Experimental Infectious Pancreatic Necrosis Virus infection via egg microinjection: effects on survival, behaviour, and early immune response in brook trout (Salvelinus fontinalis) and brook trout × rainbow trout hybrids (S. fontinalis × Oncorhynchus mykiss)

Viral infections remain a persistent challenge in salmonid aquaculture, with infectious pancreatic necrosis virus (IPNV) causing substantial economic losses worldwide. Interspecific crossbreeding has been explored as a strategy to increase disease resistance, but its effectiveness remains uncertain. In this study, brook trout (Salvelinus fontinalis) and brook trout × rainbow trout hybrids (S. fontinalis × Oncorhynchus mykiss) were experimentally infected with IPNV via a microinjection method to compare species-specific responses. Survival was assessed via Kaplan-Meier curves and Cox models. Behavioural changes, including locomotion, spatial preference, and social interactions, were analysed via automated tracking software. Gene expression of selected immune markers (IL-1β, IL-6, IL-8, TNFα, IFN2, IFNγ, and lysozyme type II) was quantified via RT-qPCR. Growth and morphological abnormalities were also examined to evaluate the physiological effects of infection. The survival of hybrid embryos decreased during incubation, suggesting increased vulnerability to developmental stressors. IPNV infection significantly increased post-hatching mortality, particularly in brook trout. Infection also altered behaviour in a species-specific manner: infected brook trout demonstrated erratic movement, avoidance behaviours, and reduced social interaction, whereas hybrids maintained more stable but reactive patterns. Gene expression profiling revealed that hybrids presented earlier immune activation, notably of IL-6 and IFN2, without improved survival. These findings indicate that interspecific hybridization does not confer consistent resistance to viral pathogens. The behavioural alterations observed during infection may serve as early indicators of disease, supporting their potential for real-time health monitoring in aquaculture. This study highlights important trade-offs between developmental acceleration and immune adaptation, with implications for hybrid viability and fish welfare management.