March 29, 2008

What is Killing the Bats of the American Northeast?

In yet another one of the apparent ecological catastrophes that are becoming more and more prevalent of late, a disease of some sort is causing massive die-offs among populations of several bat species in the northeastern US. The disease, labeled "white-nose syndrome," is symptomized by the proliferation of a Fusarium fungus around the animals' noses. Bats with the disease are waking up too early from hibernation, are emaciated, and are leaving their winter shelters in a desperate search for food. The end result is that they freeze to death. Nobody knows yet what actually causes this syndrome. Fusarium can be a pathogen, of course, but in this case it appears to be a secondary infection and a symptom rather than a cause. The Hartford Courant is reporting today that the disease, already known in Massachusetts, New York and Vermont, has now appeared in Connecticut as well.

The presence of the fungal infection may be a dead giveaway as to the mechanism that's causing the bats to become emaciated, though. I have a hypothesis that I admit is based in part on my ignorance of what specific investigations have already ruled out. As I don't know, though, I'll advance a possible cause. First, though, a little bit of today's Hartford Courant article.

Epidemic Strikes Bats
White-Nose Syndrome Spreads To Connecticut

A mysterious condition that is already decimating bat populations in New York, Massachusetts and Vermont has spread to Connecticut, with vast implications not only for bats but for the vital role they play in controlling mosquito populations.

Jenny Dickson, a wildlife biologist with the Connecticut Department of Environmental Protection who entered a major bat hibernaculum (or bat cave) in Litchfield County on Thursday, has confirmed the presence of the usually fatal "white-nose syndrome" on numerous hibernating bats.

The syndrome, first detected in New York state caves in the winter of 2006-07, is so named for the white fungus that coats the noses, legs and wings of afflicted bats. The condition generally causes death before the bats emerge from their winter hibernation. In New York state, more than 80 percent of the bats in affected caves have died over the past two winters...

"Bats are our single largest predator of night-flying insects and provide an important form of natural insect control. Any significant depletion in their numbers will also result in a significant effect in other parts of our ecosystem," she said...

The bat plague was first noticed about a year ago, when hikers and cave enthusiasts in New York state observed bats that were still supposed to be hibernating congregating near cave entrances. Some were even flying out in frigid daytime temperatures to die on the snow. Responding to these reports, wildlife biologists entered known bat caves — as many as 250,000 bats can hibernate in a single large cave — and began documenting the telltale white-nose fungus on the sleeping bats, many of whom also showed signs of emaciation.

An impromptu network of federal and state agencies, and teams of veterinary pathologists — from the University of Connecticut, Cornell and the University of Wisconsin — have been meeting all winter via teleconferences to share information and the results of necropsies of affected bats. But so far they have not been able to determine the cause of the syndrome. The scientists mostly agree, however, that the fungus found on the bodies of infected bats is probably a symptom and not a cause of the condition.

"We just don't know yet what the cause of [white-nose syndrome] is — whether a virus, a bacteria or a toxin," said Dr. Randall Nelson, Connecticut's public health veterinarian...

Experts also agree that the affected bats have extremely low fat levels in their bodies...

Dr. Richard A. French, a veterinary pathologist at UConn who has supervised the necropsies of bats with the fungus, points to another difficulty that is likely to delay a rapid solution to discovering the cause of the syndrome.

"We've never done a comprehensive study on hibernating bats, so we don't have a lot of norms to start with," said French.

"We have found on the bats we've examined so far, for instance, significant amounts of parasites. But we don't even know if that's normal or not. It's possible this could take years of study to understand..."
I don't know if investigators have already examined the gut bacterial flora in these bats, but from the little bit I've been reading about white-nose syndrome it would seem a crucial possibility that there may be a change there.

As readers may know, one of the things I've been researching myself is the role of chitinases in nature. In my research, I ran across a paper a few months back in which it was demonstrated that bats harbor several chitinase-producing bacteria in their intestines:
Whitaker JO, Dannelly HK and Prentice DA. 2004. Chitinase in Insectivorous Bats. Journal of Mammalogy 85:15–18. DOI: 10.1644/1545-1542(2004)085<0015:CIIB>2.0.CO;2
In the study, nine bat species were found to harbor a diversity of at least ten different chitinase-producing bacteria. It was also shown that as bats feed during the warm season, they build up residual insect chitin in their gut. During hibernation, the chitinase-producing bacteria proliferate and break the chitin down into carbohydrates. Mammals can't normally digest chitin; it requires the presence of symbionts that manufacture chitinase for them to use the normally indigestible material as a source of carbohydrates. Mammals do produce lysozymes, another group of enzymes that possess a binding domain much like the active sites in chitinases, but these are not nearly as efficient at slicing up chitin polymers. They just break them into chunks and are involved with preventing fungal infection by preventing cell wall coherency in a potential pathogen. If you're going to derive nutritional benefit from leftover chitin, you've got to have chitinases in your intestine. It's the same principle we see in symbioses such as that in the gut flora of termites; remove the lignase-producing microflora from termite guts and you wind up with termites that can no longer digest wood. They starve to death no matter how much they eat.

If insectivorous bats somehow lost their intestinal symbionts, a similar thing would happen. During their active season, the bats might be just fine because they don't rely on the digestion of chitin for nutrients. However, when they stopped actively feeding and went into hibernation, some other bacterium might proliferate in the gut. The chitin "store" that provides energy during the winter would no longer be accessible. The bats would starve in their sleep, essentially. That's exactly what is being observed in white-nose syndrome.

I find the presence of the Fusarium interesting in light of this. It may be that the bats' immune system is being compromised by lack of nutrition, but it could also be the case that the loss of the chitinase-producers means that the organisms that are responsible for keeping fungi from infecting the bats aren't around to digest the pathogen. Bacteria the dwell in the digestive tract could easily have a role in this. Chitin is chitin for the most part; the same organism that produces chitinase responsible for breaking down insect chitin is very likely to be producing chitinase that breaks down fungal chitin. It's very typical that a given organism that digests chitin produces multiple chitinases, each of which is a variant on the basic theme of family 18 glucosyl hydrolase.

As I've admitted, I don't have a clue as to what white-nose investigators have examined, and for all I know the loss or replacement of bacterial flora may have already been discarded as the cause of the bat die-off. If it hasn't been, however, I hope it will be. I doubt that we have the luxury of years in figuring out how to stop this die-off. Without bats to control insect populations, there could be serious human health consequences due to the proliferation of mosquitoes carrying diseases such as West Nile and Eastern Equine Encephalitis. Besides, bats are amazingly cool creatures and it would be a pity to lose them. I hate to think of the suffering; these are social, relatively intelligent animals. For that reason alone, investigators must find a way to stop this disease.

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