The efficacy of new oral vaccine feeds against Salmonid novirhabdovirus in rainbow trout

Salmonid novirhabdovirus (IHNV) causes infectious haematopoietic necrosis (IHN) in salmonid species. Despite an injectable plasmid-based DNA vaccine of the glycoprotein (G) gene is effective, there are no oral vaccines for mass vaccination of rainbow trout (Oncorhynchus mykiss) fry. Recombinant baculoviruses were generated, used in cabbage looper (Trichoplusia ni) insect larvae to produce IHNV G and IHNV G-C5a proteins. Western blotting and chemiluminescence assays confirmed the expression of recombinant proteins, which were added to the fish feeding and top-coated with unflavored gelatin binder. Commercial rainbow trout were fed with experimental diets containing either IHNV G or IHNV G-C5a proteins for 2 weeks, and boosted 4 weeks after. Four weeks post-booster, fish were challenged with IHNV by immersion. Survival upon the infection challenge was evaluated. Spleen were sampled at 7 and 14 days post infection (dpi). Non-vaccinated and IHNV G fed trout reached a mortality of 91.7 and 97.6%, and 70.9 and 88.4%, respectively at 8 and 15 dpi. The IHNV G-C5a fed group exhibited a reduced mortality of 51.2% at 8 dpi, reaching 81.7% at 15 dpi, suggesting some level of antiviral protection. The individual viral load was measured by RT-qPCR detection of IHNV N gene, showing no significant difference across experimental groups. The transcription modulation of selected immune response markers was evaluated across experimental groups, including Type I IFN-a, Mx-1, CD4, and IgM. Further study is needed to assess how new oral vaccines may become effective to mitigate IHNV pathogenesis in juvenile trout by modulating the host immune response to protect towards IHNV exposure.