Citing an "unprecedented" crisis of bats dying off from West Virginia to New England, federal officials on Thursday asked would-be spelunkers to stay out of caves in states struck by "white-nose syndrome."

The U.S. Fish and Wildlife Service made the request to guard against the possibility that visitors are unwittingly spreading the mysterious affliction when they explore multiple caves. There is no evidence that white nose threatens humans.

Named for the sugary smudges of fungus on the noses and wings of hibernating bats, the syndrome pushes those affected to run through their winter fat stores before spring. It was confirmed in eight states this winter from New Hampshire to West Virginia and there is evidence it might have spread to Virginia, wildlife service spokeswoman Diana Weaver said.

Some death-count estimates run as high as 500,000 bats. Researchers worry about a mass die-off of bats, which help control the populations of insects that can damage wheat, apples and dozens of other crops.

The advisory seeking a voluntary caving moratorium also would cover states adjacent to affected states - a swath of the nation stretching from Maine to North Carolina and west to Tennessee, Kentucky and Ohio, Weaver said.

Recreational cavers, who have enthusiastically supported past white-nose control efforts, seemed bewildered by the breadth of the request. Peter Youngbaer, white-nose syndrome liaison for the National Speleological Society, said the advisory covers tens of thousands of caves and would affect everything from organized caving events to equipment sales.

"The ramifications are mind-boggling, and I guess we're all just trying figure out what to do," said Youngbaer, who is based in Vermont. "I think to great extent it will be followed, but there will be a lot of discussion and tweaking about it."

Bob Handley, president of the West Virginia Cave Conservancy, said the four West Virginia caves owned or leased by the conservancy that had been open to caving would be closed "until this emergency is over."

The conservancy owns five caves, leases one and manages three others, most of them in the Lewisburg area. One cave, which lies on the Virginia side of the border, is already under closure orders until April 14 - the approximate end of the hibernation season - following a recommendation by the Virginia Cave Board, after suspected white-nose syndrome was detected in that state.

Handley, who has been caving for more than 60 years, said the advisory would potentially affect thousands of people who go caving in West Virginia.

"We have about 4,400 caves in the state, and 1,300 in Greenbrier County alone," he said. "We have 10 caves in Greenbrier County that are 10 miles or more long, and several that are 20 miles long. We're one of the major cave areas in the United States. People are caving here ... almost every weekend."

Handley said that while he hopes most cavers will heed the advisory, "I can't really say for certain that they will. ... No one really knows if cavers are spreading white-nose syndrome, or if it's the bats themselves, or something else."

The U.S. Fish and Wildlife Service made the request to guard against the possibility that visitors are unwittingly spreading the mysterious affliction when they explore multiple caves. There is no evidence that white nose threatens humans.

Named for the sugary smudges of fungus on the noses and wings of hibernating bats, the syndrome pushes those affected to run through their winter fat stores before spring. It was confirmed in eight states this winter from New Hampshire to West Virginia and there is evidence it might have spread to Virginia, wildlife service spokeswoman Diana Weaver said.

Some death-count estimates run as high as 500,000 bats. Researchers worry about a mass die-off of bats, which help control the populations of insects that can damage wheat, apples and dozens of other crops.

The advisory seeking a voluntary caving moratorium also would cover states adjacent to affected states - a swath of the nation stretching from Maine to North Carolina and west to Tennessee, Kentucky and Ohio, Weaver said.

Recreational cavers, who have enthusiastically supported past white-nose control efforts, seemed bewildered by the breadth of the request. Peter Youngbaer, white-nose syndrome liaison for the National Speleological Society, said the advisory covers tens of thousands of caves and would affect everything from organized caving events to equipment sales.

"The ramifications are mind-boggling, and I guess we're all just trying figure out what to do," said Youngbaer, who is based in Vermont. "I think to great extent it will be followed, but there will be a lot of discussion and tweaking about it."

Bob Handley, president of the West Virginia Cave Conservancy, said the four West Virginia caves owned or leased by the conservancy that had been open to caving would be closed "until this emergency is over."

The conservancy owns five caves, leases one and manages three others, most of them in the Lewisburg area. One cave, which lies on the Virginia side of the border, is already under closure orders until April 14 - the approximate end of the hibernation season - following a recommendation by the Virginia Cave Board, after suspected white-nose syndrome was detected in that state.

Handley, who has been caving for more than 60 years, said the advisory would potentially affect thousands of people who go caving in West Virginia.

"We have about 4,400 caves in the state, and 1,300 in Greenbrier County alone," he said. "We have 10 caves in Greenbrier County that are 10 miles or more long, and several that are 20 miles long. We're one of the major cave areas in the United States. People are caving here ... almost every weekend."

Handley said that while he hopes most cavers will heed the advisory, "I can't really say for certain that they will. ... No one really knows if cavers are spreading white-nose syndrome, or if it's the bats themselves, or something else."