streptococcus agalactiae oral vaccine for Nile tilapia (Oreochromis niloticus)

• Novel oral vaccine delivery material for aquaculture, using chitosan/ β-glucan conjugate nanoparticles. • Chitosan/β-glucan nanoparticles significantly boost immunity in Nile tilapia. • Elevated level of immune genes in the primary lymphoid organ was observed. • The vaccine reduces mortality and disease severity against Streptococcus agalactiae. In recent decades, high disease outbreaks in Nile tilapia, mainly in low- to middle-income countries, have affected the economic grade and created a threat of antimicrobial resistance (AMR). Overusing antibiotics leads to water contamination and creates a pathway for AMR. To overcome the AMR, a vaccine is a very effective measure to save cultured aquatic crops. Streptococcus agalactiae is a group B Streptococcus causing high mortality in reared Nile tilapia. Due to antigen instability and degradation, oral vaccine delivery in fish has several limitations, but oral delivery of the vaccine is an easy and non-invasive method to immunize fish. The work is designed to administer an encapsulated inactivated vaccine with the Chitosan/β-glucan through commercial feed. Formalin-inactivated vaccine feed was fed to fingerling-size Nile Tilapia for 7 days, following a booster dose for 7 days. In between the initial vaccination and the booster dose, non-vaccinated feed was fed for 7 days. On 1,2,3,4-week post-vaccination, blood samples were collected from the vaccinated fish, and innate immune responses were evaluated by lysozyme activity, myeloperoxidase, antiprotease activity, and superoxide dismutase activity. The titre value of IgM in serum was determined by the Indirect ELISA method. RT-qPCR was performed to evaluate gene expression in the primary lymphoid organ, spleen, on 14- and 28-day post-vaccination. Vaccinated fish were challenged with the LD 50 of S. agalactiae on the 30 th day post-vaccination (D.p.v) to check the ability of the orally administered vaccine by calculating the RPS (Relative percentage survival) on 15 days of challenging. The results indicate a survival rate ranging from 33.33 % to 75.56 % across Groups 1 to 4, respectively. These findings demonstrate that the highly stable, environmentally friendly encapsulated oral vaccine effectively upregulates the innate immune response and stimulates specific antibody production against bacterial pathogens, resulting in an improved survival rate.