Systemic and mucosal antibody response of freshwater cultured Asian seabass (Lates calcarifer) to monovalent and bivalent vaccines against Streptococcus agalactiae and Streptococcus iniae.

Asian seabass, Lates calcarifer farming in Southeast Asia, encounters serious disease challenges caused by Streptococcus agalactiae and Streptococcus iniae. However, a vaccine for disease prevention is not yet available. In this study, we investigated the mucosal and systemic antibody (IgM) response kinetics of the Asian seabass following primary immunization with oil-based formalin-killed vaccines (FKVs) prepared from S. agalactiae and S. iniae (monovalent Sa, monovalent Si, and bivalent Sa-Si) and secondary booster with the respective water-based FKVs. The efficacy of vaccines was subsequently evaluated by an experimental challenge. The results revealed similar antibody response kinetics in both systemic and mucosal systems. However, the immune response in the fish vaccinated with the monovalent vaccines was superior to those fish received the bivalent vaccine in terms of specific antibody titer. The fish that received monovalent vaccines required 1-2 weeks to raise a significant level of specific antibody titer in both systemic and mucosal systems while those vaccinated with bivalent vaccine required three weeks. Following booster at day 21, both systemic and mucosal antibody titers in all vaccinated groups had reached the peak at day 28 and gradually declined in the following weeks but remained significantly higher than control until the end of the experiment (day 63). In the challenge test, both monovalent and bivalent vaccines were found to be highly efficacious, with the relative percentage survival (RPS) ranging from 75 to 85%. In summary, this study explored the 63-days antibody response kinetics (both mucosal and systemic systems) of Asian seabass to monovalent and bivalent inactivated vaccines and confirmed that the combination of S. agalactiae and S. iniae in a single injectable vaccine is possible.