Priming route and age at first vaccination shape prime and boost vaccine responses in rainbow trout (Oncorhynchus mykiss).

Abstract

Infectious Hematopoietic Necrosis Virus (IHNV) is a significant economic threat to salmonid aquaculture worldwide. Understanding the factors that influence vaccine effectiveness is essential for controlling current and future IHN outbreaks. Because long-term protection often requires prime-boost vaccination, optimizing each vaccine dose is critical. This study evaluated the protective efficacy of different homologous prime-boost vaccination regimes in rainbow trout 5 months after primary vaccination using a live attenuated vaccine against IHNV. Rainbow trout (1000 or 1500 degree-days, DD) were primed intranasally (IN) or by immersion (Imm) vaccination, and boosted 28 days later via IN, Imm, or intramuscular injection (IM). Fish were challenged 5 months post-primary vaccination with virulent IHNV (500 PFU/fish for 1000 DD and 1000 PFU/fish for 1500 DD), and survival and specific serum IgM antibody titers were measured. Results show that the IN route consistently provided superior survival against IHNV, with the IN/IN prime-boost group showing the highest survival rates (97-98%) of all vaccine combinations tested, regardless of fish age but differences were only significant in the 1500 DD age group. When 1500 DD trout were prime vaccinated by Imm, boosting by IM injection resulted in significantly higher survival compared to IN and Imm boosters. Specific serum IgM titers were highest in the IN/IN prime-boost group and lowest in the Imm/Imm groups, often mirroring vaccine efficacy. Age-dependent differences in specific humoral immune responses were observed with significantly higher specific IgM antibody titers in 1000 DD compared to 1500 DD fish in the IN/IN and Imm/IN groups. Our study reveals that both priming route and age at first vaccination are important determinants of prime-boost efficacy, providing practical guidance for IHNV vaccination in rainbow trout aquaculture with potential applications to other host-pathogen systems.