On the outskirts of Madison, Wisconsin at the United States Geological Survey’s National Wildlife Health Center, David Blehert sits in an office that overlooks a prairie restoration project. Swallows—looking to my amateur eyes an awful lot like the bats Blehert studies—nosedive over the tall grass, missing his window by mere inches.
Blehert is the branch chief of the Wildlife Disease Diagnostic Laboratories, where he and his colleagues investigate the causes of death in wildlife brought to their facilities from across the United States. Their goal is to diagnose and minimize the impact of disease on wildlife.
I visited Blehert to talk about his work with bats with White-Nose Syndrome, a fungal disease that’s killed over six million bats in the past nine years. Blehert and his colleagues isolated the pathogen that causes the disease here in 2008. The following is an edited version of our conversation.
Catherine Halley: Tell me about your work with bats.
David Blehert: I’ve been at the USGS-National Wildlife Health Center for about 12 years now. White-Nose Syndrome in bats has been a large part of my research program here since 2008, when we described the fungus that causes the disease.
White-Nose Syndrome is a wildlife disease impacting only hibernating bats. An agency like the USGS-National Wildlife Health Center has expertise in wildlife population, structure, and management, so understanding wildlife diseases like White-Nose Syndrome is central to our mission.
White-Nose Syndrome is caused by a fungus called Pseudogymnoascus destructans… Is it coincidental that the species name sounds like destruction?
No.
Why did this disease emerge now?
Think about all the global travel and trade we see today. I believe we’re building a very strong case that this is likely a pathogen of European origin—with which European bats for the most part co-exist—that was inadvertently brought to the US either by a tourist or through trade. It falls under that guise of pathogen pollution or introduction of a novel pathogen into a naïve host ecosystem. Think of it like an invasive species that’s behaving out of check.
In the winter of 2005–2006, a recreational caver took a photograph of a bat that had white fungus around its nose, but he didn’t know the significance of his photograph of it at the time.
In the winter of 2006–2007, New York State biologists were conducting a semi-annual survey of endangered Indiana bats, and they saw in five caves near Albany, New York either that the bats were missing or they were dead and on the floor.
The next winter, in January 2008, we started getting samples. Four or five months later, by April or May of 2008, we identified the pathogen. I think our first publication in Science came out online in October 2008, and then ended up in the Charles Darwin anniversary print issue, in January of 2009.
Here we are a mere ten years later, and you’ve mentioned that six to seven million bats have died.
Yes, it really has been unprecedented in terms of the rapidity with which this disease has spread… I’m very proud of our track record in terms of defining the basic biology of this pathogen and answering fundamental questions to demonstrate in the laboratory that it causes the disease.
We have some ideas now on the mechanisms by which it kills bats. We’ve made great strides in terms of characterizing how the pathogen has the potential to exist perpetually in these caves so that bats pick it up.
We’ve also defined transmission pathways that spread it bat to bat, though they likely also pick it up in the environment as well. That means that if you are a biologist or recreational caver that goes into a cave and steps in soil that harbors these long-living spores of the fungus, you need to decontaminate your shoes so that you don’t track it to a new cave.
So humans spread it around?
Yes. Since we can’t control the movement of wild animals, we’ve looked to what we can do to enact bio-security measures so that a human does not inadvertently transport spores to a site at greater distance than where you would expect bats to gradually move the pathogen on their own. To date, we have not observed or documented any long-distance jumps of the fungus, and so I think that’s good evidence that what management we can enact is working.
You mentioned that White-Nose Syndrome is “unprecedented” in terms of its destructive force. How would it compare to, say, Colony Collapse Disorder in bees?
It’s quite different, actually. First of all, after as many years, I still don’t think we have a clear cause for Colony Collapse Disorder. Numerous causes have been proposed and it could be some combination of those.
The other major difference is that the bees that are most susceptible, honey bees, are a domesticated, non-native bee species. They’re not “wildlife.”
European honey bees were imported to the United States over 100 years ago and are still to this day maintained under husbandry conditions. You’re talking about something that’s raised in captivity and harbored or maintained by humans, so there are direct management applications.
You’ve been able to identify which fungus is causing White-Nose Syndrome in the bats, but how does it actually kill them?
One of the interesting properties and something that makes this fungus unique, is that it cannot grow above 68 degrees Fahrenheit.
A bat’s body temperature is about 98 to a 100 degrees Fahrenheit when it’s metabolically active. But in the wintertime, bats in temperate regions of North America build up fat reserves and go into a hibernation period for about six months of the year from late October through April.
While they’re hibernating, their body temperature is about the same as the inside of your refrigerator, or close to about 44 degrees Fahrenheit. This lower body temperature allows the fungus to grow.
But hibernation is complicated. Bats go into hibernation for a period of about two to three weeks, and then they come out of hibernation for an hour and then they go back into hibernation for two to three weeks, and then come out for about an hour. They even mate sometimes during those arousal periods.
These arousals, even though they’re not well understood, are believed to be essential to their physiological health.
One of the things people have noticed is that when bats get White-Nose Syndrome, they come out of hibernation more frequently. Their hibernation periods are shortened and their arousal frequency increases.
This disruption to the normal rhythms of hibernation causes them to consume the fat reserves required for them to survive winter.
So could they be starving to death when they have White Nose Syndrome?
That’s one theory.
The disease is called White-Nose Syndrome because they get blooms of fungal growth on their noses. But the primary area where the fungus colonizes the bat is on their un-haired wings. Their wings are basically just skin… The scientific Order bat is chiroptera, which means “hand wing.”
That’s where the fungus starts to grow, on the wing?
Right, and the skin that comprises the wing comprises over 80% of all skin on the bat, so it’s this massive surface area and it’s literally two cell layers thick. It’s exquisitely delicate, and it’s full of nerves and blood vessels and muscle. It’s quite a remarkable structure. It’s the only mammal that is capable of self-powered flight, and it’s a very different flight strategy than that used by birds, even in terms of their basic musculature.
The fungus colonizes this exquisitely delicate skin… and causes profound damage, impacting the ability of the animal to fly, which it has to do in order to feed itself. In addition, these wings also mediate functions like release of CO2 while the animals are hibernating and otherwise breathing at very low rates.
They might only be taking two, three, four breaths a minute during hibernation, and so they can passively off-load some percentage of the CO2 that accumulates in their blood through their wing skin.
There are all sorts of complex physiological functions [of the wing membrane], and that’s what we believe is the heart of the issue, that White-Nose Syndrome is disrupting this delicately balanced physiology of an animal that has to survive for six months of every year without eating.
So the fungus colonizes the skin on their wings, interfering with their ability to fly, and therefore to eat. That sounds mighty grim. Is there any hope?
We have shown both through laboratory and animal work that we did in collaboration with a bat rehabilitator that if you take a bat sick with White-Nose Syndrome out of hibernation, give it a warm environment and feed it, it will get better on its own, so further medical intervention is not required. Other people have gone in to caves and put wing-band markers on hibernating bats that visibly had White-Nose Syndrome, and they’ve later recaptured those bats in the springtime, and they’ve apparently healed or not shown signs of the fungus.
It’s not necessarily an easy route for a bat to cure the infection on its own, but it is possible. We’ve seen that European bats commonly have the fungus on them, and they even develop lesions that under the microscope are indistinguishable from those that we see in North American bats, but they just don’t progress to the point that they kill the bat.
That could be a consequence of those bats having some immunological resistance to the fungus or the environmental conditions under which those European bats hibernate being less conducive to rapid growth and progression of the fungal disease.
European bat populations tend to be much smaller than North American bat populations. The bat populations most heavily impacted by White-Nose Syndrome in eastern North America often numbered tens to even hundreds of thousands of bats per large cave system. In Europe those bat populations, instead of being tens to hundreds of thousands are tens to hundreds of bats.
It could be that as our US populations are drastically reduced to similar levels… there’s lesser amplification of the fungus, so that when bats do get infected, they get infected later in the year, with fewer spores, and the disease does not progress to that highly lethal level.
Then, maybe in the future the bat populations will rebound to some point where they reach whatever balance they can maintain with the fungus.
But a big challenge here [in terms of conservation] is that bats are very, very different from birds or even rodents. Some people call them flying mice, but they are not. Unlike birds or rodents, bats have a very low reproductive rate, producing only one offspring per year, so population recovery, if possible, will likely be slow.
Yes, bats have a bad reputation.
I think that genetically, they may be more closely related to marine mammals than they are to mice. You wouldn’t necessarily think this from looking at them… The ones that are heavily impacted in North America, they weigh six to eight grams, which is about as much as two or three pennies.
Oh, wow, these bats are tiny.
Yes, and their bodies are about two inches, total wingspan is approximately 6 to 8 inches.
Two inches? Their bodies are two inches?
Yes.
Everybody’s so afraid of them!
I know. They’re really cute actually. Nonetheless, these animals live 10 to 20 years and they only have one baby per year, so for an animal that has a high level of parental care and low reproductive rates, their populations do not recover quickly.
When you’re managing wildlife, you have to think in terms of the population because you can’t readily manage populations by treating individual animals. That’s something we do for our pets or our kids or ourselves. But if your goal is to find an economically viable solution to maintaining entire species, you have to look towards something that benefits more than individuals.
What are some of the things people are trying to do to counter the devastation of White-Nose Syndrome?
Something that we’re looking at now is to develop a better understanding of how environmental conditions in hibernation sites contribute to the environmental reservoir of the fungus and to progression of the disease. Is there, for example, a way that you could subtly change temperature profiles of underground hibernation sites? You could propose to do this in an artificial mine, for example, which often are occupied by large numbers of bats.
Right here in the state of Wisconsin, the three largest hibernacula in the state are two sand mines that I think have been in operation since the 1940s, and an abandoned underground iron mine that may harbor up to a half million animals each winter.
You mentioned that one way you’ve tried to stop the spread or deal with this, is by changing the temperature of caves…
Actually lowering the temperature. We’re still developing the data to support this idea, but that’s what we are investigating.
Raising the temperature to the point that it would preclude growth of the fungus would also preclude the ability of the bats to hibernate.
A postdoc in my laboratory, Dr. Michelle Verant, has published a paper in PLOSONE about this.
How on earth would you drop the temperature of a cave?
Well, if it’s a mine, you can drill ventilation tunnels to change airflow patterns.
Are there other possible solutions?
Researchers, including people in my laboratory, are also looking at various bio-control or chemical control strategies, but a lot of these are based on novel ideas and the reality is that novel ideas take time to go through testing and approvals and licensing so that they can be released in the environment. … You’re looking at a decade-off solution.
A disease-management strategy that works in humans and in domestic animals, and that has also been proven to work in wildlife is vaccination. Vaccination is currently used to control the spread of rabies in wild carnivores like raccoons, skunks, foxes. They do that by dropping vaccine that’s put in edible baits from airplanes.
Oh, interesting. So you’d basically be trying to feed the bats something that would vaccinate them against White-Nose Syndrome?
I think it also has much broader implications. Bats, especially in the tropics, have been identified as the reservoirs for some of the horrendous viral diseases you read about in the paper like Ebola, Marburg, SARS, and MERS. So there is the potential that you could do widespread vaccination against zoonotic diseases and start to eliminate them just like we’ve eliminated small pox.
In the meantime, bats are still dying at an alarming rate from White-Nose Syndrome?
What we know about high mortality comes largely from the Northeastern United States. There seemed to be a delay in spread over the Appalachian Mountains into the Southern Midwestern areas, Kentucky, Tennessee, Ohio. There was a delay that I think we can define as about three years from when we first detected the fungus in this group of animals until we started seeing mortality. Now we’re definitely, as of last winter, seeing high bat mortality in those areas, but we don’t yet know how it’s going to compare to what we have seen in the Northeast.
We’ve just had our first detection in Wisconsin, but it’s colder, and maybe a little drier here in the winter so we don’t yet fully understand what the ultimate impacts may be. They could be bad, but then again as you move further into the arid West, once you cross the Mississippi River and the 100th meridian, the country starts transitioning to a drier climate, and there’s different population dynamics. The bats become more dispersed as opposed to hibernating in large caves.
My hope is that some of these species, which also exist west of the Mississippi, will be impacted to a lesser degree. That’s just speculation on my part, but there clearly are some environmental differences.
I know little brown bats are affected, but what other major bat species get White-Nose Syndrome?
One called the Eastern pipistrelle (some call it the Tri-colored bat) is heavily impacted. The Northern long-eared bat is a candidate for the endangered species listing because of this disease. The Indiana bat, which is on the endangered species list, is definitely susceptible, and then there’s some other, less common species in which infection has been documented but has not necessarily had adverse impacts, like the Eastern small-footed bat. The Big Brown bat does get infected, but also seems to be somewhat resistant, and it’s also less frequently found in caves. It really is the cave hibernating bats that are most at risk.
What can people do to try to help stop the spread of this?
Following what are called “Universal Precautions” in disease epidemiology—not moving from one cave to another without decontaminating—will help. If you’re a recreational caver, do not bring gear from an infected site to an uninfected site. Also, anything that supports bat habitat will help the bats.
Bats have such a bad reputation. So many people are afraid of them. They think of Dracula or they think of rabies or even Ebola. Some people might say “Good riddance,” what does it matter that bats are dying?
Bats are an integral component of our ecosystem. They consume vast amounts of insects. Those bats consuming insects can be referred to as providing an “ecosystem service,” and people have valued the “ecosystem services” provided by bats to US agriculture in the tens of billions of dollars a year.
We don’t understand all of contributions that bats provide to our ecosystem, but if you take too many bricks out of the bottom course of the wall, eventually the whole wall can collapse. I think it’s important to recognize that bats have intrinsic value in and of themselves as unique element of our world.
