Influence of temperature on the vaccine efficacy against Streptococcus agalactiae in Nile tilapia (Oreochromis niloticus)

Abstract

Temperature is an important external parameter which can affect the immune response in fish. Although Oreochromis niloticus can tolerate a wide range of temperature, the immunoreactions are undoubtedly affected when the fluctuations in temperature levels are beyond the optimum growth temperature. This study aims to assess the effects of temperature on the vaccine efficacy of O. niloticus vaccinated with inactivated Streptococcus agalactiae. O. niloticus were reared at 21, 25, 29, and 33 °C, and then immunized intraperitoneally with inactivated S. agalactiae vaccine. Among the vaccinated fish, total WBC counts, absolute counts of neutrophil, monocyte, and lymphocyte, respiratory burst activity level, serum bactericidal activity level, and antibody level were lowest at 21 °C. Significantly lower level of antibody and significantly lower counts of lymphocyte were also found in vaccinated fish reared at 33 °C, as compared to vaccinated fish reared at 25 and 29 °C. In fish vaccinated at 21 and 33 °C, four of immune genes (IgM, IL-1β, TNF-α, and IFN-γ) expression levels were significantly inhibited. Furthermore, the lower survival rate (70.00 ± 4.08%) after challenged with S. agalactiae in fish vaccinated at 21 °C also further indicated that innate and adapted immune responses were weakened when vaccinated fish were reared at 21 °C. Although the survival rate of vaccinated fish reared at 33 °C (81.67 ± 6.24%) was lower than those of vaccinated fish reared at 25 °C (91.67 ± 2.36%) and 29 °C (90.00 ± 4.08%), there were no significant differences in survival rate among vaccinated fish reared at 25, 29, and 33 °C. These results suggest that the immune responses and vaccine efficacy following vaccination were markedly suppressed in vaccinated O. niloticus reared at 21 °C.