Juvenile olive flounder immersed in live VHSV at 17 °C and 20 °C shows resistance against VHSV infection at 10 °C.

Viral hemorrhagic septicemia (VHS) causes serious economic loss in olive flounder aquaculture industry in Korea. Water temperature is known to play a critical role in VHS disease outbreak. Here, we assessed the potential efficacy of VHSV immersion treatment in relation to resistance conferred at differential water temperatures in olive flounder. VHSV acquired resistance was compared between formalin-killed VHSV immersion treatment and live VHSV immersion treatment at three different water temperatures viz., 10 °C, 17 °C, and 20 °C. At 10 °C, cumulative mortality was around 80% in live VHSV immersed group while 30% cumulative mortality was observed in formalin-killed VHSV treated group. After 4 weeks, surviving olive flounder at 17 °C and 20 °C were challenged with VHSV at 10 °C following which the VHS outbreaks took place at host susceptible water temperature. For the pre-treated flounder at 17 °C, survival rates were 80% and 30% after challenge at 10 °C in live VHSV immersed group and formalin-killed VHSV immersed group, respectively. Whereas, the pre-treated flounder at 20 °C showed survival rate of 75% and 20% after challenge at 10 °C in live VHSV immersed group and formalin-killed VHSV immersed group, respectively. Our results propose the fact that live VHSV immersion using non-susceptible water temperature has the potential to protect olive flounder against VHSV infection. Moreover, the protective efficacy of live immersion treatment in a non-excited immune state without the use of an adjuvant combined with water temperature adjustment was investigated for the first time at 17 °C. Further studies should be targeted to explore the host-associated immune factors responsible for the protective effect and acquired resistance in olive flounder after live VHSV immersion treatment.