Abstract
Vaccination is the most effective way to prevent diseases in aquaculture. Long-lasting immunity often requires the delivery of booster vaccines, yet the optimal interval time between prime and boost vaccination for fish vaccines is often not experimentally determined. The goal of this study was to compare three different prime-boost intervals (4, 8, and 12 weeks) in rainbow trout (Oncorhynchus mykiss) using a live attenuated infectious hematopoietic necrosis virus (IHNV) vaccine. Two independent experiments were conducted when fish were either 1000 or 1500 degree-days, dd. Fish received an immersion prime followed by intranasal boost at designated intervals and were challenged with IHNV 5 months after primary vaccination. Two different challenge pressures were used. For the 1000 dd fish, we used an LD50 for the mock-vaccinated group whereas for the 1500 dd fish it was an LD90. Following challenge at 1000 dd, the 8-week interval elicited optimal protection (95% survival), outperforming both shorter (90%, 4 weeks) and longer (75%, 12 weeks) intervals. Similarly, at 1500 dd, survival was highest (65%) in the 8-week interval group, compared to the 4- and 12-weeks interval groups (55% and 50%, respectively). Serum specific IgM titers as measured by ELISA supported these findings, revealing significantly higher specific IgM titers in the 8-week interval groups for both age classes. Our results demonstrate that an 8-week prime-boost interval maximizes the efficacy of mucosal vaccines against IHN regardless of the challenge dose and highlights the importance of testing interval doses in aquaculture vaccinology.