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Abstract Taiwan marine fish suffers the epidemic infection of iridovirus. In this research, infected fish of Epinephelus. lanceolatus was diagnozed by polymerase chain reaction. Transmission electron microscopy showed the morphology of viral particles from tissue lysate. The nucleotide sequence of major capsid protein (MCP) was compared to OSGIV, thereafter the novel virus from Epinephelus lanceolatus is named OSGIV-like iridovirus. By cloning expression of the MCP of TGIV and OSGIV-like, the MCP of 51 kDa was insoluable although minor virus-like substance was observed in TEM. Triton X-100 of 2.5% improved the solubility of protein to 30.95 £gg/£gl. Chloroform treatment enhanced the antibody-detection for the MCP of virus in the tissue. We inference that iridovirus MCP is associated with the envelope because of its hydrophobicity.

Iridovirus infections have led to serious economic loss in the aquaculture industry in Kaohsiung County as well as the whole Southern Taiwan region. Identified susceptible host species in this region includes hybrid grouper (Epinephelus hybrid), giant seaperch (Lates calcarifer), largemouth bass (Micropterum salmoides), king grouper (Epinephelus lanceolatus), spotted butter fish (Scatophagus argus), yellow-wax pomfrat (Trachinotus blochii), goldlined seabream (Sparus sarba), humpback grouper (Chromileptes altivelis), Mangrove red snapper (Lutjanus argentimaculatus). In this study, a diagnostic PCR primer pair CY15n-F/CY15nR, and its nested primer pair RY16-F/RY16-R, were designed and applied to amplify virus-specific products of 1339 bp and 305 bp, respectively. This primer set did not amplify products from lymphocystis disease virus, largemouth bass virus, or healthy control fish DNA. This sensitive technique can detect the presence of 50 fg plasmid with viral DNA insert in the presence of 100 ng/£gl host DNA, or 0.05 fg DNA from infected fish. Comparing sequences of CY15 fragment, ATPase gene, predicted major capsid protein and partial DNA polymerase genes among iridoviruses, it is suggested that the viruses found in this area should be classified as the Megalocytivirus of Iridoviridae. These viruses are clearly different from the Ranavirus, another fish-pathogenic iridovirus. Those iridoviruses can be classified into two genotypes: the CY630 type, which is the Taiwan grouper iridovirus; and the CY113 type, which is similar to red seabream iridovirus (RSIV). The identity in CY15 fragment sequences is about 91%. Microscopically, enlarged cells can be found in organs of infected fishes. They appear in the spleen, head kidney, and trunk kidney in infected groupers. The enlarged cells may be relocated from other organs. In giant seaperch, the enlarged cells appeared in the above mentioned organs, and also in the digestive tract and the heart. Two kinds of the enlarged cells in grouper can be distinguished by their H&E staining properties: the basophilic and the eosinophilic enlarged cells. The result from in situ hybridization and electron microscopy suggest that the viruses only appear in the basophilic enlarged cells. Both nucleus and cell volume increase in basophilic enlarged cells, while only the cell volume increases in the eosinophilic enlarged cells. The viruses appeared first in the nuclei of the basophilic enlarged cells, after the mid-phase of the infection they distributed into the whole cells. Judging from the results of phagocytosis, acid phosphatase activity and the ultrastructure of infected cells, it is suggested that this target cell is macrophage or monocyte. The viral capsid is assembled in the viromatrix, and the virogenic stroma can be either ring-shped or disc-shaped. The diameter of mature virus is 120-130 nm from side to side, or 160-170 nm from apex to apex. The electron-lucent space between the capsid and the envelope is about 20-50 nm. The virus particles can be found in (1) lysosome-like vesicles in the cytoplasm, if the host cell still has its nucleus; or (2) the viromatrix, When the host nucleus is dissolved or only has some vestige nuclear membrane left.