St. Louis • They hang upside down in caves. They come out only at night. They evoke blood-sucking vampires.

But bats, for all their nocturnal peculiarity, are among the most critical creatures in the ecosystem — and they are dying.

A mysterious condition, called White Nose Syndrome, has killed more than a million bats in North America and appears poised to sweep westward. So today at the St. Louis Zoo, a group of experts is gathering to discuss a potential last-resort solution: taking some bats into captivity in an effort to preserve them.

"We've got some very tough decisions to make" said Jeremy Coleman, the national White Nose coordinator for the U.S. Fish and Wildlife Service, which organized the meeting. "This is an attempt to start a conversation."

This spring scientists found evidence of White Nose in two Missouri counties, and more recently as far west as Oklahoma, bringing the total reach of the problem to 13 states and two Canadian provinces. Scientists say White Nose is responsible for the most dramatic decline of North American wildlife in 100 years, one with potentially dire environmental consequences.

"It's devastating," said Bill Elliott, the state's cave biologist. "They're a very fundamental control on night-flying insects. They eat tons a year in Missouri, and some of them are pests — forest pests, agricultural pests, human pests. Some of these insects are gobbling up our forests, some are spreading West Nile Virus."

Researchers first discovered White Nose in New York four years ago. But in the past year particularly, as the disease has spread, researchers and conservation groups have ramped up efforts to find out how the affliction is transmitted and how, exactly, it is killing bats. They still don't have definite answers.

So far, scientists know this: The faces and wings of hibernating bats become covered with a white fungus, causing their winter sleep to be disturbed. The agitation depletes the bats' fat reserves, making them leave their caves in search of food. Once outside, many die from the cold or starvation.

But researchers don't know where the fungus comes from. And although they know the condition is spread from bat to bat, they aren't sure how. They also believe the fungus, which does not affect humans, could be carried by cavers on their clothes and equipment as they explore one cave and move on to the next.

That hypothesis has prompted federal authorities to close caves in national parks and forests, including the Mark Twain National Forest. In Missouri — dubbed the "Cave State" for its 6,300 caves — the state's Department of Conservation closed more than 100 caves and is urging private cave owners to limit entry into theirs. Federal authorities also have issued a decontamination protocol for all cavers to limit the potential transmission of the fungus.

Some cavers say these restrictions could be unnecessary. In congressional testimony on White Nose, Peter Youngbaer of the National Speleological Society said last year that cave closures had cost areas that host caving conventions thousands of dollars, while students have been unable to pursue cave studies. Youngbaer pointed out that, given the lack of understanding about where the fungus comes from, these measures could be futile.

"If [the fungus] is already present," he said, "then efforts to contain via decontamination and limiting human access may be moot."

For avid cavers, the closures have been tough to take.

Jo Schaper, of Pacific, has been caving for nearly 25 years and is a former president of two area chapters of the speleological society, called "grottos."

"I was married in a cave to a caver by a caver, and I have speleothems on my ring," Schaper said, using the formal term for cave formations. "You bet your belief I miss this."

Schaper said members of the speleological society were concerned about the bats' welfare and were heeding the closures, but some wonder how long the closures will last.

"We're willing to do disinfection; we're willing to abide by closures in the winters. We're not willing to quit caving for the rest of our lives," she said, adding, "The bats are trumping all activity in caves. My background is in geology. I like bats, but I want to see my rocks."

Government researchers say, however, that most caves — not including commercial or 'show" caves — will remain off limits as they try to figure out how to contain the White Nose plague.

"We will have to have a lot of measurements and data before we can make a determination," Elliott said. "We've got to understand a lot more about this disease before we can even think about opening caves."

In the meantime, as scientists struggle to isolate the causes of White Nose, bat researchers are looking at taking bat colonies into captivity or even freezing genetic material.

At the St. Louis Zoo meeting, 35 academics, veterinarians, geneticists, conservationists, biologists and bat rehabilitators will put their heads together over the next three days in preparation for the worst.

"The issue is loaded with lots of ethical and sociological concerns, let alone the scientific challenges," Coleman said. "These animals are very sensitive. They stress easily. They're very hard to maintain. But it's a difficult situation. We're looking at the extinction of multiple species, and we need to know what we should be doing."