Isolation and characterization of viral hemorrhagic septicemia virus from farmed bastard halibut

Viral hemorrhagic septicemia virus (VHSV) was detected and identified in healthy bastard halibut Paralichthys olivaceus on the east coast of China through a combination of virus isolation, real-time reverse transcription polymerase chain reaction (RT-PCR) and conventional RT-PCR followed by sequencing. Analysis using MEGA11 software revealed that the isolated VHSV belongs to genotype IVa. Bastard halibut fry were experimentally infected by immersion and gill inoculation with infected cell culture medium and exhibited clinical signs. Cytopathic effects (CPE) were observed again when the epithelioma papulosum cyprini (EPC) cells were inoculated with homogenized tissue samples from the diseased fish. Real-time RT-PCR confirmed the existence of the virus in the infected fish tissues. Transmission electron microscopy (TEM) revealed the presence of numerous virus particles in the infected EPC cells, both extracellularly and intracellularly. The combination of clinical signs, typical CPE, and genetic analysis identified the isolated virus as VHSV.