Variable Infection Outcomes in Repeated Experimental Challenges of Atlantic Salmon ( Salmo salar ) With an Infectious Salmon Anaemia Virus Isolate

ABSTRACT Infectious salmon anaemia (ISA) affects Atlantic salmon ( Salmo salar ) and is caused by the ISA virus (ISAV). The disease is associated with ISAV strains with deletions in the hemagglutinin‐esterase protein, named HPRΔ variants. The present study was initially designed as a vaccine efficacy study against ISA. Considerable variability in mortality in virus‐injected fish was observed in repeated experimental challenges using the NO/Finnmark/NVI‐70‐72/2020 isolate. Four independent experimental challenge trials were conducted. In two experiments, high cumulative mortality was seen in intraperitoneally (i.p.) injected fish. In contrast, considerably lower cumulative mortality was observed in i.p. injected fish in two other experiments. Mortality among cohabitant fish was moderate in one experiment and low or absent in two others. The observed inconsistency precluded reliable assessment of vaccine efficacy and implied investigation into factors underlying the outcome reproducibility between the experimental challenges. To investigate the observed variation in outcomes, fish, virus, and physical conditions were compared in detail. One notable difference among the challenge experiments was tank volume. Mortality was higher among fish injected with virus in smaller tanks than in larger ones. Further examination of the physical and biological variables failed to identify any other definitive determining factors. In addition, to investigate whether the comparatively delayed mortality induced by the injected NO/Finnmark/NVI‐70‐72/2020 isolate was associated with early virus–host interactions, a time‐course study was conducted to quantify relative viral RNA levels and innate immune responses in ASK‐II cells. A highly virulent isolate induced greater expression of innate immune response genes, which may be attributable to the higher amount of viral RNA present in the inoculum despite comparable levels of infectious virus. Collectively, these findings suggest that subtle differences in experimental conditions may contribute to variation in disease outcome and indicate that further research is required to improve understanding of fundamental ISAV biological characteristics.