Implementation of formalin-inactivated Streptococcus agalactiae vaccine in Nile tilapia Oreochromis niloticus broodstock: Efficient vaccination regime, antibody response, and immune-related genes

Nile tilapia broodstock were given formalin-killed Streptococcus agalactiae vaccine. Group A received a single dose, and four other groups (groups B, C, D, and E) received a booster dose with intervals of 2, 4, 12, and 24 weeks post-primary vaccination (ppv), respectively. The specific antibody titers from each vaccinated group increased within 2–4 weeks. At 8 weeks ppv, groups A, B, and C demonstrated relative percentage survival (RPS) of 88.8%, 92.6%, and 92.6%, respectively. At 16 weeks, RPS for groups A, B, and D were 60.7%, 60.7%, and 88%, while group C maintained an RPS of 92.3%. At 28 weeks, groups A and B exhibited significantly lower antibody titers (log 10 < 2) and reduced RPS (40.7% and 44.4%) compared to the booster groups (66.6–92.5%). These findings suggest that booster vaccinations at 4 or 12 weeks ppv can prolong the adequate protection titer in the broodstock at least 28 weeks ppv. Moreover, the expressions of both innate ( IL-1β, IL-4/13B, IL-10, CCL4, MHC-I , and MHC-IIβ ) and adaptive immune genes (IgM, CD4, and CD8-α ) were significantly upregulated within 12 h post-challenge (hpc) in the vaccinated fish, suggesting the vaccine efficacy to prime the immune system for robust response upon pathogen exposure.