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

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.