Investigating Increased Mortality In Pacific Oysters

The mandate terms of reference specifically requested to assess the relative importance of possible causes, including infectious agents with special focus on Ostreid Herpesvirus-1(OsHV-1) µvar), as well as environmental factors. It was also requested to assess which other mollusc species could be involved and finally the risk of infection posed by the transference of adult Pacific oysters when they are sourced from an area affected by the increased mortality event.

The following approach was taken: 1) a description of the production systems and husbandry methods for Pacific oyster production in Europe 2) a review of abnormal mortality events prior to 2008 in Pacific oyster 3) a description of the surveillance methods in place for investigation of abnormal mortality in European member states 4) a collection and analysis of the available information on mortality events during the period of 2008-2010 5) a description of infectious agents, host and environmental factors that have been associated with increased mortality. The question regarding the possible involvement of other mollusc species in “increased mortality” was addressed by presenting evidence of susceptibility (EFSA 2008) to OsHV-1 for certain mollusc species.

The Animal Health and Welfare (AHAW) panel concluded that OsHV-1 (reference strain and µvar) has been predominantly demonstrated in Pacific oysters spat and juveniles from events of increased mortality. The available evidence suggests that OSHV-1 infection is a necessary cause but may not be sufficient by itself as other factors appear to be important. OsHV-1 µvar has not been sufficiently characterised to be defined as a new genotype but may be considered as a different strain. OsHV-1 µvar seems to be the dominant viral strain in the 2008-2010 increased mortality events but it is not clear if this is a result of increased virulence or other epidemiological factors. The sensitivity and specificity of current diagnostic methods for OsHV-1 is not known. The role of other pathogenic agents such as Vibrio spp. has not yet been resolved.

Climatic and seasonal factors alone are not likely to be a sufficient cause for the increased Pacific oyster mortality reported in France, United Kingdom and Republic of Ireland in 2008-2009 although these events are seasonal in their occurrence. An increase or a sudden change in the temperature of the water around oysters has been shown to be an important risk factor predisposing for the disease. Up to now no outbreaks have been reported when the water temperature is below 16ºC. Husbandry practices such as introduction of non certified possibly infected spat, movements and mixing of populations and age groups are probably important risk factors.

Events of increased mortality have been only observed in Pacific oyster. In addition to C. gigas, there is evidence of susceptibility to OsHV-1 in Ostrea edulis, Pecten maximus and Ruditapes philippinarum. There was no investigation on susceptibility to OsHV-1 µvar made on other mollusc species other than C. gigas.

OsHV-1 (reference strain and µvar) was detected by PCR in Pacific oyster older than 18 months associated with increased mortality. Therefore it is concluded that oysters older than 18 months can be a source of virus and it is not safe to transfer oysters older than 18 months from affected areas to areas not affected by increased mortality events.

The AHAW panel recommended that a clear case definition for “increased mortality” in Pacific oyster needs to be developed. An acceptable baseline mortality and “increased mortality” in Pacific oyster needs to be defined where unit of concern (population, lease area, batches and/or production units), season, age/size, observation period should be taken into account as well as a description of an appropriate method for mortality estimation.

A description of the Pacific oyster aquaculture industry in Europe namely regarding number of farms, production figures, and traceability on movements/transfers both on hatchery and grow out sites should be achieved in accordance to the requirements by Council Directive 2006/88/EC.

The panel recommended that to best promote and preserve 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 to check for the presence of OsHV-1 µvar and other strains.The methods for detection of OsHV-1 (including different strains) need to be validated and harmonized. Relevant genomic information of the OsHV-1 µvar virus should be obtained for a better characterization of the strain in order to i) perform phylogenetic studies, ii) improve diagnosis iii) investigate potential for increased infectivity and virulence. The phylogenetic relationship of OsHV-1 strains should be investigated. Clear criteria for viral strain differentiation taking in account genotype and epidemiological criteria are necessary.

To better understand existing and emerging health problems, a robust health surveillance system for Pacific oyster production in Europe is needed. Well designed epidemiological research studies, including comparison studies, in order to determine the potential importance of infectious agents and other environmental factors on increased mortality in pacific oyster are necessary.