Bats are dying off by the thousands as they hibernate in caves and mines around New York and Vermont, sending researchers scrambling to find the cause of mysterious condition dubbed "white nose syndrome."

The ailment - named for the white circle of fungus found around the noses of affected bats - was first noticed last January in four caves west of Albany. It has now spread to eight hibernation sites in the state and another in Vermont.

Alan Hicks, a bat specialist with New York's Department of Environmental Conservation, called the quick-spreading disorder the "gravest threat" to bats he had ever seen. Up to 11,000 bats were found dead last winter and many more are showing signs illness this winter. One hard-hit cave went from more than 15,000 bats two years ago to 1,500 now, he said.

"We do not know what the cause is and we do not know how it was spread, either from cave to cave, or bat to bat," said Hicks. "You have this potential for this huge spread."

The white fungus ring around bats' noses is a symptom, but not necessarily the cause. For some unknown reason, the bats deplete their fat reserves and die months before they would normally emerge from hibernation.

New York and Vermont environmental officials are asking people not to enter caves or mines with bats until researchers figure out how the infection is spread. There is no evidence it is a threat to humans, but officials want to take every precaution to avoid it spreading from cave to cave.

Bats are considered particularly vulnerable when they hibernate, a time when they can hang together tightly by the thousands. Indiana bats, a federally endangered species, are considered particularly vulnerable, though the highest death count has been among little brown bats.

Researchers with Cornell University and the National Wildlife Health Center in Madison, Wis., are among those helping state environmental officials.

The bat die-off has some eerie similarities with "colony collapse disorder," the baffling affliction that began decimating honeybee colonies years ago. Scientists last fall said they suspected a virus previously unknown in the United States.

"I'm very concerned," Hicks said. "I can only hope that what we're seeing today will dissipate in the future."