A virulent form of herpes is killing countless Pacific oysters this summer, according to a new study accepted for publication in the journal Virus Research.

The deadly virus has already destroyed anywhere from 20 to 100 percent of its victims in French oyster beds, and has spread to oysters in Britain. Farmed oysters in California also have tested positive for herpes, although not this latest, highly infectious strain.

(The oyster Crassostrea gigas; Credit: David Monniaux)

"Herpes and herpes-like viruses are known to infect a wide range of bivalve mollusk species throughout the world," according to co-author Tristan Renault and his colleagues. Renault is director of the French Research Institute for Exploitation of the Sea.

He and his team note that "abnormal summer mortalities" associated with the new herpes virus Ostreid herpesvirus 1 (OsHV-1) have been reported among the Pacific cupped Crassostrea gigas, an oyster important to commercial harvests worldwide. It is the number one shellfish resource in Washington State, for example.

The scientists, in a prior study, demonstrated how quickly this new particular strain of herpes reproduces within oysters. It attacks young oysters at times when their immune systems are compromised.

Renault told National Geographic that global warming "could be an explanation of the

appearance of this particular type of the virus." Kevin Denham of Britain's Fish Health Inspectorate explained that the virus remains dormant until water temperatures exceed 61

degrees F. That's been happening a lot this summer.

The "good" news here is that the oyster herpes virus is thought to only infect shellfish, such as clams, scallops, and other mollusks. When a person gets herpes, it's another strain, usually forms of Herpes simplex viruses, which can cause oral and genital blistering. 

So far, there are no known visible symptoms associated with herpes in shellfish.