Abstract:“Helong grouper” is the hybrid progeny produced by female longtooth grouper (Epinephelus bruneus) and male giant grouper (E. lanceolatus). As a new breed of hybrid grouper, there is no report on the diseases of the Helong grouper, at home or abroad. In July 2017, the Helong grouper on a fish farm in Shandong Province suddenly died, and the cumulative mortality was as high as 80% within 10 d. A field survey found that the water temperature was 28℃ and the salinity was 31 during epidemics. The appearance of diseased fish was normal. However, they were unresponsive, and died at the bottom of the tank. Clinical examination and necropsies revealed that the spleen and kidney of diseased fish were severely swollen and crispy. Histopathology studies indicated that tissues of diseased fish were severely damaged. A large number of basophilic, homogeneous cytoplasmic, swollen cells, with a diameter of 10~15 μm were observed in these tissues. In ultra-thin tissue sections of the spleen and head kidney, there are a large number of iridovirus-like particles, with a diameter of 130~150 nm in the cytoplasm of the swollen cells. Using specific PCR primers, severe infections of red sea bream iridovirus (RSIV) were detected in the spleen, head kidney, kidney, and liver tissues of diseased fish, which was consistent with electron microscopy observations. The complete coding sequence (CDS) of the major capsid protein (MCP) gene of the virus was 1362 bp in length. Based on the CDS of the MCP gene, a phylogenetic tree of 19 species (isolates) of the iridovirus was constructed. The tree showed that the iridovirus infecting the Helong grouper belongs to the RSIV cluster of the genus Megalocytivirus, family Iridovirus. This article describes for the first time the histopathological features of the iridoviral disease of the Helong grouper and confirmed that RSIV can naturally infect the hybrid grouper. This study not only revealed that the Helong grouper is a new susceptible host of RSIV, but also provided an important reference for the diagnosis and control of hybrid grouper viral disease.
