Isolation and identification of a new isolate of lymphocystis disease virus isolated from black rockfish (Sebastes schlegelii) in China

Iridoviruses consist of five genera as follows: Ranavirus, Lymphocystivirus, Megalocytivirus, Iridovirus and Chloriridovirus. Lymphocystis disease virus (LCDV) belongs to Lymphocystivirus, and it has been identified as the causative agent of lymphocystis disease (LCD) in more than 100 different species of seawater and freshwater fish. In recent years, the types of Lymphocystivirus-infected fish have been rapidly increased. In the present study, we isolated an iridovirus from nodules of LCDV-infected black rockfish, and it was named as LCDV-ss. On the skin, fins and mouth of infected black rockfish, we observed a large number of irregular white nodules. Histophological analysis indicated that the nodules contained encapsulated hypertrophic cells and an enlarged nucleus. Through transmission electron microscopy, numerous viral particles with a diameter of 200 nm were observed in the cytoplasm, showing a hexagonally arranged pattern. Based on the major capsid protein (MCP) and DNA polymerase genes, our phylogenetic analysis revealed that LCDV-ss was closely related to LCDV from China (LCDV-C), followed by LCDV-1. In summary, we, for the first time, provided molecular evidence that LCDV-ss was a novel emerging iridovirus pathogen in cultured black rockfish.