Properties of viral hemorrhagic septicemia virus (VHSV) isolated from Japanese flounder Paralichthys olivaceus

Physiological properties and pathogenicity of three isolates of viral hemorrhagic sepicemia virus (VHSV), Obama25 isolated from wild Japanese flounder Paralichthys olivaceus in 1999, JF00Ehi1 from farmed diseased Japanese flounder in 2000 and KRRV9601 from farmed diseased Japanese flounder in 1996, were studied. The former two isolates belong to American genotype (genogroup 1) and the latter one to European genotype (genogroup 111). Among three fish cell lines tested, the three isolates multiplied best in FHM cells, moderately in EPC cells but hardly in RTG-2 cells. In FHM cells, these isolates multiplied at 10, 15, and 20degreesC but not at 25degreesC. The optimum temperature was 15degreesC for KRRV9601 and 20degreesC for the other two isolates. In in vitro stability tests in different waters at different temperatures, the viral infectivity decreased rapidly in non-treated seawater with increasing water temperature. The two American genotype isolates, Obama25 and JIF00Ehi1, exhibited a similar level of virulence in flounder at 13degreesC causing disease signs similar to those observed in naturally affected flounder. On the other hand, no mortality was produced by KRRV9601, though the virus was re-isolated from surviving fish. These results in the pathogenicity test support the fact that the American genotype of VHSV has been prevailing among wild and farmed flounder in Japan.