Susceptibility of juvenile humpback grouper Cromileptes altivelis to grouper sleepy disease iridovirus (GSDIV)

Abstract

Susceptibility of juvenile humpback grouper Cromileptes altivelis to the grouper sleepy disease iridovirus (GSDIV) was examined. GSDIV-containing inocula for challenge were obtained using a filtrate of spleen tissues from donor fish (orange-spotted grouper Epinephelus coioides) infected with GSDIV. Groups injected with the primary filtrate showed lower mortalities (30 to 60%) than groups receiving the 10-4 diluted inoculum (90 to 100% mortality). This result was contrary to the expectation that fish challenged with a higher concentration of virus would show higher mortality. Electron microscopy revealed that moribund fish receiving the 10-4 diluted inoculum displayed massive formation of typical inclusion body-bearing cells (IBCs) containing an intracytoplasmic inclusion body with many virions in the 180-200 nm size range propagated within a virus assembly site. In contrast, survivors in fish receiving the primary filtrate showed the formation of unusual IBCs containing an abnormal inclusion body that was characterized by the assembly of a small number of deformed virions. This impaired virus assembly appeared to prevent mortality in the challenged fish and was assumed to be due to an interferon-like effect of a previously unknown substance that was passed on to the challenged fish with the tissue filtrate from the donor fish.