Protection of Pacific oyster (Crassostrea gigas) spat from mortality due to ostreid herpesvirus 1 (OsHV-1 μVar) using simple treatments of incoming seawater in land-based upwellers

The microvariant genotype of Ostreid herpesvirus 1 (OsHV-1 μVar) has severely disrupted the production of Pacific oysters Crassostrea gigas in Europe, New Zealand, and Australia since its first detection in France in 2008. The disease occurs in the warmer months, recurs annually, and requires new management strategies. Larvae and spat are the most susceptible life history stages, which poses a threat to hatchery production. This is the first study to examine strategies to enable survival of spat in treated seawater in an OsHV-1 μVar endemic area. In 2013 and 2014, seven controlled experiments were conducted in which potentially infected estuarine water was pumped through an upweller nursery system housing spat. Controls died of OsHV-1 infection in 6 of 7 experiments. Filtration of seawater to 5 μm was protective, whereas filtration to 30 μm or 55 μm was not. UV irradiation was not required for protection after 5 μm filtration. Chilling water by 2-3 °C delayed the onset and reduced the mortality rate. Aging seawater for 48 h prior to use prevented mortality, but the mechanism, either sedimentation of particles or inactivation of OsHV-1, was not determined. Some oysters in treatments in which mortality was prevented contained low quantities of OsHV-1 DNA, suggesting that infections may have occurred. The results support a hypothesis that OsHV-1 is carried on particles rather than being uniformly distributed in water; the removal of the putative particulate vector of OsHV-1 from seawater using aging/sedimentation of water or filtration to 5 μm enabled C. gigas spat to survive despite the presence of OsHV-1 μVar in the water supply.