The limestone room carved by water and time had a deep, impenetrable darkness. My words echoed, but sounded distorted and muffled as they stretched into the expanse. The air felt thick like the hottest humid day, but my coat couldn't keep out the chill.
I had gone there willingly, trailing a small team of scientists down the long visitor ramp at the main entrance to central Kentucky's Mammoth Cave, which is part of the US National Park system and one of America's natural treasures. When we reached the end of the handrails and lights marking the guided tour routes, we kept walking, further into the depths of the longest known cave system in the world.
It was October 2016. We were looking for bats -- or rather, a lack of bats.
Dark markings on the ceiling of one section proved the nocturnal creatures had once gathered there in the thousands to hibernate. That doesn't happen anymore.
One culprit, along with rising cave temperatures, is a deadly fungal disease called white-nose syndrome (WNS).
Bats are major pollinators for bananas, agave and other plants in the southwest. In the eastern US, they contribute more than $3.7 billion in pest control annually by eating insects that destroy farm crops. One bat can eat thousands of bugs, including mosquitoes, in a single night. Areas with WNS outbreaks are seeing an increase in mosquitoes and mosquito-borne diseases like West Nile Virus, explains Hazel Barton, a professor of microbiology at the University of Akron in Ohio.
"I've been in some caves where there were once 300,000 bats and we walked through the cave and were ankle deep in carcasses," Barton says with a resigned tone, tinged with sadness. The cause? White-nose syndrome.
Barton says she doesn't think much can be done to stop the disease from spreading, but there's still a lot to learn from it. About 20% of new disease outbreaks are fungal, she tells me.
Other researchers aren't giving up on the bats just yet. They're trekking into caves with thermal imaging cameras to study bats in hibernation; they're treating sick bats in labs with UV light therapies and topical medicines; and they're testing the efficacy of different vaccine delivery methods -- all in an effort to protect these elusive nighttime flyers.
In 2007, scientists surveying hibernating bats in Hailes Cave, roughly 20 miles west of Albany, New York, discovered thousands of dead bats. Some of them had a curious white fuzz on their faces. The team took pictures and filed a report that would eventually confirm the first recorded cases of WNS in North America.
"Dead bats were also occasionally observed up on the walls or ledges on the ceiling," Nancy Heaslip, a wildlife biologist at the New York State Department of Environmental Conservation, wrote in the report from the Hailes Cave trip. "These bats appeared to simply have died while hanging. None were forced into crevices as if by flood water, they were just dead."
The name "white-nose syndrome" is literal. Infected bats develop a fuzzy white ring around their noses and ears. The disease is caused by Pseudogymnoascus destructans, a fungus that thrives in cold temperatures. During hibernation, a bat's body temperature drops from around 94 degrees Fahrenheit to near freezing, stimulating the growth of Pseudogymnoascus destructans.
Bats hibernate in large clusters numbering hundreds, and sometimes thousands -- and WNS spreads by touch. As the fungus invades a bat's skin, it causes dehydration and irritation. The infected bat will wake up, disoriented, and rub against other bats, unwittingly spreading the disease. When the bat leaves the cluster to search for water, it typically starves or freezes to death, burning the crucial energy reserves it needed to survive the winter months.
White-nose syndrome is an "old-world disease," Barton explains. Bats in Europe have it, but they don't get sick from it. It was likely brought to the US from Europe by a bat that crossed the ocean in a shipping container.
Since reaching North America, WNS has spread to 33 US states, seven Canadian provinces and 13 bat species out of the 47 that live in the US and Canada.
Bat mortality from white-nose syndrome varies, but typically ranges between 70 and 90 percent for susceptible species. Certain species, like the Indiana bats once common in Mammoth Cave, are almost completely gone. Barton estimates tens of millions of bats have died in North America since the first cases were reported by Heaslip.
There's no evidence that people contract white-nose syndrome, but they can spread it. WNS mainly spreads from bat to bat, but it also spreads from a person to a bat -- or from a surface to a bat.
"Pathogens are very specific. There has to be a specific set of traits for it to cause damage," Barton explains.
It works similarly with bats and coronaviruses. Bats can carry coronaviruses, the spiky ring-shaped viruses responsible for SARS and MERS in humans, without getting sick. But if it makes the leap from bat to humans, it can result in disease.
The recent coronavirus outbreak, responsible for the respiratory disease COVID-19 in humans, may have started with close contact between people and bats or pangolins (scaly mammals that look like a cross between an armadillo and an anteater), according to a report published in Nature Medicine.
Scientists can detect white-nose syndrome in bats because they practically glow in the dark. The fungus that infects the bats contains a fluorescent compound that glows under certain wavelengths of light. When scientists shine the specialized light on a bat's wing or a cave ceiling and it "looks like it was flecked with fluorescent paint," they can be sure the bat has white-nose syndrome.
Bats might also have the telltale white nose, but that isn't always present, depending on the stage of the infection.
Scientists can also collect samples directly from bats, excrement and cave soil to run polymerase chain reaction tests, which definitively diagnose WNS. The PCR test looks for specific fungal genes and is the same process scientists use to diagnose humans with diseases like COVID-19, HIV and tuberculosis.
"The PCR test allows scientists to understand the level of infection as well as provide timely feedback to the field biologists," says Jeffrey Lorch, a microbiologist with the US Geological Survey. Having information about the extent of the disease on a certain bat or in a certain cave -- or section of a cave -- makes it easier for the team to decide where to focus their research efforts.
Teams also take remote-controlled thermal imaging cameras into caves to keep an eye on hibernating bats during the winter.
A continent-wide program called "NABat," or the North American Bat Monitoring Program, uses special ultrasonic equipment in the summer to track bats and estimate their numbers both before and after WNS reaches an area.
Paul Cryan, a bat research biologist with the USGS Fort Collins Science Center, refers to NABat as "coordinated ultrasonic bat call detection."
"We have never seen anything like WNS. Looking back over the 30 years I've been researching both historic and emerging threats to bat populations, this ghastly new fungal disease ravaging our continent's many hibernating bats has overshadowed much of our prior understanding and protection efforts," Cryan says.
Researchers have had to get creative.
In an effort to save bats, multiple scientific agencies and universities have joined forces, explains Jonathan Reichard, national assistant coordinator for white-nose syndrome with the US Fish and Wildlife Service.
Together, the groups are experimenting with topical solutions, gels, probiotics and other ointments to help treat bats with WNS. They're also testing out a UV light exposure treatment; it's a different wavelength than the one used to detect the glowing presence of the fungus that causes WNS, Reichard notes.
Then there's the vaccine. The USGS has had some success with WNS vaccines in lab environments, but have run into challenges testing it with bat populations in the wild. That's partly because they need a large sample size of bats -- and there aren't as many bats as there used to be.
"I'd love to say we learned how to stop these kinds of things, but I think we learned more about how difficult they are to control once they establish," Barton, the microbiology professor at the University of Akron, says. "If we see something like this again, we're just gonna burn the house down immediately."
It would have been unthinkable in 2007 for scientists to seal off Hailes Cave and fumigate all the infected bats, she explains, "but ultimately that might've been the only thing that we could've done."
With millions of bats dead, it's difficult to blame Barton for feeling this way.
"It's not easy," Reichard admits when I ask how he keeps doing this work. He was a Ph.D. student back in 2007 when the first cases of WNS were reported in the US.
He leans on the community of people dedicated to keeping bats safe today, despite the challenges -- and is hopeful they're one great idea away from stopping the spread.
"People often think that scientists already have most of the answers and that most discoveries have been made," Cryan says. "WNS is a great example of how there are still many surprises out there, but also how science is an incredibly powerful and effective way to approach and address new and unexpected problems as they arise."