Vaccine-induced antibody level as the parameter of the influence of environmental salinity on vaccine efficacy in Nile tilapia

To effectively increase production and improve economic returns, the co-culture of Nile tilapia (Oreochromis niloticus) and marine shrimp has been adopted in many countries, including China. Although O. niloticus is an euryhaline fish that can tolerate elevated salinities and even full-strength seawater, fluctuations in salinity levels can undoubtedly induce stress and affect the immune response of this fish. Therefore, this study assessed the impact of salinity on vaccine efficacy in Nile tilapia, which used serum antibody level as a surrogate marker to detect vaccine efficacy. Nile tilapia were acclimatized to 0, 10, 20, or 30 ppt salinity, and then immunized with a formalin-inactivated Streptococcus agalactiae vaccine. Significantly lower levels of antibody in vaccinated fish were found at 20 and 30 ppt salinity compared to 0 and 10 ppt salinity. White blood cell counts, absolute blood lymphocyte counts, and serum bactericidal activity levels were all significantly lower in vaccinated fish at 20 and 30 ppt salinity. Elevated cortisol levels were detected in all of the fish exposure to salinity. Concentrations of serum electrolytes (Na+ and Cl−) were significantly higher in fish at 30 ppt salinity, as compared to fish at lower salinities. Furthermore, the mRNA transcription levels of three of the immune-related genes analyzed (IgM, IL-1β, and IFN-γ, but not Hsp70) were significantly inhibited in the vaccinated fish at 20 and 30 ppt salinity. A suppressed immune response and decreased vaccine efficacy were also indicated by the lower survival rate of vaccinated fish at 20 ppt salinity when challenged with S. agalactiae. Therefore, salinities ≥20 ppt negatively affected antibody production in Nile tilapia, ultimately affecting vaccine efficacy.