Oyster Herpes Virus (OsHV-1)
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
16C. The highest mortalities occur in juvenile oysters, however all life stages
are thought to be susceptible to infection.
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.
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 16C (60.8F).
Control/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