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Scientists have identified a form of herpes as the culprit in a widespread viral outbreak that has killed as many as 8 billion French oysters in recent weeks. Experts warn that the great oyster devastation of 2008 will result in a shortage in supplies of the shellfish over the next two years, and could push many people who cultivate and sell the creatures to financial ruin.

Researchers have been scrambling to respond to a crisis that has rocked France's ostreiculture industry since last month: the discovery that 40% to 100% of oysters aged 12 and 18 months being raised in France's Atlantic cultivation beds had died. The reason, officials at the French Institute for Research Into Use of the Sea (Ifremer) say, is Oyster Herpes Virus type 1 (OsHV-1). That virus, has proliferated along France's Atlantic coast due to a mild winter and abundant rains that allowed ocean water to remain warm, scientists believe. Those same conditions have also created an abundance of plankton — a cornucopia of nutrition that the shellfish have gorged on.

Fully fed and assured of more food whenever they wanted it, the youthful oysters turned their energies and attention to their sexual organs, leaving the rest of their system vulnerable to herpes infection. "Once sufficiently nourished to ensure for survival, oysters focus on development of sexual organs — creating sperm and ovaries mostly — and ignore their defense systems," explains Tristan Renault, the director of Ifremer's genetic and pathology laboratory. "Oysters are capable of only one function at once, especially in springtime. So if a threat is posed during this period of concentrated sexual development, they're very susceptible."

To make matters worse, the same climatic changes that caused the abundance of herpes and plankton on the Atlantic coast — and which contributed to an explosion of jellyfish in Mediterranean waters have also caused a proliferation of Vibrio splendidus bacterium. The effects of that bacteria left younger oysters both more vulnerable to herpes infection, and less capable of battling the virus as it killed them. Scientists fear that as waters heat up thanks to global warming oysters may regularly face such conditions in the future, disrupting France's annual oyster production of 120,000 tons — the largest in Europe and fourth biggest in the world.

To avoid that, Renault and his Ifremer colleagues are looking for ways to battle the OsHV-1 threat. They want to know why cultivations in the Arcachon basin southwest of Bordeaux have been spared while virtually all others in France have seen most or all of their young oysters decimate. Experts will also look for reasons why a few individuals in those otherwise devastated populations survived. "Since we can't vaccinate shellfish, one way of protecting future generations is to use oysters that are resistant to OsHV-1 and generally more robust in reproduction [to produce] future oyster generations that may better withstand the virus," Renault explains.

The good news for oyster farmers and consumers alike is that OsHv-1 has no impact on older mollusks, nor on humans who consume them. With China producing over 80% of the world's oyster supply-Japan and South Korea are the next biggest growers — there's no great risk of a global shortage.

• Bruce Crumley / Paris
• An outbreak of Oyster Herpes virus type 1 has lovers of the salty delicacy crying