IDENTIFICATION AND CHARACTERIZATION ANALYSIS ON A MEGALOCYTIVIRUS ISOLATED FROM DISEASED MARBLED SLEEPY GOBY, OXYELEOTRIS MARMORATUS

An unknown disease with highly mortality of 85% occurred in marbled sleepy goby, Oxyeleotris marmoratus, cultured in Shunde area, Guangdong Province, in Oct., 2009. The diseased gobies were mainly 15-18 cm in body length and showed abdominal distension, lethargy and unusual swimming behavior such as floating on the pond surface until died. The postmortem examination revealed that liver, spleen and kidney were obviously swollen with petechial haemorrhages. No pathogenic bacterium was isolated from organs including liver, spleen and kidney. After filtration treatment, the tissue suspension was injected intraperitoneally with 20 gobies. The affected gobies began died without any obvious signs at 7 days post-infection (p.i.), and all gobies died until 10 days p.i., whereas there were no deaths in the control group. The section samples of liver, spleen and kidney including diseased and artificial affected gobies exhibited similar histopathological changes, showing many enlargement and necrosis cells. Under electron microscope (EM), many icosahedral viral particles were observed in the cytoplasm from liver, spleen and kidney. The hexagonal virus particles measured an average 135 nm from side to side, and the shape was highly similar to that of iridovirus. With the primers designed according to the sequence of major capsid protein (MCP) from the GenBank database, specific products with predicted size were obtained from all diseased and artificial affected gobies. Comparative analysis of nucleotide sequences was performed with the GenBank databases using BLAST database network service. The resultsshowed that the putative gene products from infected gobies shared high identity with homologous 98.8%, 98.1% and 94.7% of infectious spleen and kidney necrosis virus (ISKNV), dwarf gourami iridovirus (DGIV) and Red sea bream iridovirus (RBIV) respectively. Based on neighbor-joining analyses of the MCP gene sequences, phylogenetic tree was constructed and the result indicated that the pathogen in present study was a species within the genus Megalocytivirus, which was named tentatively marbled sleepy goby iridovirus (MSGIV).