Effect of water temperature shifting on mortality of Japanese flounder Paralichthys olivaceus experimentally infected with viral hemorrhagic septicemia virus

Viral hemorrhagic septicemia virus (VHSV) infection has been one of the major obstacles for Japanese flounder Paralichthys olivaceus culture both in inland tank systems and open sea net-pens in Japan. The mortality due to the disease in cultured Japanese flounder occurs in the cold-water season at temperatures mostly below 15 °C. We examined the effect on the disease by shifting the rearing-water temperature to a higher temperature (20 °C) as a potential control measure. Japanese flounder reared in aquariums were infected with a VHSV isolate JF00Ehi1, and the water temperature was shifted from 14 °C to 20 °C (Experiment I) and from 20 °C to 15 °C (Experiment II). In Experiment I, earlier shifting resulted in lower cumulative mortality rates. In Experiment II, the group of fish reared for a longer period at 20 °C showed lower cumulative mortalities. In a third set of experiments, fish inoculated with the virus were reared at 20 °C or at 25 °C (a viral non-permissive temperature), observing no mortality in either group. Viral multiplication was detected at a low level in the fish at 20 °C but not in those at 25 °C. At 21 dpi, these fish were challenged with the virus and reared at 14 °C, yielding no mortality in the 20 °C group, and 92% in the 25 °C group. These results indicate that rearing at 20 °C reduced mortality in the Japanese flounder experimentally infected with VHSV, probably inducing protective immune response against subsequent infection.