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The viral hemorrhagic septicemia virus (VHSV) is a viral pathogen to cause severe losses in farming industry of olive flounder. Among six-encoding VHSV proteins, NV protein has been shown to have an impact on virulence. Thirteen VHSV strains are isolated from olive flounders showing VHS symptoms and mortality in Korea, and then molecular variation of nucleotide and amino acid sequences of the NV genes were compared. Although common mutations were detected at four sites in nucleotide sequences of NV gene, there was no difference in the amino acid sequences among 13 VHSV variants. We identified substitution of one base of 34 positions within the NV ORF of 13 VHSV isolates and these point mutations resulted in changes in amino acid sequences at 6 residues of NV (8, 56, 81, 88, 117, and 119). Based on finding of NV-mediated decrease of intracellular ATP level, protein expression of six NV variants in flounder cells decreased more than control NV protein. The strongest decreasing activity on cellular ATP level was shown by No. 56 (serine → leucine) of NV variation. The variation changes the hydropathy index significantly. The results imply that we can use appearance of some amino acid variations as virulence markers in VHSV-derived mortality. Statement of relevance 1. Recently VHSV has infected > 80 kinds of fish species. The number of infection host of VHSV is rapidly increased. Among the VHSV-infected fishes, olive flounder is one of the most important fish of aquaculture in Korea, Japan, and China. In every year, we found the massive VHSV-derived mortality of olive flounder aquaculture in Korea. To overcome this VHSV pathogenesis, we need to identify high-pathogenic VHSV appearance at aquaculture in earlier time. 2. Since VHSV gene is highly mutated in time-dependent manner, the efficient vaccine for VHSV-derived disease prevention is not well achieved. Therefore, the other approaches for prevention of epidemics by VHSV should be required. Our research goal is to develop prevention of epidemics propagation of VHSV infection by high pathogenic VHSV prediction with gene variation analysis. 3. In this study, we identified several gene variations of VHSV NV protein from 13 VHSV specimens of severe VHS-showing olive flounders in Korea. The gene variations are associated with amino acid changes of VHSV NV proteins. The amino acid variants of NV protein show the inhibitory activity of fish cellular energy (ATP) generation. The cellular energy deprivation might give a big impact on fish health and mortality. The DNA mutation on specific amino acid changes is possibly applied to develop the PCR diagnosis system for early detection of high pathogenic VHSV appearance. The early pathogenic gene diagnosis of VHSV gives more efficient prevention of epidemics. [ABSTRACT FROM AUTHOR]

A batch of wild common carp and largemouth bass died in Andong, Gyeongsangbuk-do province, South Korea, in 2016. Moribund fish showed typical signs of spring viremia of carp (SVC) disease, which causes acute hemorrhage in the skin and ascites. Thus far, SVC disease has been detected in several regions of the world but never in South Korea. Suspecting the infectious agent to be the SCV virus (SVCV), the moribund fish were sampled and screened. The isolated virus developed a cytopathic effect in EPC cells. Both viral isolates from the common carp (ADC-SVC2016-1) and largemouth bass (ADC-SVC2016-3) were identical in terms of their genome sequence, which were 11,034 bp nucleotides in length. Genome comparison exhibited greater sequence similarity with the Asian SVCV sequences available at NCBI. Phylogenetic analysis revealed that the Korean SVCV isolates were clustered within the Asian clade. More specifically, evolutionary analysis by using the P gene sequences showed that the Korean isolates were sub-cladded within the Iai genogroup but diverged from Chinese strains of SH150514 and SH160901. The Korean isolates shared more than 98% sequence similarity with the two Chinese SVCV isolates, suggesting that the spread of SVCV originated from China. The isolated virus had cytopathic effects on EPC cells. Virus transmission studies showed that the virus exhibited the highest virulence at 15 °C, which was also dependent on the method used, with the injection method being better than the immersion and cohabitation methods. This is the first study to document that Korean SVCV isolates may be epizootic in wild common carp and other susceptible animal populations in South Korea. [ABSTRACT FROM AUTHOR]