Madison, Wisconsin—Damage to bat wings from the fungus associated with white-nose syndrome (WNS) may cause catastrophic imbalance in life-support processes, according to newly published research.
This imbalance may be to blame for the more than 1 million deaths of bats due to WNS thus far, proposes Carol Meteyer, a pathologist with the U.S. Geological Survey’s National Wildlife Health Center and a lead author of the research published in BMC Biology.
Physiological problems caused by the novel fungus, may, in fact, represent a completely new disease paradigm for mammals, Meteyer and her colleagues wrote. Other skin infections in mammals due to fungi (ringworm, athlete’s foot) remain superficial and do not invade living tissue—typically they only affect the surface of skin, hair and nails.
Not so for the aptly named Geomyces destructans.
“This fungus is amazingly destructive — it digests, erodes, and invades the skin — particularly the wings — of hibernating bats,” said Meteyer. “The ability of this fungus to invade bats’ wing skin is unlike that of any known skin fungal pathogen in land mammals.”
The authors examined nearly 200 bats that had died from WNS, and also reviewed the critical function and physiology of bat wings during hibernation. As a result, they propose that G. destructans may cause unsustainable dehydration in hibernating bats, triggering thirst-associated arousals. In addition to the direct damage to the wings that would alter flight control, the erosion and invasion of skin may also cause significant changes in circulation, body-temperature regulation and respiratory function.
Since signs of the disease were first observed in New York during the winter of 2006-07, the fungus has spread through 11 states and 2 Canadian provinces, resulting in the first sustained high-mortality disease affecting bats in recorded history. Biologists assume that as the disease spreads to new areas, cave-hibernating bats in those areas will also be at risk, including some that are endangered.
“The high number of bat deaths and range of species being affected far exceeds the rate and magnitude of any previously known natural or human-caused mortality event in bats, and possibly in any other mammals,” said Paul Cryan, a lead author of the paper and a USGS bat ecologist at the Fort Collins Science Center.
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 Cryan. “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.
The researchers compare the ability of this novel bat fungus to destabilize internal functions with the electrolyte imbalance that occurs in frogs infected by chytrid fungus, which, like G. destructans, is a novel disease of vertebrates. Chytrid infection impairs the ability of frog skin to regulate hydration and internal equilibrium, causing electrolyte imbalance and ultimately cardiac arrest.
“The skin plays a critical role in the physiology of both amphibians and bats,” Meteyer said. “We suggest that a similar, but less subtle, disturbance could be occurring in the wing membranes of bats with WNS.”
The journal article can be accessed online.
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