Studies On Virial Reddish Booy Syndrome Of Farmed Turbot Scophthalmus Maximus In China

Turbot, Scophthalmus maximus, is a traditionally farmed fish species in Europe. Although it was introduced to China only 10 years ago, it has become an important aquaculture species in coastal areas of northern China, especially in the Jiaodong Peninsula, Shandong Province. The annual production of farmed turbot in China achieved 7000 metric tons and valued US$150 million in 2002.Recently, more and more epizootic diseases of farmed turbot in China occurred because of high density stocking and improper management. The pathogens were assumed to be parasites or bacteria, although some of them may have been virus. In a recent survey of farmed turbot diseases, we found a 'reddish body syndrome' (RBS) in both juveniles and adults. It has caused severe mortalities of farmed turbot and leaded to serious economic losses.The gross signs of diseased fish were pale gills with local haemorrhages, petechial haemorrhages in fins and fin bases, especially haemorrhages in muscle and skin. Parasites were absent in smears and wet mounts of RBS turbot tissues. Bacteria were not consistently isolated from diseased fish, too. These examination results suggested that the disease might be caused by a viral agent.By light microscope, many enlarged cells were observed in the spleen and kidney of diseased turbot. Many 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 onlypresent in the cytoplasm of infected cells. TEM images of virions showed them to be icosahedral in symmetry, measuring 120 to 130 nm from vertex to vertex and 110 to 116 nm from face to face. Complete virions consisted of three layers: the outer layer was an icosahedral capsid 10 to 14 nm in thickness, the intermediate layer was an 11 to 15 nm thick translucent space and the inner spheroid core was a homogeneously electron-dense nucleoid measuring 64 to 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. In the affected splenic cells, cytoplasmic vacuolation was distinct while ballooning endoplasmic reticula also can be seen. The mitochondria were mild enlarged with disrupted cristae.Intraperitoneal inoculation of the filtrate (450 nm) of spleen homogenate of RBS fish to turbot juveniles induced similar gross signs of RBS and the cumulative mortalities reached 85.7% within 3 weeks, while no turbot died in the control groups. Cytopathic effects (CPE) in the form of cell rounding were induced in confluent mono-layers cells of FEC (flounder embryonic cell line) 4 days after being inoculation with filtrate from RBS turbots. Numerous iridovirus-like particles were revealed from the ultra-thin sections of the infected FEC cells. Some viral particles measured 115 to 125 nm in diameter and had a hexagonal profile were purified from spleen and kidney tissues of RBS turbots after centrifugation on sucrose gradients.Based on above researches of epidemiology, aetiology, histopathology and cell pathology, we concluded that the iridovirus-like virus is the causative agent of reddish body syndrome that afflicts farmed turbot in China. The virus is subsequently referred to as turbot reddish body iridovirus (TRBIV) and RBS should be named as viral reddish body syndrome of turbot.According to the shape, size and target tissues of the virus, we recognized that TRBIV is different from the documented turbot iridovirus and it may be a new piscine iridovirus for cultured turbot. To determine the taxonomic position of the virus, a pair of PCR primers corresponding to the consensus sequence of major capsid protein (MCP) gene in red sea bream iridovirus (RSIV) was applied to amplify TRBIV g...