Experimental Infection of Ostreid Herpesvirus 1 (OsHV-1) JPType1, a Japanese Variant, in Pacific Oyster Crassostrea gigas Larvae and Spats

In many locations worldwide, ostreid herpesvirus 1 (OsHV-1) infections are associated with mortalities in Pacific oyster Crassostrea gigas primarily during the summer months. In the present study, experimental infections were performed to investigate the pathogenicity of OsHV・1JPType1 (a Japanese variant) in hatchery-reared Pacific oyster larvae and spats. The JPType1 virus was prepared from dead oyster spats obtained from a sea cage and was once passed in oyster larvae as the inoculum source of the infection experiments. Compared to pediveliger larvae, D-shaped larvae were more susceptible to infections when the larvae were exposed to serially diluted virus solutions at the concentrations of 3.2 x 106, 3.2 x 105 and 3.2×104 virus DNA copies/ml. Real-time PCR analysis revealed 1.6× 108 virus DNA copies/ng total DNA in a pooled sample of D-shaped larvae. No mortality was observed after 5 days when nine spat groups of differently sizes (mean shell height from 1.8 mm to 6.1 mm) were immersed in seawater containing 2.5×106 virus DNA copies/ml; however, 6.3×105 virus DNA copies/mg whole weight were detected in a group of spats. Virus concentrations were higher in smaller spat groups than in larger spat groups. The pathogenicity of OsHV司 1JPType1 in larvae and spats was confirmed; however, i t declined with the growth of the oyster.