Protection of Nile tilapia against Aeromonas hydrophila using a cobalt oxide nanoparticle vaccine containing inactivated whole cell bacteria

Aeromonas hydrophila is a prevalent bacterial pathogen causing disease outbreaks in tilapia aquaculture. Vaccination has proved to be effective in protecting farmed tilapia against infectious diseases. In this study, we aimed to assess the effectiveness of a novel nanovaccine in protecting Nile tilapia (Oreochromis niloticus) against A. hydrophila using different routes of administration. The vaccine consisted of cobalt oxide coupled to formalin-killed A. hydrophila. The biophysical characterisation of the synthesized nanovaccine containing formalin-killed A. hydrophila was examined with transmission electron microscopy and by establishing its zeta potential. The particles were found as aggregates or were dispersed throughout the water. The nanovaccine was administered to fish by intraperitoneal injection (IP), immersion and oral delivery, followed by a booster dose 21 days post-vaccination (dpv) for the vaccinated groups. The vaccinated fish were challenged with an autologous strain of A. hydrophila at 36 dpv and monitored for three weeks to assess the development of a clinical infection. Histopathological examination of tissues from immunized fish showed infiltration of lymphocytes, suggestive of gill and gut mucosal immune responses. RT-PCR analysis of spleen from all vaccinated fish showed downregulation MHC-I, IgM, and IFN-γ expression relative to unvaccinated fish at 36 dpv. The relative percentage survival (RPS) for the group given the nanovaccine by IP injection was 66.7 %, which was higher than the group receiving the inactivated vaccine in adjuvant by IP injection, with an RPS of 44.4 %. The groups that received the nanovaccine via immersion and orally had RPS values of 11.1 % and 22.2 %, respectively. Significant levels of specific IgM were seen in fish vaccinated with the nanovaccine by IP injection at 36 dpi. In conclusion, the nanovaccine showed potential to confer immune protection against A. hydrophila infection when administered by IP injection, but not when administered by immersion or orally.