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Ostreid Herpesvirus-1 (OsHV-1) Update

A relatively new virus is causing havoc in the international oyster industry. Charlotte Johnston, TheFishSite editor speaks with David Jarrad from the Shellfish Association of Great Britain about the fatal oyster herpes virus.

Officially identified in France no more than 24 months ago, the Ostreid herpesvirus-1 (OsHV-1) is one that has spread fast.

The disease only affects the Pacific oyster, native oysters remain uninfected, says Mr Jarrad. To date the disease has been associated with large scale mortalities. It is likely the disease has been present for a lot longer, says Mr Jarrad, but has been unknown to the industry.

In Europe, the virus is mostly affecting younger stock, although some adult stocks have been affected, he says.

Adult mortality varies between 10 - 30 per cent, however juvenile mortality is a lot higher, between 60 - 100 per cent.

The cause of the virus is still unknown, although it has been suggested that there are a number of causative agents. One of these is varying climates and temperatures. A French scientist, Tristan Renault, from the genetics and pathology lab at the IFREMER (French Research Institute for Exploitation of the Sea), has suggested in recent publications that one of the contributing factors to the appearance of the disease could be global warming and a subsequent significant rise in the oceans temperature.

Only very occassionally have mortalities occurred when the temperature has been less than 15-16 degrees Celsius.

However Mr Jarrad says that there is a lot of uncertainty about the disease, and until more research is completed, nothing can be set in stone.

Large scale mortalities began in France four to five years ago, with the disease then spreading to Southern Ireland. In early 2010, oyster farms in South England were infected, with smaller mortalities in the Netherlands. Later on in the year, farms in New Zealand became aware of the disease and in January 2011, Australia confirmed an outbreak of the virus.

Prevention

Mr Jarrad says that the most efficient method to prevent the spread of the disease is too minimise the movement of stock. However this is not always successful. In Whitstable, South England, a closed farming system was in place, however oysters were still found to be infected with the virus.

The government and seafood bodies have advised producers not to bring any oysters for re-watering into the UK from affected areas.

In Whitstable, it has been discovered that some stocks are still infected with the virus, but no further mortalities have occurred. It is still unknown whether stock can hold the virus dormant and then rid themselves of it.

To date, all mortalities have been connected back to presence of the virus in oyster stocks.

In 2010 the EU introduced the regulation 175/2010, which require all member states to test areas of large scale mortality for presence of the pathogen. If the virus is present, containment zones are established and movement in and out of that zone prohibited.

Mr Jarrad says that this regulation was due to expire on 31 December 2010, however due to situation continuing to be of great concern to the industry, it has been extended until 30th April 2011.

Research is ongoing, allowing member states more time to look into the disease and its effects on oyster populations.

He says that the Shellfish Association of Great Britain would like to see more regulation, to prevent the spread of the virus. Whilst the UK is keen to adopt certain measures, other countries aren't as interested, he suggested.

Effects on the industry

The virus has started to cause a shortage of oysters across Europe. With France being the number one producer of oysters in Europe, French growers have been forced to approach UK seed hatcheries. This has meant that a lot of UK production has gone to France, which Mr Jarrad says leaves the UK in a precarious situation.

If the virus continues to spread and wipe out stocks, the sourcing of future stocks is an issue that the industry will have to address. In the meantime, prices of oysters have been on the rise.

In New Zealand, it is suspected that the disease has killed off half the juvenile crops in the North Island. The financial loss caused by what the NZ Ministry of Agriculture identifies as Ostreid herpesvirus-1 (OsHV-1) has so far been estimated at NZ$ 30 million.

The Ostreid herpesvirus-1 presents no threat to humans.

February 2011

the Fish Site Editor

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