Tattered Wings: Bats Grounded by White-Nose Syndrome's Lethal Effects on Life-Support Functions of Wings
Damage to bat wings from the fungus associated with white-nose syndrome (WNS) may cause catastrophic imbalance in life-support processes, and this imbalance may be to blame for the more than 1 million deaths of bats due to WNS thus far. Paul Cryan, USGS bat ecologist at the Fort Collins Science Center, discusses this newly published USGS research.
Juliette Wilson: Hello and welcome to the USGS CoreCast. I'm Juliette Wilson and today I'm talking with USGS bat ecologist, Dr. Paul Cryan. Paul is one of the lead authors on just-published research about white-nose syndrome in bats. White-nose syndrome is a disease of hibernating bats now thought to be caused by a cold-living fungus aptly named Geomyces destructans. Since the disease emerged in the winter of 2006 to 2007, white-nose syndrome has spread more than 800 miles, is affecting several species of bats, and has resulted in the deaths of more than 1 million bats.
Paul, thank you for being here today and talking with us, especially since winter is about to begin. Doesn't that mean you're maybe seeing more cases of white-nose syndrome?
Paul Cryan: Yeah. Thanks, Juliette. It's likely that this disease is going to continue to wreak havoc on communities of hibernating bats in the northeastern United States. And this winter is going to be one when we find out whether white-nose syndrome is going to continue moving across the continent, imperiling even more species of hibernating bats.
As far as we can tell at this point, white-nose is a disease that primarily affects hibernating bats during winter, when they're hiding out in these cold, wet, underground sites likes caves and mines, what we call hibernacula. In North America, there are roughly 25 species of bats that hibernate to survive the winter. So, there's a lot at stake here.
Since the disease first emerged in a small area of upstate New York about three winters ago, we've seen it kill staggering numbers of bats in a growing area of eastern North America. The syndrome right now is affecting all six species of hibernating bats in the area where it has occurred for at least two winters, one of those species being the endangered Indiana bat. But it's not just killing endangered bats. We're seeing species that were once common in eastern North America suffering what may be catastrophic population losses. We don't expect these populations to recover very quickly.
Out ahead of the spread of the disease, white-nose syndrome, we're seeing the spread of a fungus now thought to cause the disease, Geomyces destructans. Late last winter, traces of the fungus were detected in three additional species of hibernating bats and in new regions of the continent as far west as Oklahoma. We haven't seen the characteristic disease or associated mortality with fungal presence in these other species or in the regions, but we suspect that those were early detections of the disease. There's a lot of uncertainty right now about how things are going to play out this winter and only time is going to tell.
There are another 14 species of hibernating bats that occur farther west of the current white-nose zone. We suspect they're in harm's way, so it's going to be a very long winter.
Juliette Wilson: So, how does this Geomyces fungus kill a bat? What is the proposal that you and your colleagues make in the new research paper in the journal, BMC Biology?
Paul Cryan: Well, Juliette, in a nutshell, existing explanations for why bats die from white-nose syndrome mostly centered on this idea that hibernating insectivorous bats starve after fungal infection causes them to deplete their winter fat reserves, mostly by triggering energy-wasting aberrant behaviors when they should be hibernating.
In our BMC paper, we take this a step further and suggest several distinct ways by which fungal damage to bat wings could also cause catastrophic imbalance in these life-support processes and outright kill bats during winter.
Juliette Wilson: How can a fungus that only infects the skin of an animal ultimately cause death? Wouldn't that be like athlete's foot causing us to die?
Paul Cryan: Two points we thought were being under-appreciated in the disease investigation of white-nose thus far. First, this fungus is very different from any we've ever seen infecting the skin of land mammals. And second, the wings of hibernating bats are unlike the organs of any other animals on Earth.
So, although it's not apparent to the naked eye, Geomyces destructans is doing incredible damage inside the living tissues of bat wings. So, my co-author Carol Meteyer, who is a wildlife pathologist at the USGS, spent a lot of time looking through a microscope at wings of bats killed by white-nose. And what she consistently sees is extensive damage to the living skin tissues caused by this fungus.
There are a lot of fungi out there that can infect the superficial layers of the skin, which is mostly dead skin cells; athlete's foot, as you mentioned, is a perfect example. Geomyces destructans is very different. Somehow, it and not the thousands of other fungi undoubtedly out there in these bat hibernation sites found a way to infiltrate, digest, and destroy the deeper layers of skin in bats when they hibernate. We don't know why, but somehow it can.
Those of us that study bat ecology and physiology, including our co-author Justin Boyles, were considering the many, many ways that wings are extremely important to bats during hibernation. Bats are unique in the animal kingdom in that they're the only animals that fly on wings made entirely of living skin.
Skin is the largest body organ. And the wing surface area of bats is more than four times larger than all their other body surfaces. So, in addition to keeping everything wrapped up and serving as a barrier against microorganisms, skin plays really important roles in sensation, temperature regulation, fluid balance, and in bats, exchanging oxygen and carbon dioxide with the environment. So, bat wings aren't just important for flying. They're proportionately huge, living dynamic surfaces that help keep bats healthy when they're coping with these harsh conditions of winter.
Then, from that, we propose several testable hypotheses for how damage to wing skin caused by Geomyces destructans may directly kill bats—perhaps by leading to unsustainable water loss and dehydration, by disrupting blood flow and making it difficult for bats to warm up from hibernation without burning too much energy, by causing the buildup of waste gases or shortage of oxygen in their bodies, or by destroying touch sensors and other tissues in their wings that are necessary for controlling the subtleties of everyday flight. There's nothing like the wings of hibernating bats. And so far as we know, there is nothing else like this fungus, which when you put those two things together, make for a nasty situation.
Juliette Wilson: In the paper, you all say that hibernation itself is one reason this emerging disease is so successful. Why is that?
Paul Cryan: This fungus, Geomyces destructans, grows best at cold temperatures, between about 34 to 60 [degrees] Fahrenheit. Bats must hibernate in cold underground sites that tend to have these same range of temperatures during the winter. So, during hibernation bats suppress their metabolism and their body temperatures drop to within the optimal growth range of this fungus. The bats must do this to survive on limited body fat when insect prey isn't available. Their immune system is probably shut down with their metabolism during hibernation. Combine those conditions with dampness, darkness, and animals that cluster and can disperse tens to hundreds of miles, and you sort of have a recipe for disaster.
Juliette Wilson: Are there other fungi that only infect the skin yet have the potential to be the sole cause of death in bats or other animals?
Paul Cryan: We don't know of any fungi that can affect only the skin of healthy, warm-blooded birds and mammals and be the sole agent of death. We do know of numerous fungal pathogens that directly cause death in so-called cold-blooded animals and plants; sudden oak death, chestnut blight, potato blight, crayfish plague, just to name a few.
And the disease caused by chytrid fungus in amphibians is another very good example. And in the case of amphibians, which like bats exchange remarkable amounts of water and gas through their skin, the infection caused by the chytrid fungus is much more superficial than what we see with Geomyces destructans in bat wings. Yet, chytrid infection is still lethal and is causing widespread population declines and extinctions of amphibians throughout the world.
Juliette Wilson: Do these examples, chytrid and white-nose syndrome, make you and other ecologists think that there is something going on that is causing fungi to become more deadly and dangerous?
Paul Cryan: I don't think we have evidence that fungi are generally becoming more dangerous. After all, we mammals have been around on Earth for only about 200 million years and fungi have been around for about a billion years. They've had their chances at us. And in general, our warm body temperatures tend to keep them out.
In the cases of chytrid and white-nose, we think it's more likely that we're seeing the results of humans inadvertently moving fungi across oceans and into contact with animals that haven't yet met those organisms or had a chance to evolve defenses. Geomyces destructans is potentially an exotic species that was recently introduced to North America, so same deal with the global spread of chytrid fungus: biosecurity is important in these cases.
Juliette Wilson: If the wings are ultimately responsible for the deaths of these bats, is there anything that can be done?
Paul Cryan: White-nose syndrome is a wildlife disease. And as with most wildlife diseases, there aren't any feasible options currently available for completely halting or curing it. There are intensive efforts underway to try and find new ways of treating the disease and efforts to find new ways of both assessing and limiting its impacts on these affected populations of bats.
Our work tends to focus on how the disease is transmitted and kill[s] bats. And by studying the chain of events that lead from exposure to the fungus to death, we may be able to find those weakest links where we can direct some intervention and management actions. We can also learn how to better respond to wildlife diseases through these experiences.
Like most emerging diseases, this one blind-sided us from out of nowhere. And there are some very important lessons we've learned about quickly organizing a response. We may eventually see bats consistently survive exposure and develop immunity to disease. And therein lies most of the hope. If and when we start seeing survivors, we can do our best to help ensure their well-being by putting into place all the really effective conservation techniques that were developed for bats and proven to be very useful before white-nose came along and changed everything.
Juliette Wilson: Earlier, you mentioned the aberrant behavior of bats—what does that look like? And if people see bats acting unnaturally like that, who should they contact?
Paul Cryan: The most obvious indication that white-nose syndrome has reached an area is when people start seeing unusual numbers of bats outside during the day in winter. Typically starting after about December, bats in hibernacula affected by white-nose often leave those sites and can be seen flying in daylight, crawling on the ground, hanging out on buildings, and generally being in other unusual places.
So, if you see something suspicious, please contact your state wildlife agency, because states are now all on high alert and have plans in place to deal with these kinds of reports.
Juliette Wilson: Paul, thank you so much again for taking the time to talk with us.
Paul Cryan: Thank you.
Announcer: CoreCast is a production of the U.S. Geological Survey, Department of the Interior. Juliette Wilson is with ASRC Management Services under contract to the USGS.