The persistence of viral hemorrhagic septicemia virus (VHSV) mRNA in juvenile olive flounder, Paralichthys olivaceus following live VHSV vaccination

Several studies have been conducted on vaccines for viral hemorrhagic septicemia virus (VHSV) in order to prevent VHSV infection. Effective live VHSV vaccines have been reported for suboptimal temperatures (17–20 °C). Previous strategies for live vaccine immunization periods required more than two weeks for fish to acquire resistance. In our study, we demonstrate the efficacy of a 10-day live VHSV vaccine immunization period at 17 °C. We also investigated the presence of viral mRNA genes in vaccinated juvenile olive flounder after challenge at 10 °C using RNA in situ hybridization (ISH) and quantitative polymerase chain reaction (qPCR). The live VHSV-vaccinated flounder exhibited a 100% survival rate for 20 days post-challenge (dpc) at 10 °C, whereas all non-vaccinated flounder had perished by 13 dpc. Viral mRNA was detected in the spleen and heart tissue of vaccinated flounder through RNA ISH visualization, whereas no signal was observed in the kidney. The viral mRNA copy number in internal organs (heart, kidney, and spleen) of vaccinated flounder was lower than that in non-vaccinated flounder. These findings support the effectiveness of a 10-day rearing period following live VHSV vaccination and suggest its potential for shortening the VHSV vaccine program.