Protection of olive flounder, Paralichthys olivaceus, against viral hemorrhagic septicemia virus (VHSV) by immunization with NV gene-knockout recombinant VHSV

The potential of the NV gene-knockout recombinant viral hemorrhagic septicemia virus (rVHSV-ΔNV-EGFP) as a live attenuated vaccine was assessed. In an in vivo virulence experiment, olive flounder (Paralichthys olivaceus) intramuscularly (i.m.) injected with 103 and 105 PFU of rVHSV-ΔNV-EGFP showed no mortality. The rVHSV-ΔNV-EGFP had ability to spread and replicate in the internal organs of olive flounder reared at either 15 °C or 20 °C. In the vaccination experiment, the cumulative mortality rates of olive flounder i.m. immunized with 103, 104, and 105 PFU of rVHSV-ΔNV-EGFP were 40–50%, 10–30%, and 0–10%, respectively, at 4 weeks after the primary immunization. In the second challenge experiment conducted at 4 weeks after the boost immunization, the cumulative mortality in fish immunized with 103 PFU of rVHSV-ΔNV-EGFP was 10–30%. No mortality was recorded in fish immunized with 104 and 105 PFU of the recombinant virus. While the cumulative mortality of fish in control group was 90–100%. The serum neutralization titers were also proportional to the survival rates of vaccine groups. The present results suggest that the rVHSV-ΔNV-EGFP has a high potential to be used as a live vaccine in olive flounder farms.