Bats have a reputation for being spooky, even though most pose no danger to humans. In fact, bats have far more reason to be scared of us.

That's true for lots of wild animals, but humans have become especially dangerous to North American bats lately. A deadly, cave-dwelling fungal disease known as white-nose syndrome is sweeping the continent, with a mortality rate as high as 100 percent in some bat colonies. After it appeared at a single New York cave in 2006, the fungus has spread to at least 33 U.S. states and five Canadian provinces, killing 6 million bats along the way. Scientists think it's an invasive species that humans accidentally brought from Eurasia, but much about it remains shrouded in mystery.

Scientists are sure, however, that it's bad news for millions of American bats, which recover slowly from population loss since many have just one offspring per year. Bat experts also worry that white-nose syndrome may already be hopping through vast cave networks underneath the U.S. Midwest and Southeast, potentially wiping out endangered species like the gray bat and the Indiana bat.

And what's bad for bats is often bad for people, too. As a top predator of flying insects, bats regulate populations of mosquitoes and other biting bugs that spread disease to humans, as well as agricultural pests like beetles and moths. Every 1 million bats can eat about 700 tons of insects per year, and insect-eating bats save U.S. corn farmers alone about $1 billion per year by eating crop pests. Their value to U.S. agriculture overall ranges from $3.7 billion to $53 billion per year.

A long winter's nap

White nose syndrome causes bats to wake often during hibernation, and thus burn up vital fat stores. White-nose syndrome causes bats to use up vital energy reserves during hibernation. (Photo: Jay Ondreicka/Shutterstock)

Bats are one of the most successful and diverse mammals on Earth, ranging from 4-inch, sub-Arctic furballs with sonar to tropical "macrobats" with 6-foot wingspans and primate-like vision. (Bats are not rodents, despite appearances, and are actually more closely related to primates than they are to squirrels or mice.)

Many North American bats pay a price for living in colder climates, though. Their frequent flapping uses a lot of energy, and freezing temperatures virtually eliminate the protein-rich insects they eat. Some species migrate south, but the majority of U.S. bats tough it out by hibernating in caves or mines until the bugs come back in spring.

Surviving a New England winter with no food isn't easy, and bats undergo extreme physiological changes so they can conserve enough energy. They slow down their heart rates, suppress their immune systems and drop their body temperatures to within one degree of the ambient air. They enter this near-death state for up to two months at a time, waking periodically to stretch, preen, relieve themselves and sometimes mate. These hibernation breaks use up about 90 percent of the energy bats have stored for winter, so it's critical they only wake up at the right times.

Despite its high stakes and risks, hibernation has worked for millennia. It wasn't until the early 20th century that it began to fail for some bats, and only then because of cavers and scientists who disturbed their hibernation without understanding the consequences. Combined with increased pesticide use, habitat loss and bats' naturally slow reproduction rate, this decimated several U.S. bat species over the decades — Indiana bats, for example, fell by 50 percent from 1967 to 2005, and now half of the species' worldwide population spends winters in just two caves.

But today, all 25 U.S. species of hibernating bats face perhaps the greatest threat to their business model they've ever seen. The seemingly safe caves and mines where they've always wintered are increasingly infected with Pseudogymnoascus destructans, a previously unknown fungus that's now implicated in "one of the most severe wildlife diseases ever recorded," as a recent study described it.

A curiosity killed the bat

white-nose syndrome A little brown bat from New York shows the telltale symptom of white-nose syndrome. (Photo: U.S. Fish and Wildlife Service)

In February 2006, a caver spelunking in upstate New York found groups of hibernating bats with a strange white fuzz growing on their snouts. Looking around further, he also noticed several dead ones. He shot some photos, but the winter soon faded with little cause for alarm.

The next winter, bats at several nearby caves started behaving strangely — waking up from hibernation too early, then flying outside as if spring had arrived — and many once again had distinctive white fuzz on their noses, ears and wings. By January 2007, the New York Department of Environmental Conservation was aware of the outbreak, and state biologists documented a few hundred bat deaths.

At least 6 million more bats have died since then, according to the U.S. Fish and Wildlife Service. White-nose syndrome has now spread north to Canada, south to Alabama, and as far west as Seattle (click map below to enlarge).

white-nose syndrome map 2017 A map showing the spread of white-nose syndrome since 2006. (Image: whitenosesyndrome.org)

Fungus among us

Scientists know the disease can be transmitted between bats within a colony, but it's not entirely clear how the fungus moves from cave to cave. A leading theory suggests people are the carriers.

"Looking at how it jumped and where it jumped to in Virginia, there may also be some human transmission — that is, a human caver transmitting it to bats, then bats to other bats," says U.S. Fish and Wildlife Service biologist Susi von Oettingen.

The decline in bat populations has been so drastic that state and federal agencies have been closing many caves, and the FWS has called for a voluntary halt to caving in affected states. Going spelunking in almost any North American bat cave is quickly becoming an ecological faux pas, and the fungus now looms over a sprawling labyrinth of caves in the Midwest and Southeast that serves millions of hibernating bats each winter, including four endangered species.

Pseudogymnoascus destructans A lab culture of P. destructans, the fungus that causes white-nose syndrome in bats. (Photo: Raudabaugh DB/Wikipedia)

So far, cave closures are about the only thing wildlife managers know to do. The fungus that's suddenly threatening virtually all hibernating bats in North America was unknown to science before 2006. Its indirect style of killing its hosts is equally novel.

"This has never been documented anywhere before, worldwide," von Oettingen says. "This is a first."

While the fungus was unknown to science before 2006, however, it definitely isn't new. It's an ancient species native to Europe, as researchers concluded in a 2015 study, and was introduced relatively recently to North America, where it has become an invasive species. It's likely "a true fungal pathogen of bats," according to another recent study, that evolved alongside Eurasian bats for millions of years — enough time for those species to develop defenses. When the fungus reached North America, however, it was able to quickly capitalize on a continent full of defenseless bats.

From cave to grave

little brown bat with white-nose syndrome This little brown bat was the first confirmed case of WNS west of the Rocky Mountains. (Photo: PAWS/FWS)

White-nose syndrome doesn't directly damage, or even infect, any of bats' internal organs. P. destructans comes from a family of cold-loving, "keratinophilic" fungi, meaning they feed on keratin, the substance that makes up skin and hair.

But if bats' vital organs are left untouched, what could be killing them? And what possesses them to fly outside during winter?

"One of the theories is, because it's invading the skin in winter, it could be an irritant, waking up the bat from hibernation because it itches and causes stress," von Oettingen says. "The bat may then leave the cave simply to try to flee the itching."

P. destructans is still poorly understood, but most affected bats seem to die from starvation, having exhausted themselves by flying around — or just being awake — when food is scarce. Their dead bodies often have little or no fat left.

Other theories range from wing infections, which might disrupt bats' temperature-regulating abilities, to disorientation and confusion, which could be what sends them flying outside. Some insect-eating Cordyceps fungi even control their host's body, send it zombie-walking to an open location and then kill it, all so a mushroom can grow from its corpse and release spores into the wind.

While mushrooms aren't likely to start sprouting from dead bats, Cordyceps is an extreme example of how aggressively adaptable such predatory fungi can be. In an online article about white-nose syndrome, U.S. Geological Survey biologist Paul Cryan says P. destructans — initially named Geomyces destructans — "appears to be exquisitely adapted to persist in caves and mines and to colonize the skin of hibernating bats."

Lights at the end of the tunnel

insectivorous bats at sunset Many bats provide free pest control, often feasting on hundreds of winged insects per night. (Photo: Andy Kainz/Flickr)

Sometimes natural immunity emerges as a ray of hope during disease outbreaks, and there have been a few hints of resistance to WNS in some of the earliest-hit states like Vermont. This hasn't significantly helped bat populations overall, however, and the versatility of P. destructans means it may be virtually impossible to eradicate from caves — even after all the bats are gone. In other words, the fact it doesn't depend on bats to survive could make it even more dangerous to bats.

Finding a cure, vaccine or treatment won't be easy, either. It would need to be something that can easily be applied to a large number of bats, is safe for bats, is safe for people, and isn't deadly to other, beneficial fungi that also live in the cave.

Scientists are still trying, though, and they have made some promising discoveries in recent years. In 2015, for example, researchers at the University of California Santa Cruz identified six bacterial strains that inhibit the growth of P. destructans, including two that suppressed fungal growth for more than 35 days. The same year, researchers in Missouri released several dozen bats after successfully ridding them of WNS using native North American soil bacteria. And in early 2018, a team of scientists revealed that P. destructans has "extreme sensitivity" to ultraviolet light, which they called a "potential Achilles' heel" for the fungus.

These findings are all encouraging, but so far they haven't yielded any large-scale treatments that can slow the historic decline of North America's bats. The urgency of white-nose syndrome is driving a flurry of similar research, though, and this could be a pivotal time for discovering the disease's secrets, and for planning how to save endangered bat species before it's too late.

Editor's Note: This story has been updated since it was first published in October 2009.