New clues are helping explain the mysteries surrounding white-nose syndrome, a devastating epidemic that has killed more than five and a half million bats in the eastern United States and Canada in just a few years.
In the latest advance, the strongest evidence yet suggests that infection with a suspected fungus causes the deadly disease. What's more, the fungus appears to have traveled to North America from Europe, most likely on a human shoe.
Once the fungus arrived here in 2006, North American bats were defenseless against the infectious killer, leading to the worst epidemic of wild mammals ever observed on this continent. Last week, the disease turned up for the first time in bats west of the Mississippi.
While many questions remain unanswered, each new detail brings scientists closer to figuring out how to stop a devastating and bewildering wildlife disease.
“Before we can even know whether a solution is possible, we have to know what the disease is doing and where it came from,” said Craig Willis, a wildlife biologist at the University of Winnipeg in Canada. “That may still be a long time away, but getting at these fundamental questions is really important.”
“This is probably our fault,” he added, referring to the likely role that tourists played in carrying the fungus from a cave in Europe to a popular cave in upstate New York, where the North American white-nose epidemic began. “For that reason, we have an obligation to figure out what is going on, to invest in understanding it, and to do our best to try and fix it or at least make it not as bad as possible.”
Ever since March 2006, when an annual survey of hibernating bats turned up thousands of dead animals in the New York cave, scientists have been racing to decode the secrets of a rapidly spreading killer.
Studies have narrowed in on a fungus called Geomyces destructans (Gd), which thrives in cold temperatures, causes lesions on bats’ wings and can be spread through direct contact from bat to bat. But proof that the fungus kills bats has remained elusive.
To investigate, Willis and colleagues collected 54 hibernating little brown bats (Myotis lucifugus) from a cave in Manitoba and transported them to a carefully protected facility at the University of Saskatchewan. All of the bats were healthy and free of disease.
Using a pipette, the researchers put Gd fungi that had been isolated from North America on the wings of one group of bats. They put European Gd on another third. And they put fungus-free liquid on a third group. Then, they placed the bats in specially designed incubators that were kept cold with high levels of humidity, which are the conditions that little brown bats prefer for hibernating.
All of the bats quickly returned to a state of torpor. The scientists waited and watched.
Pretty soon, both groups of infected bats developed lesions on their wings. By day 70, some of the bats infected with the European fungus began to die, the researchers report today in the journal Proceedings of the National Academy of Sciences. By day 90, some of those with the North American version followed -- showing for the first time that infection with the fungus alone was enough to kill the animals.
Even while still alive, infected bats displayed some unusual behaviors. Normally during hibernation, healthy bats periodically warm their bodies up for about an hour before dropping back into a chilly torpor again -- probably as a way to get rid of wastes.
All of the bats in the study experienced occasional temperature spikes, but infected bats warmed up significantly more often. Each warming event takes energy, and extra warnings caused infected bats to deplete their fat stores before hibernation was over.
Besides helping to explain how white-nose syndrome develops, the study was also able to show where the disease came from. Because both geographic strains of fungus caused similar symptoms, the study ruled out the possibility that white-nose syndrome arose from a random mutation in fungus that already existed in North America. Instead, the fungus appears to have traveled here from Europe.
European bats aren’t dying en masse from the disease, though, which might mean that overseas, flying mammals have developed immune defenses against the infectious fungus. That would explain why the European strain hit North American bats harder in the new study. Once on this side of the Atlantic, the fungus may have mutated to become less virulent because bats here were already so vulnerable.
“As much as it breaks my heart, this tells us that people inadvertently brought it from one place to another by not cleaning their boots or pants,” said Brock Fenton, a bat biologist at Western University in London, Ontario. “Before, you could at least say here’s a calamity but it was not caused by humans. Now, I don’t think you can believe that anymore.”
Theoretically, understanding how the disease works and spreads could lead to interventions that would slow or stop it. But there are still many unanswered questions, and a cure remains elusive.
“I don’t think this paper gives us any better handle on solving the problem than we had before, which was no handle at all,” Fenton said. “Right now, the outlook is really bleak.”