Characterization of a megalocytivirus from cultured rock bream,Oplegnathus fasciatus (Temminck & Schlege), in China

Megalocytivirus belongs to the Iridoviridae family and is known to affect finfish. Megalocytivirus epizootics have been reported to occur in several cultured fish species in China; however, no megalocytivirus associated with rock bream, Oplegnathus fasciatus (Temminck & Schlege), has been documented. In this study, we characterized for the first time a megalocytivirus, rock bream iridovirus (RBIV)‐C1, detected in cultured rock bream in a fish farm in China that had been inflicted with a high‐mortality disease outbreak. Sequence analysis of three conserved genes showed that RBIV‐C1 shares over 90% overall identities with a number of known megalocytiviruses. Electron microscopic examination revealed RBIV‐C1 as hexagonal particles similar to those reported for megalocytiviruses. In vivo infection study indicated that, following inoculation into rock bream, RBIV‐C1 induced 100% mortality and upregulated the expression of Mx, IL‐1β and IL‐8. The infected fish exhibited pathological signs similar to those observed in naturally diseased fish. Furthermore, studies in a turbot (Scophthalmus maximus L.) model indicated that RBIV‐C1 induced acute infection in turbot that led to 100% mortality. These results indicate that RBIV‐C1 is highly virulent to rock bream as well as turbot and that RBIV‐C1 is closely related to a number of previously reported megalocytivirus and likely a genetic variant of the latter.