Isolation, Identification, and Pathogenicity Characterization of a Largemouth Bass Ranavirus from Fujian, China

This study aimed to isolate and characterize the pathogenic agent of a suspected viral disease of largemouth bass (Micropeterus salmoides), which broke out in an aquatic farm in Fujian, China. The diseased bass had ulcer skin and muscles, bleeding fin, lesion liver, and enlarged spleen. Histopathological analysis showed multifocal necrosis on the gill, hepatopancreas spleen, and kidney. In the epithelioma papulosum cyprinid (EPC) cell line, stable cytopathic effects (CPE) were observed. Electron microscopy analysis showed icosahedral virus particles in the cytoplasm with a 150-160 nm diameter. PCR amplification revealed that the isolated virus belongs to largemouth bass ranavirus (LMBV) and was designated as LMBV FJ_22109. Further phylogenetic analysis determined that LMBV FJ_22109 is a variant of European and American strains. The intraperitoneal injection of LMBV FJ_22109 resulted in a regress infection, demonstrating its ability to induce pathogenicity in largemouth bass. The cumulative mortality rate was 100% at a culture temperature of 29°C, and 47% at a culture temperature of 17°C. Thus, it confirmed that LMBV FJ_22109 is indeed the causative agent of the suspected viral disease, and its virulence could be influenced by temperature, which will be beneficial for preventing and managing the disease.