Comparison of susceptibility in olive flounder (Paralichthys olivaceus) and rainbow trout (Oncorhynchus mykiss) against viral hemorrhagic septicemia virus (VHSV) genotypes I, II, III, IVa, and IVb

Viral hemorrhagic septicemia (VHS) is a significant disease affecting farmed rainbow trout and olive flounder. Of the four major viral hemorrhagic septicemia virus (VHSV) genotypes (I–IV), genotype IVa is the only genotype reported in East Asia. However, increasing international trade in aquatic animals poses a potential risk of introducing other VHSV genotypes into the region. To assess this risk, we compared the susceptibility of olive flounder and rainbow trout to infection with VHSV genotypes I, II, III, IVa, and IVb using in vitro cell culture assays and in vivo challenge experiments with intramuscular injections of 10 3 , 10 4 , and 10 5 copies/fish. In rainbow trout, genotype I caused high mortality (>90%) at all challenge doses, while genotype IVa caused dose-dependent mortality ranging from 36% to 92%. Genotypes III and IVb caused relatively low mortality, whereas genotype II caused no mortality. In olive flounder, genotypes III, IVa, and IVb caused high mortality, genotype I caused up to 92% mortality, and genotype II caused no mortality. Tissue viral loads generally reflected the genotype-dependent differences in mortality, and in vitro infectivity in fish cell lines showed a similar pattern to in vivo pathogenicity. These findings suggest that cell culture-based infectivity assays may serve as a useful preliminary screening tool for estimating the potential pathogenicity of VHSV isolates. Overall, VHSV genotypes I, III, and IVb may pose a potential threat to the olive flounder industry in Asia, whereas genotype IVa may pose a risk to rainbow trout production outside Asia.