Histopathological analysis and molecular detection of iridovirus in cultured Pampus argenteus

In July 2018, an outbreak of unknown disease occurred in the silver pomfret (Pampus argenteus) cultured in a farm in Xiangshan, Ningbo, with a cumulative mortality rate of over 80%. To investigate the cause of disease outbreak, histopathological analysis, transmission electron microscope observation and molecular biological detection and analysis were carried out to identify the pathogen, so as to help us to understand and control the disease.The clinical manifestations of sick P. argenteus are anorexia, body imbalance, swelling of the spleen, and abnormal color of the liver. Histopathological analysis showed the presence of basophilic enlarged cells with a diameter of 10~15 μm in diseased fish spleen, liver and kidney. Further transmission electron microscopic observation revealed the presence of inclusion bodies and a large number of virus particles (a diameter of 140-160 nm) in cells from spleen and kidney tissues. Moreover, results of the iridovirus-specific PCR test showed that all these tissue samples collected from diseased fish were positive for iridovirus. To further detect the presence of iridovirus, the major capsid protein (MCP) gene of virus in P. argenteus was amplified, cloned and sequenced. Sequence similarity analysis showed that silver pomfret-derived virus (SPDV) shared the highest homology with the large yellow croaker iridovirus (LYCIV) (GenBank accession number: AY779031.1) MCP, with a sequence similarity of 99.76%. In a phylogenetic tree, SPDV and LYCIV are closely grouped with red sea bream iridovirus (RSIV) strains. Thus, we concluded that the virus that infected silver pomfret belongs to the Iridovirus family, Megalocytivirus genus, RSIV group. In summary, this paper reported Megalocytivirus infection in cultured P. argenteus for the first time. This study will provide an important reference for the diagnosis and prevention of P. argenteus iridescent virus disease.