Disease of the Month: White-nose Syndrome in Bats

Monday, January 18, 2010 at 1:13 PM Bookmark and Share
I've been trying to come up with a nice recurring theme related to my research interests, so this is the first of what will hopefully become a series of posts on transmissible wildlife diseases.  If you're into that sort of thing (which you should be - either because their natural history is so unbelievable and amazing, or maybe because they're wiping out their host population) then definitely check out the blog Parasite of the Day, whose recent post on Geomyces destructans reminded me to finish up this draft and get it off of my to-do list! 

So without further ado, here's one of the nastiest little germs that has flared up recently: the scourge of cave dwelling Chiropterans, White-nose Syndrome (WNS).


Figure1: Little brown bat; close-up of nose with fungus, New York, Oct. 2008.
Photo courtesy Ryan von Linden/NY Department of Environmental Conservation.


In short, while bats hibernate the fungal pathogen somehow manages to subvert their immune defenses (somehow the fungal hyphae that penetrate the skin don't elicit an inflammatory response), and infect their skin - especially on the muzzle.  Infection often causes them to come out of hibernation early (I would imagine this is a consequence of their taxed energy reserves and/or the infection triggering some sort of "wake up and turn on your immune system!" response, though that's purely speculation on my part) and seems to almost always result in death.

It is still unclear whether the sole cause of the disease is Geomyces destructans or if there might be a third factor facilitating the progression and spread of WNS.  Initially researchers were cautious -- the G. destructans infections could have been a symptom of some other less visible disease (e.g. some sort of bat immunodeficiency virus or a build-up of some new environmental toxin) or the primary cause. Recent work suggests the fungus is likely the culprit, but folks are still working to rule out other factors. From this page on the fungal pathogen ...
Another set of experiments is underway to try to prove that G. destructans is the actual cause of death. Koch's postulates have not yet been fulfilled, so it has not been strictly proven that Geomyces destructans causes death in bats. The histopathology has been well documented, and it is very clear that this fungus is the causal agent of white-nose syndrome cutaneous infection. So far, the correlation between the presence of the fungus and bat death is quite substantial, but *cause* has not yet been proven.
Even without knowing the causal details one thing is clear: WNS is decimating bat populations in the northeast.  Once a hibernaculum is infected, winter mortality rates seem to be very high: from near 80% up to 97% in some locations. Even worse, it's possible fungal spores can persist in these caves across multiple winters, and the list of infected hibernacula seems to be increasing in number and extending into previously uninfected areas (click here for a map from this page).

To get a better feel for the real severity of this disease, here's a video by Gerrit Vyn based on his experiences following field researchers during the winter of 2008-2009.



From Gerrit's blog...
White-nose Syndrome is a mysterious disease that is currently decimating bat populations in the northeastern United States. First observed near Albany, New York in 2006, it had spread as far as Virginia by 2009. Scientists fear this disease could rapidly push an entire group of species to extinction and will trigger unknown ecological consequences. Why the bats are dying remains a mystery.

I accompanied researchers to several caves and mines in New York and Vermont during the winter of 2009 in an effort to document the situation. This piece is a result of that effort and aims to communicate the urgency of the situation to the public and policy-makers and to elicit an emotional response to a group of species that are often disregarded or disliked.

Related Links

13 comments:

Posted by: prophit1970 | 1/21/2010 5:29 AM

Is it my fancy alone, that wind farm barotrauma, known to cause a bat's lung tissue to rip fatally, would rip other bat tissues as well?

A host population immunologically weakened by tissue-barrier compromise could have set the entire WNS event into motion. The pathogen penetrates, and gains a new environment within which to potentially adapt.

The largest wind farm in New York State went on-line in January 2006; one month later and 100 miles away, WNS was first spotted.

Please help to promote the investigation of this possibility.

Posted by: Paul | 1/21/2010 12:17 PM

Interesting idea, though there have probably always been immunologically weakened individuals around as a result of numerous factors: genetic variation, other diseases, nutrition, pollution, injuries resulting from predator attacks, collisions with automobiles or other objects, etc. The presence or absence of wind farms (I would guess) probably hasn't contributed much to the expected background level of injured bats going into hibernation.

That said, the idea of variation in immune defenses is an important one. The large mortality rates being observed now may be imposing strong selection for bats that can fight off or recover from a G. destructans infection.

If that ability is sufficiently heritable, we could see an evolutionary shift in the bats that saves them from what otherwise looks like impending doom. This, unfortunately, would be more likely to happen in species with large populations (e.g. the Little Brown Bat) as there might not be enough genetic variation and/or survivors among species with smaller populations (e.g. those listed as endangered or threatened, like the Indiana Bat).

Posted by: prophit1970 | 1/21/2010 5:20 PM

While it's true that bats have suffered many assaults, the Turbine-Bends would be a genuinely new addition to their existing burdens. If this is a wholly new evolutionary selection pressure, suddenly exerted across a sub-population of mammals, I would imagine that any genetic differences in tissue strength would be insufficiently protective. Are any humans genetically more resistant of barotrauma than the rest?

We see soft-tissue wing damage, drying and cracking in WNS bats; but which began first? We need comparative imaging, necropsies, histology by bat researchers and those who study human barotrauma.

Whom should I be asking these questions? I have no degree or standing in the field, but until I hear a counter-argument, I will continue to post on blogs and write cold to researchers. Any contacts you might suggest would be appreciated.

Posted by: Paul | 1/21/2010 5:41 PM

Just to play devil's advocate, here's a counter argument to consider. First, what is the (estimated) percentage of bats entering hibernacula that have suffered barotrauma or similar injuries, and what percentage of those are due to close-calls with wind turbines? Second, what evidence is there that such trauma makes individuals markedly more susceptible to this particular pathogen?

Here's why I'm a bit skeptical that an increase in the number of injured or otherwise immunologically weak bats resulted in the WNS outbreak. From the looks of things -- in particular the rapid spread of the disease and high mortality rates -- there was either a recent, widespread change in immune function in nearly all individuals across multiple species of bats (it's unlikely that nearly all bats suffer from turbine-related barotrauma). One of a few other more likely cases is that there was the introduction of a new fungal pathogen into a cave used by bats from far outside the north eastern U.S. (e.g. by cavers, wildlife, etc.) or perhaps a mutation in an existing fungus resulting in this kind of virulent pathogen.

That said, if you're interested in pursuing this hypothesis, maybe check the links above? I'm unfortunately not in direct contact with any WNS researchers.

Posted by: prophit1970 | 1/24/2010 11:32 AM

If a Devil won't step up, who will advocate for robust theses?

"what is the (estimated) percentage of bats entering hibernacula that have suffered barotrauma or similar injuries"

- I look forward to a time when extensive necropsy results become available. Are they even being compiled?

"and what percentage of those are due to close-calls with wind turbines?"

- I strongly suspect that outside of turbine exposure, the opportunities for bats to experience turbine-similar pressure changes are very rare; few in type; and more likely to be lethal at the moment of the barotrauma.

- Can anyone describe a second source of barotrauma for bats? Weather microbursts, explosions, or jet engines would be too lethal, largely all-or-nothing, and/or affect very few bats.

Posted by: prophit1970 | 1/24/2010 11:50 AM

"Second, what evidence is there that such trauma makes individuals markedly more susceptible to this particular pathogen?"

- No evidence: an argument from basic principles 1) barotrauma tears tissue, 2) the full effectiveness of active mammalian immunity is predicated on the integrity of passive tissue barriers.

- I would not think that Geomyces destructans has a particular, i.e. exclusive or unique, affinity for exploiting barotrauma. Whatever the cause(s), this fungus got to the bats before another pathogen could as fully exploit the breach of barrier tissues.

Posted by: prophit1970 | 1/24/2010 12:02 PM

"(it's unlikely that nearly all bats suffer from turbine-related barotrauma)"

- Patterns of bat behavior around turbines are now being documented. We'll see. I can't imagine why some species would better instinctively tend, or learn to avoid turbines, than other local bat species; but if researchers fully document their behaviors near turbines, we'll see if I suffer from a limited imagination in this regard.

Posted by: prophit1970 | 1/24/2010 12:21 PM

"From the looks of things -- in particular the rapid spread of the disease and high mortality rates -- there was either a recent, widespread change in immune function in nearly all individuals across multiple species of bats"

-I disagree. An initial, limited pool of barotrauma-damaged bats could have provided the opportunity for a pathogen to take hold, and penetrate deeper into the bats' tissues than before. New tissue exposure, and higher concentrations, would constitute a new opportunity for the pathogen to adapt and mutate. The bat-adapted pathogen would then spread widely to bats that were unaffected directly by the turbine-Bends.

"One of a few other more likely cases is that there was the introduction of a new fungal pathogen into a cave used by bats from far outside the north eastern U.S. (e.g. by cavers, wildlife, etc.) or perhaps a mutation in an existing fungus resulting in this kind of virulent pathogen."

-I'm sure these are already under active and wide consideration.

-I picked out links above which might lead to researchers who could directly evaluate aspects of this concept; and so, on I go.

Posted by: Paul | 1/24/2010 1:00 PM

Your argument that wind farms contributed significantly to the spread of WNS is not convincing.

It seems highly UNlikely (given my limited knowledge of bat-windfarm mortality) that a large proportion of bats suffer from barotrauma just prior to hibernation, and that such injured individuals were necessary for initiating the current WNS outbreak.

"A host population immunologically weakened by tissue-barrier compromise could have set the entire WNS event into motion. The pathogen penetrates, and gains a new environment within which to potentially adapt."

This is plausible, but such trauma could come from a variety of sources - so it's a stretch to blame windfarms. Such adaptation could just as easily occur in the bodies of bats that every year die during hibernation. Furthermore, bats need to cool down before the fungus invades and when it does so it goes straight through intact tissues. Even if injured or sick bats did kick off WNS, it seems like a BIG stretch to blame collisions with wind turbines.

Hibernation alone seems sufficient -- so why the focus on wind farms?

Overall, while it is an interesting hypothesis, it seems like there's enough evidence to rule out wind farms as a major initiator or driver of WNS -- at least via the mechanisms you described above.

"The largest wind farm in New York State went on-line in January 2006; one month later and 100 miles away, WNS was first spotted."

As many a statistician has cautioned "correlation does not imply causation." From your comments above, it looks like you are struggling to blame windfarms for WNS despite evidence and good reason they likely play(ed) little or no effect at all? Indeed, you should realize that a windfarm that starts up mid-winter 100 miles away from a group of hibernating bats probably didn't cause any injuries in those bats?

Such oversights really make me think you are more interested in wind farms than WNS?

This is likely my last comment on this thread until I see a good argument or good evidence that (1) over 50% of hibernating bats have injuries and that such injuries strongly increase susceptiblity to disease, or (2) evidence that G. Destructans has always been present in these caves, and that the pathogen is just a recently mutated strain of a previously existing G. Destructans population -- and not an introduced pathogen.

Posted by: prophit1970 | 1/24/2010 6:59 PM

I'm sorry that you don't appreciate my argument. You certainly fail to appreciate that the primary thrust of my advocacy is for an investigation. Barotrauma is not being considered as a factor in WNS. I do not need to assert a proof, to point to a potentially useful and novel hypothesis.

I'm sorry that it angers you, that my perception of bat elasticity and immunological function doesn't align with yours. The thought that turbines could cause such an effect is disturbing; and so was your last rant ("correlation...") Really? That rhymey jingle is better suited for hardening proofs than it is as a justification for tossing aside an untested hypothesis. I offered the correlation as food for thought, not naively as evidence. Besides, the many wind turbines installed in NY State before Maple Ridge could have tenderized a sub-population of bats just as easily.

Posted by: Wayne Liston | 1/18/2011 1:58 PM

Perhaps ask Paul Cryan what he thinks:
"Researchers still don’t know why bats are attracted to wind turbines, however. Some hypothesize that they mistake them for trees, which explains why 75 percent of bat deaths are among tree-roosting species. Most bat deaths at wind turbines happen in early fall, which is mating season; the “tall tree hypothesis” suggests that bats think wind turbine towers are attractive mating sites. “Tall things are mistaken for singles bars,” said Craig Willis, a biologist at the University of Manitoba."

To study this, Willis and Cryan examined the mating readiness of bats killed by wind turbines in New York, Manitoba and Alberta, and found most male bats were ready to reproduce. The researchers couldn’t find evidence that the bats were copulating at the wind farm, but their genitalia indicated it was the right time of year, Cryan said."

“It’s easy to come up with these hypotheses, but this is one I hope we can disprove,” he said. “If you are selectively causing the death of the reproductive class, you are in trouble from a conservation standpoint.”
Some wind sites are killing hundreds to thousands of bats in a single fall migration season,” said Paul Cryan, a research biologist with the US Geological Survey. One wind farm in New York is estimated to kill more hoary bats every year than have ever been collected for scientific studies, he said.

Posted by: prophit1970 | 1/24/2011 12:03 AM

Thank you, Wayne Liston, for your suggestion. Cryan seems ideally suited to consider the possibilities.

Posted by: Paul | 1/24/2011 2:12 PM

Thanks for the info, Wayne. :)

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