Oyster herpesvirus is a virulent viral disease of the Pacific oyster Crassostrea gigas.

OsHV-1 (Oyster Herpesvirus type 1) infection causes mortality in the larvae and juveniles.

Infected larvae show a reduction in feeding and swimming activities and mortality can reach 100% in a few days. 

Affected spat show sudden and high mortalities mainly during summer time. 

The virus is associated with abnormal nuclei through connective tissues, especially in mantle, labial palps, gills, and digestive gland.