Scientific Opinion on the increased mortality events in Pacific oysters, <em>Crassostrea gigas</em>
In the summer of 2008 and 2009, severe mortality events in cultured Pacific oyster were reported from the main European producing countries. The European Commission requested EFSA to assess the relative importance of possible causes, including infectious agents with special focus on Ostreid herpesvirus 1 (OsHV-1) µvar and environmental factors. An assessment of the role of other mollusc species and the risks posed by transference of adult Pacific oysters from affected to unaffected areas was also requested. The available evidence suggests that OSHV-1 infection is a necessary cause but may not be a sufficient cause, the strain; OsHV-1 µvar seems to be dominant. An increase or a sudden change in the temperature was shown to be a risk factor. Husbandry practices such as introduction of non certified possibly infected spat, movements and mixing of populations and age groups are also important risk factors. In addition to C. gigas, there is evidence of susceptibility to OsHV-1 in Ostrea edulis, Pecten maximus and Ruditapes philippinarum. It was concluded that it is not safe to transfer oysters older than 18 months from affected areas to areas not affected. The panel recommended that to promote and preserve a high health status and in particular to prevent and/or control “increased mortality” measures are urgently needed to improve the general level of biosecurity in the oyster aquaculture industry in Europe. Furthermore to minimize the risk of subsequent transfer of infectious agents from hatcheries and wild-caught spat, there is a need to establish the health status of oyster spat at source. An assessment of the health status should include results of regular batch laboratory testing (at least in regards to OsHV-1, ref strain and µvar, Vibrio species, and histopathological examination) and epidemiological assessment. Improved diagnostic methods should be developed and clear criteria for viral strain differentiation taking in account genotype and epidemiological criteria are necessary.