Abstract
Effective prime-boost vaccination is critical for generating robust immunity against aquatic pathogens such as infectious hematopoietic necrosis virus (IHNV). The optimal combination of administration routes for priming and boosting, however, is often unknown. The goal of this study is to evaluate six different prime-boost combinations in rainbow trout (Oncorhynchus mykiss) at two developmental stages (1000 and 1500 degree-days, DD) using a live attenuated IHNV vaccine. Fish (n=300/group; n=3 tanks/group) were divided into the following groups: G1 (control), G2 (IN/IN), G3 (IN/IMM), G4 (IN/IM), G5 (IMM/IM), G6 (IMM/IMM), and G7 (IMM/IN), where IMM (immersion), IN (intranasal), and IM (intramuscular). Specific IgM titers were measured 5 months after prime vaccination, and fish were challenged with IHN at this time point. At 1000 DD, IN priming consistently enhanced protection (G2-G4: 85–95%) compared to IMM priming (G5-G7: 75–90%), with the highest survival (95%) occurring after IN prime/boost (G2). Similar results were observed in 1500 DD trout, where IMM priming, regardless of the boosting route, always showed the lowest protection among all groups. Survival outcomes strongly correlated with humoral immunity, as ELISA revealed significantly higher serum antibody titers in intranasal-primed groups and the lowest titers in all IMM-primed groups. Our findings demonstrate that mucosal priming (intranasal) is superior to immersion for inducing immunity against IHNV, regardless of the age at first vaccination.