The first report of an iridovirus-like agent infection in farmed turbot, Scophthalmus maximus, in China

An iridovirus-like agent associated with the ‘reddish body syndrome’ (RBS) of farmed turbot Scophthalmus maximus was found in China. By light microscope, many enlarged cells were observed in the spleen and kidney of diseased turbot. Iridovirus-like particles were detected by transmission electron microscopic (TEM) examination in various organs of diseased turbot. The virion had an envelope and viral particles were only present in the cytoplasm of infected cells. TEM images of virions showed them to be icosahedral in symmetry, measuring 120–130 nm from vertex to vertex and 110–116 nm from face to face. Complete virions consisted of three layers: the outer layer was an icosahedral capsid 10–14 nm in thickness, the intermediate layer was an 11–15 nm thick translucent space and the inner spheroid core was a homogeneously electron-dense nucleoid measuring 64–70 nm in diameter. The virus particle acquired an envelope through budding from the cytoplasm into a cytoplasmic vesicle. Infected cells became hypertrophic and the cytoplasm was homogeneous. According to the TEM examination, the virions were mostly in cells of gill, intestinal submucosa, spleen and basement membranes of capillaries in glomeruli of the diseased fish. The spleen was the major target organ for the virus. Further analysis using PCR suggested that the molecular characterization of the virus was different from that of red sea bream iridovirus (RSIV) in Japan, Korea, Taiwan and infectious spleen and kidney necrosis virus (ISKNV) in China. This is the first report of iridovirus infection in turbot in China.