Oyster Herpes: Latest Symptom of Global Warming?

New strain can kill 80 percent of an oyster bed in a week, experts say.
Rachel Kaufman, National Geographic News 6 Aug 10;

Don't worry—oyster herpes isn't a new side effect of eating "the food of love."

The incurable, deadly virus is, however, alarming fishing communities in Europe, where oyster herpes seems to be spreading—and could go on spreading as seas continue to warm, experts say.

In July lab testing of farmed oysters detected the first known United Kingdom cases of herpes in the shellfish. The virus has already killed between 20 to 100 percent of breeding Pacific oysters in some French beds in 2008, 2009, and 2010, according to the French Research Institute for Exploitation of the Sea (Ifremer).

The reason for oyster herpes's emergence in Pacific oysters off England remains a mystery, though global warming may have played a part, experts speculate.

A new strain named Ostreid herpesvirus 1 (OsHV-1) μvar (mew-var), the virus remains dormant until water temperatures exceed 16 C [61 degrees F], which U.K. waters reach in the height of summer, according to Kevin Denham of the British government's Fish Health Inspectorate.

With that in mind, Tristan Renault, director of Ifremer's genetic and pathology lab, said that global warming "could be an explanation of the appearance of this particular type of the virus."

Though all herpes strains are DNA-based viruses, herpes, which infects everything from cows to clams to monkeys, comes in a wide variety of species, each with their own unique set of symptoms. Among humans, perhaps the best known forms are the Herpes simplex viruses, which are spread through close contact and can manifest themselves as oral and genital blisters.

Ostreid herpes viruses are known to affect not only oysters but also clams, scallops, and other mollusks, according to Renault.

The New Oyster Herpes

Herpes-infected shellfish aren't new to science, but in 2008—the first year a huge increase in mortality rates was detected in France—Ifremer detected a new variation of the virus.

Like the other strains of herpes that affect mollusks, OsHV-1 μvar attacks young oysters during breeding season, when the mollusks' bodies are so focused on producing sperm and eggs that the oysters have no energy to maintain an immune system, Renault said.

But OsHV-1 μvar is "more virulent than strains we identified before," Renault said, adding that the virus is so efficient at killing its hosts that it can wipe out 80 percent of the oysters in a bed within a week.

That death rate is the only outward sign something's wrong, he added, because a oyster herpes have no visible symptoms, and diagnosis is possible only through lab testing.

Oyster Herpes Appears in Britain

Though oyster herpes can't be transmitted to humans, it does threaten the fishing industry, since dead oysters are unsafe for eating—and that's exactly what worries oyster harvesters such as Seasalter Shellfish.

Based in the southeastern English city of Whitstable, where oysters have been harvested for centuries, Seasalter this summer became the first company to discover the herpes-ravaged oysters in the U.K.

The finding prompted an investigation by the Fish Health Inspectorate, which detected the virus and learned that Seasalter had employed equipment previously used in France to refurbish oyster beds.

"We were told it had been out of the water for a number of years," Denham said. "Nevertheless there's still a possibility" that the virus could have traveled from infected French beds via the gear. Possible culprits also include other reused equipment or water transferred from an infected area.

Could Oyster Herpes Spread?

To keep the U.K. oyster-herpes outbreak from spreading, the British government has banned the shipping of oysters out of affected areas, most of which, like Whitstable, are around the mouth of the River Thames in southeastern England.

No matter what measures are taken, Denham said, oyster herpes is going to be tough to kick. Even if all the infected Pacific oysters are removed from oyster farms, wild Pacific oysters will still be present in surrounding waters, perhaps acting as "a reservoir for infection."

It's unlikely, though, that OsHV-1 μvar would end up in U.S. oyster beds, Renault said, because the United States doesn't typically import oysters from Europe.

But a less virulent, herpes-like virus has been detected in farmed oysters off California. If sea temperatures continue to rise, he said, perhaps μvar or something like it could emerge in U.S. waters too.