Marine Institute

Update for Irish Oyster Producers

August 7, 2009

The Marine Institute has received reports of mortalities in Pacific oysters (Crassostrea gigas) from 13 Irish bays since the 22nd June. In the majority of bays the mortality now seems to have either slowed or stopped altogether.    

Results to date from seven bays have shown the presence of herpes virus, OsHV1. Further analysis is being carried out on these samples to identify if there are any other pathogens present. Analysis is ongoing on samples from a further six bays affected by oyster losses.  

Mortalities have affected principally seed and juvenile oysters brought into Ireland in 2009. Mortalities in this stock have ranged from 15 to 95 percent. There have also been mortalities in half grown and adult oysters in a small number of bays. The mortality in these stocks has been much less severe with reported levels of around 30 percent in half-grown oysters and even lower in adult oysters. Different bays have experience varying ranges of mortalities -  in some bays there was a small peak of mortality in seed, which stopped relatively quickly, whilst in others the mortality was more severe and persisted for longer, sometimes affecting the majority of stock in the bay.  

The role of OsHV1 in the mortality is unclear as a number of parameters remain to be explored.  It is however, probable that the herpes virus has played some part in mortalities, possibly in conjunction with certain bacterial pathogens and probably in conjunction with other environmental parameters. For example, heavy rains, and high temperatures are all likely to have had an impact on the overall levels of mortalities.  

Update on oyster mortality in France and advice on importing seed.

Oyster mortalities in France appear to have ceased in most areas and the majority of areas have re-opened for export of seed to Ireland. The Marine Institute must be notified prior to any imports in line with fish health regulations, to ensure that the Institute can check the clinical status of the site / area of origin prior to import. The main hatcheries in France have undergone voluntary tests for OsHV1 and importers should request the results of this testing before importing. It is also recommended that importers request that screening is carried out on the batch of oysters they intend to import for additional reassurance. As mentioned above, the disease process observed in C.gigas both here and in France is likely to be multi-factoral in nature and whilst screening for OSHV1 is not a guarantee that oysters are free of disease, it will provide a greater measure of assurance than importing without testing.

Herpes virus OsHV1 has no known impact on human health.  

For more information on fish health regulations visit www.marine.ie/fishhealth