What is it?
Oyster herpesvirus is a virulent viral disease of the Pacific oyster Crassostrea gigas (also known as the rock or cupped oyster). This is the only species of shellfish that is currently known to be susceptible to this virus.
The disease is temperature dependent only occurring when water temperatures exceed 16˚C. The highest mortalities occur in juvenile oysters, however all life stages are thought to be susceptible to infection.
Adult mortality varies between 10 to 30 percent, however juvenile mortality is a lot higher, between 60 to 100 percent.
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.
Where and when might it occur?
Oyster herpesvirus has been found in many parts of the world including the USA, Europe, Japan and China where it occasionally causes a condition in Pacific oysters called summer mortality.
However a new variant of oyster herpesvirus (OsHV-1 var) associated with very high levels of mortality was recognised in France causing exceptional mortality events in Pacific oysters in 2008 and 2009.
This disease has spread throughout the major oyster growing areas in France, including both the Atlantic and Mediterranean coasts, and has also occurred in Jersey and in parts of Ireland.
In 2010 the UK experienced the first outbreak of oyster herpesvirus at a shellfish farm in Whitstable, Kent.
Diagnosis
High levels of mortality usually occurs in in water temperatures exceeding 16˚C (60.8˚F).
Control and treatment
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.
Producers are advised not to re-water any oysters from infected areas.
There is no cure currently available.
Source: Defra