Saturday, 29 October 2011

It's Official: Fungus Causes Bat-Killing White-Nose Syndrome

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Experimental infections prove that Geomyces destructans is responsible for white-nose syndrome

little-brown-batLITTLE BROWN BAT: A fungus is responsible for the white-nose syndrome that has killed more than 1 million bats of various species, particularly the little brown bat pictured here. Image: ? Alan Hicks

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A fungus known as Geomyces destructans is indeed responsible for the dusting of white across bat noses and wings that has wiped out entire populations of the flying mammals, new research shows. By purposefully infecting healthy bats with the fungus?and confirming that seemingly healthy "control" bats from the same population did not get sick from a prior but hidden fungal infection?microbiologist David Blehert of the U.S. Geological Survey and his colleagues showed in a paper published online October 26 in Nature that G. destructans is in fact responsible for the disease known as white-nose syndrome (WNS), which has devastated bat populations across the northeastern U.S., killing an estimated one million of the animals. (Scientific American is part of Nature Publishing Group.)

"It is specifically during hibernation that bats are infected with this fungus," Blehert notes. "The greatest damage it does to bats is to wing membranes."

Such membranes, in addition to enabling flight, help control physiological functions such as water retention and blood flow, and even "release CO2 when the respiratory rate is just a couple of breaths per minute," Blehert says. At the same time, it is not clear why a skin infection with G. destructans would prove directly lethal to the animals?the bats in this controlled experiment had not died from the disease by the time the experiment ended after 102 days. Nor had the fungus invaded the bats' vital organs, the researchers found.

In addition, it appears that G. destructans has been a part of the European cave-scape for some time and it has been isolated from cave walls there as well as from bats roosting in those caves. Thus far, however, the fungus has not proved lethal for those species. "It could be that European bats have evolved over a longer period of time and are immune or have a different way of coping with the fungus during hibernation," says mycologist Vishnu Chaturvedi of the New York State Department of Health, who is also studying the fungus and disease, which he calls geomycosis, but was not involved in this study. "Or the fungus in the U.S. has subtle variations that we have not even started looking at."

The core problem seems to be that G. destructans is depleting the fat layers?and thus the body mass?of very small North American bat species, such as the little brown bats used in this experiment. Blehert speculates that the lack of mass mortality in Europe may derive from the fact that European bat species are generally larger in size or the fact that they tend to hibernate in much smaller groups. "In the northeastern U.S. there are many very large hibernaculums, with upwards of 1,000 bats," Blehert notes. "The bat is providing food for the fungus and serving as an amplification host."

In fact, Blehert's experiments show that bats are quite effective at spreading the destructive fungal disease to their neighbors. "Bats are very good agents of transmission of the disease," Chaturvedi says. And that?plus the European analysis?may suggest that G. destructans is an invasive species, according to Blehert, which possibly traveled to the U.S. on a European who visited a public cavern in New York State. WNS was first observed in a wild cave connected to that commercial cave complex near Albany, N.Y. Chaturvedi's work has shown that G. destructans in North America is genetically similar wherever it is found.

There is hope for the bats. Another experiment by Blehert and his colleagues showed that bats artificially removed from hibernation, put in a warm environment, and provided with food and water could recover from WNS. "Bats can rapidly clear the infection in just a matter of weeks," Blehert says. It may be that G. destructans relies on the turning down or shutting off of the bat's immune system during hibernation?as is common to most hibernating mammals?to wreak havoc. The fungus seems to grow best at cold temperatures between 4 and 15 degrees Celsius. "It could be that hibernation is the Achilles' heel that is predisposing bats to G. destructans infection," Blehert adds.

But keeping hundreds of thousands of bats from hibernating is hardly plausible. "You can't just wake them up and shoo them out," Blehert notes, nor is it possible to feed them in mass quantities to restore fat levels. Restricting human access to caves where susceptible bats hibernate?as has been done by the U.S. Fish and Wildlife Service?and following decontamination protocols when such access is necessary will at least reduce the risk of humans further spreading the disease, which has now spread to infect bats in 11 states and Canada. "Segregating healthy animals from diseased ones to the extent possible does seem to be able to control this infection," Chaturvedi adds.

Regardless, the G. destructans epidemic is just another example of fungal disease on the march: Chytridiomycosis is wiping out amphibians worldwide and fungi may be playing a role in the colony-collapse disorder plaguing honeybees. One effort to protect frogs from this fungal plague are so-called amphibian arks, where small populations are taken into captivity to ensure their survival. That approach may become necessary for certain endangered bat species as well to protect them from the white-nose syndrome caused by G. destructans. As Blehert and his colleagues wrote: "Fungal pathogens have the unique capacity to drive host populations to extinction because of their ability to survive in host-free environments."


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