Evaluation of humoral immunity and maternal antibody transfer in Nile tilapia (Oreochromis niloticus) broodstock following immunization with a bivalent vaccine

Abstract

Aeromonas veronii and Streptococcus agalactiae are significant bacterial pathogens threatening Nile tilapia (Oreochromis niloticus) aquaculture globally. Coinfections with these pathogens are prevalent in aquaculture environments, underscoring the need for the application of a bivalent vaccine. This study investigated the humoral immune response kinetics of Nile tilapia broodstock post vaccination, examined maternal antibody (immunoglobulin M-IgM) transfer to their offspring and assessed the vaccine's protective efficacy through a passive immunization and pathogen challenges. Broodstock were immunized with an optimized bivalent vaccine, followed by two booster injections, and serum IgM levels were monitored using the enzyme-linked immunosorbent assay (ELISA). The results revealed that the vaccinated fish elicited specific IgM against both pathogens, with higher levels of anti-A. veronii antibodies compared to anti-S. agalactiae antibodies following primary and booster doses. The detection of specific IgM in fertilized eggs indicated the transfer of maternal immunity. However, the decline of these antibodies to undetectable levels in fry following yolk sac absorption indicated a transient persistence of the transferred antibodies. In the passive immunization study, fish administered sera from vaccinated broodstock demonstrated relative percent survival (RPS) of 35.7 % and 83.3 % against A. veronii and S. agalactiae, respectively. These results implied the substantial contribution of humoral immunity in defense against both pathogens. In conclusion, the findings of this study provided valuable insights into the humoral immune response in tilapia broodstock and confirmed maternal immunity transfer following bivalent vaccination. Broodstock vaccination may therefore serve as a promising biosecurity strategy to prevent infection in broodstock and enhance fry immunity, warranting further investigation.