Osyter beds. (Photo: Stock File/FIS)

Panel asked for advice on oyster deaths

It was requested that the Panel on Animal Health and Welfare (AHAW) give a scientific opinion on the increased mortalities of Pacific oysters (Crassostrea gigas) after a call from the European Commission (EC), said the European Food Safety Authority (EFSA).

Specifically, the request asked for an assessment of possible causes, such as infectious agents and particularly Ostreid Herpesvirus-1 (OsHV-1) µvar) plus environmental factors. Also assessed was which other mollusc species could be involved and the risk of infection posed by the transfer of adult Pacific oysters when coming from an area of high mortalities.

The AHAW panel found that OsHV-1 has appeared predominantly in Pacific oysters spat and juveniles from events of increased mortality and that other factors may be significant. 

In the 2008-10 increased mortality events, it appears that OsHV-1 µvar is the dominant viral strain, although it is unclear whether this has resulted from greater virulence or different epidemiological factors. Climatic and seasonal factors on their own are probably not enough to have caused the increased mortality in France, UK and Republic of Ireland in 2008-9, though these events are seasonal. 

It has been determined that an increase or a sudden change in the water temperature is an important risk factor. No outbreaks have been observed with a water temperature below 16ºC. 

The introduction of non certified possibly infected spat, movements and mixing of populations and age groups, among other husbandry practices, may constitute key risk factors.

Apart from the Pacific oyster, evidence of susceptibility to OsHV-1 exists in Ostrea edulis, Pecten maximus and Ruditapes philippinarum. Susceptibility to OsHV-1 µvar was not investigated on other mollusc species other than C. gigas.

Because OsHV-1 (reference strain and µvar) was identified in Pacific oysters older than 18 months linked to increased mortality, it was determined that oysters older than 18 months can be a source of the virus, such that it is unsafe to move them from affected areas.

The panel said measures are immediately needed to advance biosecurity in the European oyster aquaculture sector to prevent or control increased mortality. 

And to curtail the risk of subsequent transfer of infectious agents from hatcheries and wild- caught spat, the health status of oyster spat at their source must be established. An assessment should comprise results of regular batch laboratory testing and epidemiological studies.

Better diagnostic methods should be created to detect OsHV-1 µvar and other strains. 

A powerful health surveillance system for European Pacific oyster production is necessary to better grasp existing and emerging issues. Comprehensive epidemiological research studies are required to resolve the potential weight of infectious agents and other environmental factors on increased mortality in Pacific oysters.

Related article:

- French virus found in UK oyster stocks 

By Natalia Real
editorial@seafood.media
www.seafood.media

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