Identification and Characterization of Megalocytivirus Type 3 Infection with Low Mortality in Starry Flounder, Platichthys stellatus, in Korea

MVSF-12 belonging to megalocytivirus type 3 was isolated from cultured starry flounder; Platichthys stellatus, at the moribund or subclinical stage with low mortalities in Korea. Of 20 apparently healthy fish in the farms, 17 were also confirmed in nested polymerase chain reaction to be infected by megalocytivirus. When starry flounder; rock bream, Oplegnathus fasciatus; and olive flounder, Paralichthys olivaceus, were artificially infected by MVSF-12 or iridovirus sachun-1 (IVS-1, megalocytivirus type 1), starry flounder and olive flounder showed no mortality until Day 24, without any clinical signs including enlarged spleen, while rock bream showed 100% mortality by IVS-1 infection within 11d but no mortality by MVSF-12. Although it was not pathogenic, MVSF-12 in infected fish at Day 24 was viable when successfully cultured in vitro using primary rock bream embryo cells and produced a cytopathic effect (CPE) with the viral copy numbers between 1.76x10(7) and 5.23x10(7)/mL of culture supernatant. In conclusion, this study demonstrates the low pathogenicity of MVSF-12 and low susceptibility of starry flounder and olive flounder to both MVSF-12 and IVS-1. Indeed, MVSF-12 at the subclinical stage could be replicated with CPEin vitro, indicating a possibility to induce pathogenic effects and mortality under adverse environment or physiologic conditions.