A new study takes a closer look at how exactly white-nose syndrome has killed 1 million bats in 11 states and two Canadian provinces as it has spread westward from New York.
Researchers with the U.S. Geological Survey now suspect the fungus damages bat wings so severely that it allows stored water to drain out of the bats' bodies, causing dehydration and unusual thirst-driven arousals from hybernation.
Researchers say the destruction caused by the fungus may represent a completely new extreme for mammals, whose skin infections typically do not invade living tissue.
"This fungus is amazingly destructive — it digests, erodes, and invades the skin — particularly the wings — of hibernating bats,” said Carol Meteyer, USGS pathologist and lead author of the research. “The ability of this fungus to invade bats’ wing skin is unlike that of any known skin fungal pathogen in land mammals.”
The study's authors looked at nearly 200 bats that had died from white-nose syndrome and reviewed the function of bat wings during hybernation. They found fungal damage to wings could affect circulation, body temperature regulation and respiratory function, as well as causing unsustainable dehydration.
"Although the powdery white muzzles of affected bats gave the disease its name, the authors believe that the skin of bat wings is the most significant, though often less obvious, target of the fungus.
The order of bats is called Chiroptera, Greek for “hand-wing,” appropriately named since bat wings are essentially modified arms. Imagine, for a moment, your human hand with its fingers spread apart. Then imagine your fingers are 6 feet long, and the whole skeletal affair is covered with two layers of thin, somewhat transparent membranes attached to the sides of your torso and legs. Sandwiched between the membranes are blood and lymphatic vessels, delicate nerves, muscles and special connective tissues that help you fly and help keep you physiologically healthy.
“The disproportionately large areas of exposed skin that make up bat wings play critical roles in maintaining safe internal body conditions during hibernation,” noted Paul Cryan, a lead author and USGS bat ecologist. “Healthy wings are essential for day-to-day survival, even during winter when bats are mostly just hanging around. Wings damaged by the fungus may not always look so bad to the naked eye, but under the microscope things get ugly fast.”
When Meteyer examined wings of diseased bats microscopically, she discovered wing damage was often so severe that it led her and her colleagues to suggest multiple life-threatening effects on hibernating bats.
“A bat’s wings,” said Meteyer, “are obviously critical for flying, but they also play a vital part in essential functions such as body temperature, blood pressure, water balance and blood and gas circulation and exchange.”
Healthy bats occasionally rouse themselves from hibernation, probably to change roosts, drink, mate and even overcome sleep deprivation, biologists think. But bats afflicted with WNS arouse much more often. In fact, a characteristic of hibernation sites with WNS is daytime flights of affected bats outside caves.
“The prevailing hypothesis is that daytime winter flight is a last-ditch effort for starving bats to find insect prey,” Cryan said. “What we propose is that thirst, and maybe not always hunger, is driving these arousals. Unusual thirst during hibernation may result from water essentially leaking out of wings damaged by the fungus.”
Anecdotally, bats at hibernacula affected by WNS are sometimes seen flying over and drinking from water surfaces or eating snow, highlighting the plausibility of this hypothesis, the authors noted.
Hibernation itself is one reason this emerging disease is so successful. During hibernation, a bat’s immune function and metabolism are dramatically reduced, and body temperature drops significantly. Also, some of the worst-affected bat species roost in humid areas in dense clusters to conserve energy and decrease moisture loss.
“These ideal environmental conditions, combined with the hibernating bat’s suppressed immune system, likely allow the fungus to invade body tissues for nutrients without resistance, making the hibernating bat a most accommodating host for this new disease,” Meteyer said.