Identification, histopathology, and phylogenetic analysis of an iridovirus from cultivated silver pomfret in Zhejiang Province, East China

Silver pomfret [Pampus argenteus (Euphrasen, 1788)] is a perciform fish that is widely distributed in coastal regions of the Indo-West Pacific, and is a favourite food species in China. A serious disease occurred in silver pomfret at a marine fish farm in Ningbo, Zhejiang Province, East China, in the summer of 2018, causing high fish mortality. Infected fish showed darkened skin, imbalanced swimming or irregular breathing, and abdominal distention with reddish ascites. When dissected, fish were found to have an enlarged spleen and pale liver. No major parasites were found during microscopic examinations of the scales, fins, and mucus of diseased pomfret. However, the spleen, kidney, liver, and gills of diseased pomfret tested positive for red sea bream iridovirus (RSIV) using primers from the OIE (Oficina International de Epizootias). Typical swollen cells were observed in the spleen of RSIV-positive pomfret during histopathological analysis, with iridovirus-like particles from 131 to 178 nm in size observed under transmission electron microscopy (TEM). Degenerated erythrocytes were observed, with decreased sizes and number of cells seen in blood smears after Wright-Giemsa staining, while the number of monocytes, thrombocytes, and lymphocytes in the blood increased compared with those in normal pomfret. The major capsid protein (MCP) and adenosine triphosphatase (ATPase) genes of the iridovirus found in silver pomfret were subjected to phylogenetic analyses. In the MCP phylogenetic tree, the iridovirus isolated from pomfret was clustered in subgroup 1 of the RSIV type of Megalocytivirus, with 99.80, 99.78, 99.71, and 94.49% homology with large yellow croaker iridovirus (LYCIV)-15 (Zhejiang strain), LYCIV (Fujian strain), RSIV, and infectious spleen and kidney necrosis virus (ISKNV), respectively, and in the ATPase tree it had 100, 100, and 97% homology with LYCIV, RSIV, and ISKNV, respectively. The genetic distance of the iridovirus from silver pomfret was 0.002 from LYCIV-15 (Zhejiang), 0.002 from LYCIV (Fujian), and 0.003 from both RSIV (Japan) and marble goby iridovirus (MGIV) (Taiwan), while the distance of LYCIV (Fujian) was 0.001 from both RSIV (Japan) and MGIV (Taiwan) and 0.003 from LYCIV-15 (Zhejiang), meaning that the silver pomfret iridovirus and LYCIV (Fujian) belong to different iridovirus branches. It could be concluded that the iridovirus isolated from silver pomfret (silver pomfret iridovirus, or SPIV) is a potentially severe pathogen of silver pomfret, and a new member of subgroup 1 of the RSIV type of the Megalocytivirus genus.