Keep calm and carry on enjoying oysters

Published on
November 15, 2013

France’s Pacific oyster (Crassostrea gigas) producers have been on a bumpy ride in recent years as a result of the widespread ostreid herpes virus-1 (OsHV-1), while across the English Channel, Britain’s oyster industry has remained relatively unscathed. However, a third outbreak in four years in U.K. waters is reason for concern.

The virus, which is harmless to humans but deadly to juvenile Pacific oysters, was first found in U.K. waters in 2010, when it was detected after a mortality event in Whitstable, Kent. A second outbreak occurred in 2012 in the River Blackwater, Essex. Both incidents were contained through movement bans.

Inspectors have now detected OsHV-1 at a farm in Poole Harbour, on the south coast. Again, a containment zone has been established to prevent the virus from spreading to other oyster-growing areas.

It is estimated the farm has lost around 35 percent of its juvenile oysters.

The Poole site is one of the largest production areas in the country, although it isn’t on the radar of many people outside the industry as most of its product is exported, said David Jarrad, director of The Shellfish Association of Great Britain (SAGB).

“The strange thing is that, like the outbreaks in Europe and Australasia, both the other two outbreaks in England were during high summer (July); this one occurred in October. This disease is ‘usually’ temperature related,” Jarrad told SeafoodSource.

He explained that on its own, the virus won’t kill an oyster; you need the virus plus specific environmental conditions, such as overcrowding, high water temperature (above 16C) or lack of oxygen — all or any of the above. To confuse matters, overcrowding, high temperature and lack of oxygen can kill oysters without the virus.

“This latest outbreak, being in October, takes temperature and oxygen out of the equation. It would be much easier if we could pinpoint what the problem is, but the likelihood is — just as we found in Whitstable and Blackwater — we won’t know.”
Jarrad stressed that consumers should continue to enjoy eating oysters and not read too much into the word “herpes.”
“OsHV-1 is not infectious to humans; the oyster herpes virus can only be caught by oysters,” he said.

In the United Kingdom, oysters cannot be moved from a site confirmed with OsHV-1 to another growing site, but live oysters can still go to depuration facilities and be sold to consumers.

A different system operates in France, Europe’s biggest oyster-producing country, where authorities decided early on that the virus was endemic throughout the industry and decided not to impose any movement restrictions.

Interestingly, the dynamics of Europe’s oyster industry have changed following the arrival of the virus in that growers used to want to buy half-grown oysters from areas that are disease-free, but now the French industry in particular is wanting to buy half-grown oysters from infected areas where they have had survival. In other words, they want oysters that have been challenged and survived.

Despite the third outbreak, the United Kingdom is still deemed by the EU as a disease-free country, as is Southern Ireland, which has a large proportion of its oyster growing sites within containment zones.

The disease-free status would likely be lost if one of the United Kingdom’s three commercial hatcheries — Whitstable, Guernsey or Morecambe Bay – tested positive for the virus. There is significant interdependence and exchange of stocks, larvae etc. between these hatcheries, such that if one has it then the other two will have it too, said Jarrad.

“We are seeking assurance from DEFRA (Department for Environment, Food and Rural Affairs) that if that happened, we could remove the disease-free status overnight. Because if we didn’t, that would mean our on-growing sites would not be able to buy any seed whatsoever.”

Putting the outbreak to one side, Jarrad confirmed that 2013 has been a good year for oyster producers. Prices are high in line with a shortage of supply and very high consumer demand.

U.K. exports have also grown and now account for around 20 percent of total production, with France the main destination market.

Contributing Editor reporting from London, UK

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