Pathogenicity of iridovirus from Japan and Thailand for the red sea bream Pagrus major in Japan, and histopathology of experimentally infected fish

The present study evaluated the pathogenicity of iridovirus from red sea bream Pagrus major and sea bass Lateolabrax sp. in Japan and one from brown-spotted grouper spawner Epinephelus malabaricus (GSIV: grouper spawner irdovirus) in Thailand to the red sea bream. The iridovirus from red sea bream in Japan caused 75% mortality by intraperitoneal (i.p.) injection with 10(3.2) TCID50/fish and 90% mortality by immersion infection with 10(2.7) TCID50/ml. Iridovirus isolated from imported sea bass caught in the South China Sea and then reared in Japan caused 100% mortality in red sea bream by i.p. injection with 10(2.3) TCID50/fish. Red sea bream had 55.6% and 55% cumulative mortalities by an i.p. injection of the spleen filtrates of GSIV from Thailand. The same histopathological changes as red sea bream iridovirus infected fish were observed in all of the experimentally infected red sea breams. The most characteristic changes were the appearance of large numbers of blast-like inflammatory cells throughout the circulatory system and the formation of markedly enlarged cells in the spleen, kidney, heart, liver, digestive tracts, pancreas, gills, swim bladder, choroid and choroidal rete of eyes, meninges, bone and musculature. Electron microscopy showed virions with an edge-to-edge diameter of 175 to 196 nm in all experimentally infected groups. Immunofluorescence positive reaction with the monoclonal antibody against red sea bream iridovirus was observed in all experimental groups.