By Laila Kearney

SAN FRANCISCO (Reuters) - Scientists are struggling to find the trigger for a disease that appears to be ravaging starfish in record numbers along the U.S. West Coast, causing the sea creatures to lose their limbs and turn to slime in a matter of days.

Marine biologists and ecologists will launch an extensive survey this week along the coasts of California, Washington state and Oregon to determine the reach and source of the deadly syndrome, known as "star wasting disease."

"It's pretty spooky because we don't have any obvious culprit for the root cause even though we know it's likely caused by a pathogen," said Pete Raimondi, chair of the Department of Ecology and Evolutionary Biology at the University of California at Santa Cruz's Long Marine Lab.

Signs of sea star wasting syndrome typically begin with white lesions on the arms of the starfish that spread inward, causing the entire animal to disintegrate in less than a week, according to a report by the Pacific Rocky Intertidal Monitoring Program at the University of California at Santa Cruz.

Starfish have suffered from the syndrome on and off for decades but have usually been reported in small numbers, isolated to Southern California and linked to a rise in seawater temperatures, which is not the case this time, Raimondi said.

Since June, wasting sea stars have been found in dozens of coastal sites ranging from southeast Alaska to Orange County, California, and the mortality rates have been higher than ever seen before, Raimondi said. In one surveyed tide pool in Santa Cruz during the current outbreak, 90 to 95 percent of hundreds of starfish were killed by the disease.

"Their tissue just melts away," said Melissa Miner, a biologist and researcher with the Multi-Agency Rocky Intertidal Network, a group of government agencies, universities and nonprofits that monitor tidal wildlife and environment along the West Coast.

Miner, based in Washington state, has studied wasting starfish locally and in Alaska since June, when only a few cases had been reported. "It has ballooned into a much bigger issue since then," she said.

The syndrome primarily affects the mussel-eating Pisaster ochraceus, a large purple and orange starfish, but Raimondi said that at least 10 species of sea stars have shown signs of the disease since June.

If the numbers of Pisaster ochraceus begin to decrease, mussels could crowd the ocean, disrupting biodiversity, he said. He has studied wasting starfish and will aid in the months-long survey of the animals, along with other state and federal researchers.

In addition to on-site sampling, scientists in the coming months will use an interactive map to spot starfish wasting location patterns and help identify a driver for the disease.

Raimondi said he could not estimate, out of the millions of starfish on the West Coast, how many have been affected or could be in the future.

"We're way at the onset now, so we just don't know how bad it's going to get," he said.

(Editing by Cynthia Johnston and Lisa Shumaker)