Dr. David Blehert, a microbiologist with the US Geological Survey’s National Wildlife Health Center, presents on white-nose syndrome, an infectious disease among bats and its impact on the ecosystem in the 21st century… this presentation took place in March, 2012.
Dr. David Blehert – Microbiologist, USGS, National Wildlife Health Center
Thank you, Bill, for the very nice introduction and thanks to Hannah Hamilton for inviting me and the USGS communications group for arranging my visit.
So today, I’m going to talk about bats and new bat disease. Let’s see if I can get my slides to advance. Can everybody hear me okay?
So let me just begin by saying a little bit about bats to get us all on the same page.
Introduction – Bats
Bats are the only mammals that are capable of self-powered flights. Most are nocturnal. Perhaps these adaptations of being one of the few animals that’s out flying in the night sky when there’s otherwise lack of aerial predators has afforded this group of animals with the evolutionary opportunity to diverge into a huge number of species.
Bats are the second most species diverse group of mammals on the planet. Only behind rodents. There are about 1,100 species of bats out of a total 5,500 or so species of mammals. And just an amazing amount of adaptations, if you look at this giant fruit bat on the screen here, this animal consumes ripe fruit. This is nectar-feeding bat that can hover in front of cactus flowers and lap the nectar.
Here’s a bat that’s got both white and black fur. It’s the spotted bat in the American Southwest. With these huge ears, I think this animal is known to forge for insects on the ground and they can say that it can actually hear their footsteps.
And here we have a bulldog bat — which actually eats fish and it catches them right out of the water.
Fungal disease and fungal pathogens
So, to transition the topic of today’s talk, a disease of bats — white-nose syndrome is a fungal disease. Of all the various pathogens or disease agents that we know of, and I’m a microbiologist so I know of a lot of these — even for microbiologist, fungi are not often the first that come to mind when talking about disease.
When it comes to disease of humans, there are really only six major recognized groups or genre of fungi that cause disease and to name them, maybe you’ve heard of some of them, includes the yeast Candida and other fungi Aspergillus, Histoplasma, Blastomyces, Coccidioides and Cryptococcus. But, that’s six out of literally hundreds of thousands of species.
Despite fungal pathogens not necessarily being the first thing that people think of when talking about disease, it’s indisputable as I’ve outlined on this slide here, that fungi have had major impacts on the world. So for example, fungal diseases transformed landscapes by ravaging both American chestnut trees and elm trees, all within the 20th century. The Irish potato famine caused by a fungal-like organism that’s now referred to as called Oomycete, caused the death and immigration of over 2 million people.
Fungi remain potent pathogens of plants, humans and wildlife today, yet fewer than 10% of all fungal species are known to science. There are limited antifungal therapeutic drugs available and they often have associated toxicity because in terms of classes of animals, fungi are actually very closely related to people. So when we target bacterial pathogens with antibiotics, we’re targeting mechanisms of replication for that microorganism that are distinctly different than those in our own bodies.
When it comes to treating a fungal pathogen, there are also what are known as eukaryotes just like us. And so it becomes challenging to kill the fungus without harming the host.
There’s no antifungal vaccines. And then when it comes in particular to plants or animals, like wildlife, that don’t have intrinsic or obvious economic value, disease management can be very challenging.
Another point of interest is that fungal diseases in humans and other mammals are most commonly associated with hosts that have compromised immune systems. So there’s something else going on. Or, in cases where that host was exposed to a huge dose of the organism, say, through – fungal diseases sometimes arise in military personnel doing training exercises where they’re crawling through soil with their faces close to the soil that’s enriched with fungal spores of certain disease agents.
This graphic shows that with respect to human health — and likely driven changes in host’s immune status — caused by the emergence of HIV, increased use of immunosuppressive medications like steroid treatments, other intensive care-based therapies that have people in hospitals with in-blowing catheters that are providing a conduit between the outside world and the inside of the body, incidents of systemic fungal disease and people has been exponentially on the rise since about the 1950s.
As I move along through this presentation, and along these same lines, I hope to convey to you the unique world that hibernation plays in white-nose syndrome as one of these predisposing factors to this novel of disease.
So what is White-nose Syndrome?
White-nose Syndrome is an emerging fungal disease of bats and it’s a very interesting — or maybe insidious would be a better word — disease because bats are specifically infected with this fungus when they hibernate – which, based on research done in other hibernating mammals, is believed to be — it’s known to be in these other mammals, and we believed it to be in bats, to be a time of natural immunosuppression. So, there is one of the potential predisposing factors.
Furthermore, a large scale disease epidemic, or we’d say epizootic when we’re talking about mammals, a large scale epizootic among bats like White-nose Syndrome is not only unprecedented among the bats that live in the United States, but among all of the 1,100 plus species of bats and perhaps even among all mammalian species around the world.
A paper came out in the journal Science two summers ago now in which the authors predicted a 99% chance for regional extinction of little brown bats — once the most numerous hibernating insect killer bats species in the Northeastern United States — within about the next 16 years as a result of this disease.
So do I think this is a feasible hypothesis?
I would say that it actually is because unlike other pathogens, say like a virus, that is not as autonomously replicating organism, or something that’s capable of survival on its own, most fungi are. Almost all fungi, including those that are pathogenic, can also live a second type of life phase, where they’re just existing as saprobes or decomposers of organic matter in soil.
And so a fungus that’s trying to compete with all of these thousands and thousands of other species for very limited nutrients in soil, if it has somehow gained the ability to infect the host, which in the case of a bat, maybe we could just think of a hibernating container of fungus food. In that case, that fungus gets a huge advantage over its counterparts in the soil and it gains the ability to replicate and make more of itself, which is effectively the evolutionary goal of a microorganism.
So, now if these fungi kill all the bats in the cave, they just go back to a more quiet reserved lifestyle in the soil and it doesn’t matter that they drove their host to extinction. If a virus kills all of its hosts, the virus goes with them.
And so that’s one of our concerns in that fungi do have this unique ability compared to other pathogens.
Progression of White-nose Syndrome
So let me talk about a little bit about the known progression of this disease.
The first evidence that we have a White-nose Syndrome comes from a photograph that was taken by a recreational caver in early 2006. The photo which has down, what would that be your right-hand corner of the screen, shows a bat with this unusual white substance or growth of what we now know to be fungus around its nose. This photo wasn’t actually made widely public until the year 2008.
What’s interesting is this photograph was taken in a cave in east central New York, that is part of a complex of caves that includes a large tourist cave that entertains about 250,000 human visitors each year.
We’ve also since learned that the fungus that causes White-nose Syndrome is surprisingly widespread on bats of Europe, where the bats seemingly co-exist with it. In other words, we’ve never — although we’ve seen the fungus on the bats, we’ve even seen that it can cause White-nose Syndrome in European bats, there’s no current or historical documentation of these unusual mortality events caused by the disease in Europe.
So, it leads to the plausible hypothesis that this disease agent may have been introduced to the United States through tourism. And that is not an unexpected scenario given the prominent role, and I’ll talk more about this later, that global travel and trade are known to play in the emergence of infectious diseases worldwide.
So the next winter, the disease was, let’s say officially discovered, by a biologist with the New York State Department of Environmental Conservation when they were conducting routine bat hibernation survey counts for the endangered Indiana bats.
And by the time they were done doing their surveys, they had found bats with either suspicious white growth on their bodies or dead bats on cave floors at five sites within two counties encompassing about a 15-kilometer radius, near Albany, New York.
The next winter, my laboratory, others at the National Wildlife Health Center, our pathology team, our field investigation team, as well as other diagnostic labs became significantly involved. So, in other words, the people from New York State, who discovered this the previous spring, had alerted us to this problem.
We had some conference calls – we need to figure out what’s going on; we’re going to be out looking for this; we’re going to be sending you bats, get ready, it’s going to be ugly.
And so, by the time the winter of 2007-2008 was over, we had now identified the disease at 33 sites extending to a circle with a radius of 210 kilometers from that index site near Albany, New York. The disease now documented in New York, Massachusetts, Vermont and Connecticut.
The next winter, things got much worse for the bats with the disease identified now in over a 100 sites extending into nine states all the way down to the southwestern tip of Virginia, at a distance of over 900 kilometers from the disease epicenter.
And at this point, there’s really no precedent for studying or responding to a wildlife disease among what we can refer to as cryptic hibernating animals like bats. Many of us, if we didn’t for other reasons, know they were there, they’re just there, people don’t know about them, they’re doing their thing, we’re doing our thing. And there’s surprisingly little known about their immune systems and their overall biology.
So then the following winter, this will be what, three winters ago now, we worked with Canadian Wildlife Health Authorities to confirm the disease at several sites in Quebec and Ontario, showing now that as disease is moving westward across the country, there’s effectively two fronts for introduction into the mid-western states and on into the western states.
At this time we also brought online some new, more sensitive molecular tests that we can use to find DNA from the fungus which led to these possible detections of the fungus on bats in Oklahoma and Missouri. Thankfully, we’ve not been able to confirm these findings, the surveillance work that we do is opportunistic, so we don’t have people out all over the place testing every bat they can find.
We have people that are doing surveys in caves in as unobtrusive of a way as possible so as not to disturb the hibernating animals. And if they see something suspicious or dead, that animal may be collected and sent to my lab or other labs for analysis.
And so, despite heightened surveillance activity, we have not yet confirmed these sightings in Missouri and Oklahoma.
So this takes us to last winter where we by the end winter 2010-2011 had confirmed the disease out into 16 states, including these new states of Indiana, Kentucky, North Carolina and Tennessee. And now in four Canadian provinces. Interestingly, we have not yet and this still holds true today, if I pull up our current map, current as of last Friday, we have not yet seen the high unusual mortality west of the Appalachian mountains. And that’s an area that we’re actively investigating.
So the question is, we know that this fungus establishes itself in environments, in the environment, and effectively can become amplified over time and the bats then become exposed to more of it earlier each hibernation season. So is it just a matter of time and perhaps by the end of March and in the April, as the bats are completing hibernation for this winter, we’ll see things get much worse in these mid-western states like Tennessee, Kentucky, Indiana, Ohio, etc., Or, are environmental conditions different such that the disease never will become as severe west of the Appalachian mountains.
So, that’s perhaps one area of the hope but, the jury is still out on that.
Bat species affected
In terms of the species affected, we’ve documented what we in the lab would call clinical disease in six different species of bats that belong to three different genre. When all of these bat species – the Little brown bat, the Big brown bat, the Northern long-eared bat, the Eastern small-footed bat, the Tricolored bat, which people have previously known as the Eastern pipistrelle, and the Indiana bat, which is an endangered species, all have in common, is that these bats all hibernate to wait out the insect food shortage during the winter time.
And so again, we’ll further discuss how I believe hibernation predisposes these animals to infection by Geomyces destructans — the fungus.
So with that, let’s transition to a brief discussion of disease ecology.
In order to understand emergence and subsequent spread of the disease, we have to understand how susceptible hosts, pathogens, and environments, come together in a way that promotes development of the disease. So, that could be: a stomach virus outbreak among passengers that are closely housed together on a cruise ship; a cold that passes through children at day care center, or a White-nose Syndrome outbreak among hibernating bats in close proximity within an underground hibernation environment like a cave or a mine.
So hopefully by now I’ve convinced you that bats are the host for this disease.
So, I’ll talk more about the pathogen later. But let’s talk about this environment, the underground hibernation site.
This is an old U.S. Geological Survey map that shows the locations of caves as red dots in the Eastern United States. And if we overlay a map showing disease occurrence, the yellow counties on the map, you can see that it exactly mirrors the cave locations. So, indicating that these caves provide that environment that brings the susceptible hosts and the pathogen together.
A similar analysis I believe was done when West Nile Virus was moving westward across the country from its initial point of introduction on the East Coast. And effectively if you could find a susceptible bird species like a crow or a blue bird, a Blue Jay, near a water source that harbored mosquitoes that harbored the West Nile Virus pathogen, that was your key for early detection of West Nile Virus in your state as the disease march westward.
So to complete this discussion of the disease triad, we’ve covered the host, we’ve covered the environment: the cool underground dark cave.
Pathogen aspects of White-nose Syndrome
Let’s transition to the pathogen aspects of the disease. So as I mentioned before, my laboratory and others began to investigate this outbreak in earnest in early 2008, after we were contacted by the state biologists in New York. Because of a clinical presentation of this disease, this white stuff around their faces, we suspected a fungal disease agent.
But our early disease investigation efforts were complicated in that this white material that I show you on the bats’ muzzles — and it is elsewhere on their bodies too, especially their wings — is very fragile. And if these bats are removed from caves it would disappear. So, by time the bat was shipped to our bio-safe, bio-secure laboratory in Madison from the East Coast, they didn’t look like anything was wrong with them, except that they were dead.
So an early breakthrough in the investigation came when a persistent New York state pathologist, Dr. Melissa Behr, who, to our good fortune, subsequently transferred to the Wisconsin Veterinary Diagnostic Lab, and we’re continuing to work with Melissa, was also troubled by that observation.
She’s getting bats from a mine that was probably within 10 miles of her lab in Albany and didn’t look like there’s anything wrong with it. So, Melissa went in to one of these caves and she collected samples from bats right on the cave wall of this white material and prepared them for both electron-microscopy and simple light-microscopy using a blue stain.
And what she saw when she scraped this white material under her microscope preparations was a pure culture of this unusual fungus with these hook spores that are actually a spore morphology that never had before been seen.
So, in the meantime at my lab, we were culturing skin samples from the hundreds of bats that we’re receiving. And if you take an animal that lives in a cold, dark cave and culture its skin, you can imagine that it’s just covered with fungi. I mean, people probably have been in a musty basement — and musty caves the same way in that musty smell comes from all of the fungi that are present.
And so how do you figure out which one of those things growing on that bats is what’s causing your problem. Well, one of our breakthroughs and other people were doing this as well, was we had to ask the question: What do we need to do in the lab to grow something that’s colonizing the skin of a hibernating bat? And the skin of a hibernating is about the temperature of the inside of your refrigerator.
And typically, when you’re doing microbiology work, you incubate your cultures at the temperature of the human body, like 98.6 degrees, or maybe at room temperature. Well, it turns out that for this fungus room temperature is too warm. Its upper cutoff for growth is around 65-66 degrees Fahrenheit, around below 20 degrees Centigrade.
So, my technician in the lab had put some of these skin samples on petri plates and stuck it in a laboratory refrigerator. And very slowly, this white fungus started to grow on those plates. And at about the time that Melissa circulated her photographs of the white scrapings right off the bats, our cultures were starting to grow and we stuck them under microscope and compared them to Melissa’s photos and found the same fungus.
And so, the question is: What is it?
It requires cold for growth, and the temperatures at which it grows unfortunately overlap perfectly with the skin and core body temperatures of hibernating bats. The fungus cannot actively grow at warm temperatures, even the temperature of this room, as I mentioned, would be too warm for it, but that doesn’t mean it would die. And so there is a concern about not accidentally transporting it somewhere.
The fungus is common on sick bats but absent from healthy bats and all of the isolates that we have subsequently characterized within some common genetic marker regions from both North America and now from Europe as well, are genetically identical. So when you put this genetic relatedness together with this radiating pattern of disease spread from a single index site, suggest that — all this circumstantial evidences is suggestive, of a single point introduction and subsequent spread of an introduced pathogen.
So based on DNA sequence analysis of some of these marker genes, we were able to determine that this fungus belonged to a genus of common soil fungi called Geomyces, which means fungus of the earth. But based on some of these other characteristics, its ability to infect bats, its curved spore shape, and its requirement for growth in cold, we identified it and named it as a new species which we designated destructans, so the destroying Geomyces.
Some work that we’ve recently completed and published was demonstration according to strict criteria in microbiology known as Koch’s Postulates, developed by a preeminent microbiology pioneer, Robert Koch, in the late 1800s, that definitively demonstrate the Geomyces destructans in the absence of other contributing factors is the pathogen that causes this disease.
And so in order to fill Koch’s Postulates, first you have to demonstrate that the microorganism is found in abundance on all organisms or all animals suffering from the disease, but absent from healthy animals. And we accomplished that through our disease investigation work.
The microorganism must then be isolated from the sick animal and grown in pure culture in the laboratory. And so again we accomplished that through our disease investigation work.
The cultured microorganism should cause disease when introduced into a healthy animal and then subsequently, you have to be able to re-isolate the introduced pathogen from the healthy animal. And so in order to do this, our first challenge was to develop a system whereby we could successfully maintain wild hibernating bats in our laboratory.
And so having achieved that, we were then able to conduct the experiment. So, what I’m showing you on this slide, these two images are called histopathology, which allows us to see how the fungus interacts with tissues from the animal. And so what we’re looking at here are cross-sections of bat wing skin and the stuff in dark purple is fungus.
If you think this is — if you recognize this is bat — and dark color is bad, you’re getting to see the picture. What this fungus does is it forms these dense aggregations in a way that we haven’t seen with other fungal skin pathogens of bats. And it invades and destroys wing tissue, as well as tissue of the muzzle and elsewhere.
And so based on this measure, we were able to demonstrate that by putting predetermined numbers of spores from this fungus on the skin of hibernating bats that we were able to cause the disease in 100% of animals that we treated. We were also able to transmit the disease from bat to bat by co-housing sick with healthy bats.
Interestingly, when we had bats in separate cages — hibernating bats in separate cages within our incubators– we did not transmit the fungal agent between bats by air. It is an interesting result, but one of my concerns is that I think that this might stem from the way in which the experiment was done.
The bats are kept in mesh cages and the mesh can obstruct the free movement of spores between cages, as well as, unlike a cave, these incubators are constantly moving air from within the incubator over the evaporator or chiller coil and recirculating it back through, so it’s possible that we effectively vacuumed spores out of the air of the system as it works. And so this still remains an active area of research.
Having achieved this objective of demonstrating causality, which is important, because now we truly can focus disease investigation efforts, as well as disease management efforts around a single pathogen, and all of those various implications for control and management strategies.
Another important thing that comes out of this system is our model system for maintaining hibernating bats in the laboratory. And this can be used for additional purposes such as consideration for long term maintenance or holding of captive colonies of endangered bat species to protect them from this disease to which they might be exposed in the wild until better management solutions are developed.
We can also use this system to conduct experiments to further understand how this fungus kills bats. And as we develop a better understanding of how the fungus works and why it’s a pathogen, we can then look for additional ways in which we could intervene and break this disease cycle.
Also, this system could serve as a way for testing different treatment or management strategies in captivity so that we can test it in the laboratory on living bats first, before we introduce it into national systems in the wild and thus avoid unintended adverse consequences.
So it still leads to the question as to why would a skin infection like White-nose Syndrome be so deadly to bats.
Why White-nose Syndrome deadly to bats?
We get athlete’s foot infection, fungal infection of our feet. Other animals are susceptible to numerous fungi that for whatever reason fall into a category where they call the infection ringworm. It’s not actually caused by a worm, it’s caused by a fungus. But it basically just creates an itchy red spot on your skin somewhere. You don’t die from it.
All of those other fungal infections are called — those fungi are called dermatophytes. And those fungi colonize the layer of dead skin cells on the surface of our skin. Geomyces destructans in White-nose Syndrome is different in that that fungus actually actively invades the skin of bats. And it penetrates through that epidermal layer and destroys the epidermis, connective tissues, blood vessels, muscles, nerve fibers, oil glands, sweat glands — everything that it encounters.
Despite the name of the disease, White-nose Syndrome, perhaps the greatest damage that it causes to the bats is to their wings. And these wings are critical to bats not only for them to be able to fly, but they also perform numerous other physiological functions shown here, including: heat dissipation, water control; so an intact wing membrane is necessary so the bats don’t lose water while they’re hibernating and otherwise unable to eat or drink.
These wings actually allow the bat, these large wings, which the skin of the wings accounts for eight times more skin than on the entire rest of the body of the animal, also passively exchange gases while they’re hibernating. When they’re hibernating they’re only breathing a couple times a minute. And even with their wings folded in on their bodies it’s been shown that 10% of their CO2 exchange occurs through theses folded wings membranes.
Also, remember these animals are spending like six months of their lives hanging upside down. These wings actually have special shunts in the blood vessels that help move blood around and function in blood pressure regulation. So numerous critical physiological functions all of which are being disrupted by this fungus.
Let me tell you a little bit about another project that we did. Fungal diseases tend to have environmental reservoirs which presents an additional challenge for managing the disease in the wild. Even if you could cure the disease in the infected animals, if they return to their infested hibernation site the next year they’ll just become re-infected. So anything that doesn’t give them life-long immunity and there are questions about whether that’s even possible in a hibernating mammal, the animals potentially subject to reinfection.
And so, we developed the study where we partnered with biologists as well as recreational cavers from across the eastern United States. And we were able to collect over 550 soil samples from over 120 caves on states bordering on in East of the Mississippi River.
And the goal was to screen these soil samples for presence of the fungus. And first our question at this time, not yet knowing whether the fungus was causing the disease, was to just ask the basic question: Is this fungus ubiquitous in all of our sites, and the disease is only occurring in the northeast for some other reason? Or is that fungus only restricted to areas where we find sick bats?
And so, first thing we accomplished through this work was we were able to culture the viable fungus from samples with hibernating bats. And what I’m showing here is just a preliminary work. We’re continuing to work with these samples, but this is the first bit that we were able to complete.
So these stars show sites from at that time where we were able to both grow the fungus and find its DNA in the soil. We’re actually able to find viable fungus in as little as 0.2 grams of soil and so you can see next to a dime that’s a very small amount of fungus.
And if I draw your attention to the picture down here, most likely, we’re growing the fungus out of soil as a result of these numerous spores, these are its reproductive structures. These are environmentally resistant structures that the fungus produces to satisfy its one goal in life again, which is to make more of itself. And so our ability to find it viable in soil highlights the need that people be very careful — that if they’re going to a site that they don’t accidentally pick up the fungus, and as a hitchhiker bring it to the next cave that they visit.
As our infectivity work showed the bats do transmit the fungus between themselves as they interact, but this work strongly infers that humans could also be mechanical vectors or transporters of the fungus. And so we want to do all that we can so that we don’t also contribute and make this problem worse.
So one of our other interesting findings is that if you want to screen a lot of samples, using a labor intensive culture method, is probably not a viable approach. And so we’ve developed a molecular diagnostic to detect the fungus in soil.
And we tested that on the skin of bats which do have a lot of fungi on them but fewer varieties of fungi than you would find in soil. And when we started applying our PCR test to soil samples, we were finding what we later identified as cross-reactivity with practically every sample that we analyzed.
And so, as an example, these are what some of the other unknown Geomyces species present in soil from bat caves look like. So the real challenge here is that, on this map, out of just an initial subset of 24 soil samples, we potentially have identified up to 11 new species of Geomyces. And all of these are very very closely related to Geomyces destructans within the marker genes that we’ve characterized. So if you look at this little spot — When you look at this filegrams, they’re called, the closer things are to one another, that the more closely they’re related. These distances have to do with genetic changes. So this Clade 10, Clade 11, Clade 12 are relatively distant from Clade 1, which is Geomyces destructans. Clade 2, the little spot on the line here, only differs from Clade 1 at four nucleotide positions, out of 624 that we looked at to develop this chart.
And so, at this point, we’re really only scratching a surface of what could very well be one of the most common groups of fungal organisms in cave soil, this cold damp environment. And that leads to another important question: What is so special about Geomyces destructans– that it’s pathogenic to bats whereas all of these other fungi that presumably bats have co-existed with for millions of years, do not cause this disease.
We’re actually doing some work, some comparative genomic analysis work, and maybe that will help us to begin to identify virulence determinants in Geomyces desrtuctans.
This slide just shows what some of these guys look like when we grow them on petri plates. Here is our type isolate of Geomyces destructans, here’s an isolate Geomyces destructans cultured directly out of cave soil. Here are some of the relatives and you can see they’re kind of similar looking.
But interestingly this one that looks like shag carpeting, genetically, is the most closely related fungus to this one. So it’s also interesting how these visual or phenotypic traits don’t seem to necessarily correspond to genetic relatedness or differences.
And here’s a point I have touched on this already, but a question that frequently arises is: Where did Geomyces destructans come from and why did White-nose Syndrome emerge in North America in 2006-2007, approximately?
So we don’t yet have a definitive answer to this question. But there’s anecdotal references in scientific literature to bats with white fungus on their muzzles in Germany, dating back to the early 1980s.
And recent surveillance work in Europe has indicated that this fungus, Geomyces destructans, genetically and morphologically indistinguishable from isolates in the United States is widespread as shown in this map, on bats in at least 12 countries.
But as I mentioned previously again, these unusual mortality events, associated with infection in the United States, have not yet been observed in Europe. We don’t yet have an answer for that, but it may stem from differences in environmental conditions that we hope to better characterize, as well as differences in population levels.
In the northeastern United States, there used to be many very large aggregations of these bats, thousands, tens of thousands, even hundreds of thousands. So if you imagine you get a fungus into a colony of 10,000 bats and each of those bats serves an amplifying host for the fungus, within a very short period of time, one or two years, there’s great potential for that fungus to make a lot of itself — remember its goal in life — and infect a lot of bats heavily and early in hibernation and cause heavy mortality.
In Europe, as far as people have recorded back, the bat populations tend to be very much smaller – 10 bats, 30 bats, in some instances 100 bats. And in a couple of rare instances, thousands. But among these smaller populations, there’s fewer animals — potentially less chance for fungal amplification, less chance for bat-to-bat interaction and spread lower fungal burdens in the environment, lower infectious doses; and, whereas these bats get colonized, perhaps the disease just doesn’t progress to the point where it kills them.
People have, most recently, even documented these lesions that we see on the skin of North American bats that we considered diagnostic for the disease in European bats. But again, they’re not dying.
Some other work that we’ve done is showing that if you remove a bat from hibernation or take a bat that’s not yet dead from the disease and just provide it with a warm environment and food and water, they’ll make a full recovery, actually remarkably quickly. They have full capacity to regenerate the skin of their wings and microscopically, it looks like there was never anything wrong with it.
And I think a lot of that just has to do with turning back on their physiology, their immune system and they can mount a recovery. And so effectively in Europe maybe things just never progressed to the degree that we see them in the United States. But again, an active area of research. But still does lead to that question of: How did the disease get here?
Civilian global aviation networks
So this slide depicts civilian global aviation networks among the 500 largest international airports in 100 countries, most intense air traffic shown in yellow. And what this slide shows is that we humans have succeeded widely in breaching historic barriers to the spread of infectious diseases around the world, namely oceans and mountain ranges. And given our incredible capacity for mobility, there’s almost no place where we and our hitchhiking pathogens cannot be on this planet, within just a matter of days –if we set our mind to it. And indeed, global travel and trade are widely recognized as the most significant drivers in global spread of infectious diseases.
So at this point I’ll start to sum up. This next series of slides were developed with a colleague from USDA Wildlife Services, Tom DeLiberto, who’s a wildlife disease specialist with that agency.
When we think of White-nose syndrome, it’s not an ordinary disease. Few diseases have affected as many species, over such a large geographic area, in such a short period of time, with as much impact on populations.
Confounding factors contributing to the severity of White-nose Syndrome likely include: high density clusters of susceptible individual animals during hibernation, the time to which they’re uniquely susceptible to this disease, both high infection and mortality rates, transmission between bats and the environment, and likely the other direction as well, and environmental persistence.
Even though these animals are hibernating, they on somewhat regular intervals, like every two weeks or so, they do arouse out of hibernation, briefly, for maybe an hour at a time, get a drink of water, urinate, various things. I would just say go to the bathroom, but I don’t think there are bathrooms in these caves.
So, even though they’re hibernating, there are frequent movements of individuals both within their clusters, among clusters. And these animals, even though they’re not officially recognized as migratory, they are known to move distances between summer and fall of up to 200 miles. And so also the tendency for these, or the potential ability for these animals to spread viable spores of this agent over relatively large distances.
And then couple that with: These animals have a very low and natural reproductive rate. These are not flying mice or flying rodents that have many, many babies. Bats exhibit very high degree of care for their young and an insectivorous bat, on average, has one pup per year. And so the potential for these species to recover is a long-term problem.
These animals are important even though we may not even see them. A bat can eat its body weight in insects each night. Insects consumed by bats include those that cause vector-born disease, as well as those that impact crop and forest health.
Free pesticide/ecosystem services by bats
A bat biologist at Boston University estimated that for the millions of bats that have been estimated to die from White-nose Syndrome, to date that number is around 5 million, that would literally amount to thousands of tons of insects per year. And a recent modeling paper estimated that the pesticide, the free pesticide, or ecosystem services provided by bats, and these a.) Don’t cost us any money; and b.) Don’t involve any input of potentially toxic chemicals, are valued between $4 billion to $50 billion per year to the U.S. agricultural industry.
White-nose Syndrome – Treatment
So some final thoughts: Why don’t we just go out and treat these animals with antifungals?
Unfortunately, there’s no precedent for using antifungals or any sort of pharmaceutical compound –short of a vaccine — for treating disease in free-ranging wildlife. Especially for a disease with an environmental reservoir, where the animals are likely to pick it up again from the environments in which they hibernate.
So managing disease in these animals is very different than strategies that we would use for our pets, for people, or for our farm animals. As I said, this environmental reservoir, as well as a very sensitive and unique ecology in every cave that these bats inhabit, presents unique challenges.
And so what I think we’re challenged with is thinking outside the box to identify practical solutions that can benefit these animals, that the population level, while first and foremost, not doing any harm to the populations.
And so, I’ll conclude here. This slide acknowledges numerous people that have worked with me on this problem – graduate students, our pathologists at the health center, technicians and people from numerous other institutions, as well as our funding source, funding from U.S. Geological Survey, United States Fish and Wildlife Service, Bat Conservation International, and others.
And I’ll just transition to this last slide and be happy to take questions.
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