We trialled simple methods for treating seawater to prevent mortalities due to OsHV-1. This follows previous epidemiological observations that suggest the virus is attached to some kind of particle. By filtering seawater to 5 µm or aging seawater for 48 hours prior to use it was possible to completely prevent mortality in spat. These results provide a solution for ongoing hatchery production of C. gigas.
Whittington RJ, Hick PM, Evans O, Rubio A, Alford B, Dhand N and Paul-Pont I (2015). 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. Aquaculture 437: 10-20.
Summary: 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. Ageing seawater for 48 hours 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 an hypothesis that OsHV-1 is carried on particles rather than being uniformly distributed in water; removal of the putative particulate vector of OsHV-1 from seawater using ageing/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.
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