Identification of giant gourami iridovirus (GGIV): a new infectious spleen and kidney necrosis virus (ISKNV) from natural outbreak in cultured Osphronemus goramy

Megalocytivirus of family Iridoviridae has been identified as a pathogen that caused the fatal systemic infection to lead to massive death of numerous fish species in both ornamental and food fish industries. It caused significant economic losses due to severe mortality of infected fish. Since 2011 in Indonesia, megalocytivirus in giant gourami (Osphronemus goramy) had been found as the main pathogen in some cases of giant gourami outbreak, especially in West Java, Central Java, and Bali. The aim of this study is to identify and characterize the pathogen known as a megalocytivirus infection in freshwater-cultured giant gourami. We identified the virus using universal and specific primers for megalocytivirus and infectious spleen and kidney necrosis virus (ISKNV). Sequencing and BLAST analysis were used to develop a phylogenetic tree. The result showed that phylogenetic analysis of major capsid protein (MCP) gene unveiled a new member of megalocytivirus, designated as giant gourami iridovirus (GGIV). GGIV-formed cluster belonged to ISKNV and has 100% homology to ISKNV complete genome. Artificial infection by intraperitoneal injection with supernatant homogenate from spleen and kidney of naturally infected fish showed 93% cumulative mortality in 12 days. Fish showed a clinical sign of infection as lethargic, loss of appetite, pale or darken body color, and hemorrhages. Internal organ on dead fish showed swollen spleen and kidney and also a pale liver. Quantitative PCR analysis on internal organs showed spleen had the highest viral DNA copy number followed by kidney, gill, and liver. Histopathological analysis showed many abnormally hypertrophied cells in spleen which is typical histopathological characteristic of megalocytivirus infection. In conclusion, GGIV belonged to ISKNV from genus megalocytivirus.