Viral hemorrhagic septicemia virus (VHSV) is a highly contagious virus leading to high mortality in a large panel of freshwater and marine fish species. VHSV isolates originating from marine fish show low pathogenicity in rainbow trout.
VHSV belongs to the family Rhabdoviridae within the order Mononegavirales. The virion is enveloped with a typical bullet-shaped morphology. The genome consists of a non-segmented negative-sense single-stranded RNA molecule of about 11 kilobases which encodes six proteins in the order 3'-N-P-M-G-NV-L-5'. The viral RNA is tightly encapsidated with a nucleoprotein (N), a polymerase-associated phosphoprotein (P) and the large RNA-dependent RNA polymerase (L) to form the helical ribonucleoprotein complex (RNP). The matrix protein (M) interacts with the RNP and the viral envelope and is involved in the budding step. Finally, the viral surface glycoprotein (G) is implicated in the entry step and thus represents the unique target for neutralizing and protective antibodies. In contrast to other rhabdoviruses, the VHSV genome possesses an additional gene, localized between the G and L genes, that encodes a small non-structural NV (Non-Virion) protein. Due to the presence of the NV gene, VHSV was classified in the genus Novirhabdovirus together with IHNV. Using reverse genetics systems, it was demonstrated that the NV protein is essential for virus pathogenicity in rainbow trout, yellow perch (Perca flavescens) and Japanese olive flounder. The NV protein functions as inhibitor of the innate immune response by targeting cellular components of the RIG-I pathway that control the induction of interferon expression.