Incurable oyster herpes behind big shellfish die-off

File photo / Bay of Plenty Times

An incurable herpes virus has been named as the likely cause of a huge increase in juvenile oyster deaths in the upper North Island.

The $30m oyster industry announced last week that about half its crops had died over November and December.

Ministry of Agriculture and Forestry (MAF) scientists today announced they have found ostreid herpesvirus-1 (OsHV-1) in oyster samples from affected farming areas.

Its response manager Dr Richard Norman says there is no health risk from the disease.

"OsHV-1 cannot be transmitted to humans. Despite the name, there is no connection between OsHV-1 and the completely different herpes viruses in reptiles, birds and mammals."

Sharp changes to water temperature could also have contributed to the dieback, he says.

NZ Oyster Industry Association exective Tom Hollings says many oyster farmers are reeling at news their 2011 income will be significantly reduced.

On some farms, up to 80 per cent of juvenile oysters have died, compared with 5 to 10 percent in a normal year.

Farmers will be looking at cutting jobs and some could even consider shutting down, he says.

"If you've lost half of next year's income and some of this years you'd be looking at those options.

"It's devastating for the oyster industry."

But he is welcoming the news there is no food safety risk from the disease.

"We're trying to get the message across that it shouldn't have a stigma.

"It's a big issue for our industry but we're licking our wounds and thinking how we're going to get on with it."

New Zealand's famous Bluff oysters have tested as negative for the disease.

A strain of the virus killed between 20 to 100 percent of breeding Pacific oysters in some French beds in 2008, 2009, and 2010.

That virus has since has spread into British waters.

Ostreid herpes viruses are known to affect not only oysters but also clams, scallops, and other molluscs, according to French Research Institute for Exploitation of the Sea pathology lab director Tristan Renault, who has suggested in Europe that global warming "could be an explanation of the appearance of this particular type of the virus".

Animal health experts at the European Food Safety Authority (EFSA) are assessing the extent to which a combination of "infectious agents" such as OsHV-1 and environmental factors are causing the die-off of Pacific oysters there, whether other shellfish species are involved and the risk of infection posed by the transfer of adult Pacific oysters from infected farms.

But MAF said today that OsHV-1 was not listed as a mollusc disease by the World Organisation for Animal Health (OIE), "meaning it is not an issue of concern in oyster trade".

"The oyster industry is clearly facing significant production issues with a predicted shortfall for next year of approximately half of next year's harvest," said Dr Norman. "MAF will continue to work closely with the industry to identify other causes of the event and ways future production can be managed."