Protective efficacy of a squalene adjuvanted oral inactivated vaccine against Streptococcus agalactiae in Nile tilapia.

Streptococcus agalactiae is one of the bacterial pathogens that leads to high rate of mortality and economic loss in the aquaculture of Nile tilapia (Oreochromis niloticus). Vaccination is an effective disease control strategy particularly as oral vaccination has gained more attention due to its field-friendly approach. This study was aimed to develop an oral inactivated vaccine using squalene as an adjuvant to increase the efficacy of vaccines for better immune response. Formalin inactivated vaccine for S. agalactiae was prepared which was incorporated into commercial feed pellets. Fish were divided into four groups; control (PBS coated feed), vaccine only (formalin- inactivated vaccine), adjuvant only (squalene) and squalene adjuvanted vaccine. Immunological parameters including superoxide dismutase (SOD), lysozyme (LYZ), myeloperoxidase (MPO), catalase (CAT) and malondialdehyde (MDA) showed elevated response in the vaccinated group. The specific IgM antibody levels was significantly improved in the vaccinated groups. A good level of protection after S. agalactiae challenge was noticed at a range from 66% to 40% in the vaccinated and control groups respectively. IgM, TCR-β, MHC-II, IFN-γ, IL6, IL8 genes were significantly altered in the vaccinated fish. The results showed that adjuvanted oral vaccination has significantly improved both innate and humoral immune responses indicating an improved protection using squalene as adjuvant in oral vaccine delivery route for sustainable aquaculture.