An outbreak of VHSV (viral hemorrhagic septicemia virus) infection in farmed olive flounder Paralichthys olivaceus in Korea

We examined the cause of a disease outbreak in juvenile and market-sized olive flounder Paralichthys olivaceus from aquaculture farms in the south sea of Korea in 2005. Principal signs included expanded abdomen, congested liver and enlarged spleen and kidney, with fish suffering heavy mortality (40 to 60%). Although no parasites or bacteria were isolated from diseased fish, tissue filtrates still produced cytopathic effects (CPE) in FHM, CHSE-214 and FSP cells. PCR reactions of tissue filtrates from diseased fish and supernatants of cell cultures showing CPE indicated specific 587 bp fragments from the glycoprotein (G) gene of the viral hemorrhagic septicemia virus (VHSV). Supporting this, the nucleotide sequences among three isolates had shown 100% homology, and 99.3% and 99.8% homology with the VHSV JY-0112 isolate from flounder in Korea and VHSV obama25 isolate from flounder in Japan, respectively. Experimental infection trials using supernatants of cell cultures showing CPE gave cumulative mortalities of 100% and 60% for virus-injected and virus-immersed flounder, respectively. The pathological signs shown were generally similar to those of naturally diseased fish. Results of this investigation indicate that VHSV was the causative agent of the natural epizootic.