White-nose syndrome (WNS), the disease decimating bat populations in the Northeastern US and into Canada, is caused by a single species of fungus and is not the result of co-infection by any other pathogen, according to researchers studying the once-mysterious scourge.
Geomyces destructans, a cold-loving fungus first described in 2009, had been isolated from bats suffering from WNS, but its role in causing the disease was controversial as fungal infections in mammals are seldom fatal in isolation, and typically exploit immune systems weakened by other pathogens. But researchers working with healthy little brown bats (Myotis lucifugus)—one of the species hit hardest by WNS—have shown, in laboratory conditions approximating those in the hibernation habitats where the disease occurs, that exposure to G. destructans alone can cause the disease. Their study is published today (October 26) in Nature.
"I think it's nice to show experimentally under some controlled conditions to verify what we're seeing in nature," said University of Northern Colorado bat biologist Rick Adams, who was not involved with the study.
Since turning up in the US in 2006, WNS has spread to 16 states and four Canadian provinces killing millions of bats belonging to 6 species, including the endangered Indiana bat (Myotis sodalis), according to the US Fish and Wildlife Service. "If you were to design a pathogen to wipe out hibernating bats, it would do just what this fungus does," said DeeAnn Reeder, a comparative ecophysiologist at Bucknell University in Pennsylvania and co-author on the Nature paper. "It's something that our bats had very little resistance to. That's why the mortality rates are staggering."
To understand the disease's cause, Reeder's colleague, microbiologist David Blehart of the US Geographical Survey's National Wildlife Health Center in Madison, Wisconsin, and collaborators, housed little brown bats in lab refrigerators that mimicked temperature and humidity conditions typically experienced by hibernating bats. The researchers exposed 29 bats to G. destructans spores, and all of the animals came down with WNS by the end of the experiment more than 100 days later. In contrast, more than 30 control animals held in identical conditions were negative for the disease at the end of the study.
Conclusively linking G. destructans as the causative agent in WNS—which is characterized by skin lesions on the wings, noses, and other exposed skin of infected bats and results in eventual starvation—will allow researchers to ramp up efforts to ascertain exactly how the fungus kills bats. The results could also lend insights for managers seeking to slow the spread of the disease or to mitigate its effect by allowing them to focus on just one pathogen, head author Blehart said. "Ultimately it really sharpens the tool, because now we have one thing to focus management on."
Kevin Castle, a wildlife veterinarian with the National Parks Service in Fort Collins, Colorado, agreed with Blehart, saying that fingering the fungus as the sole causative agent is an important piece of the three-part puzzle—comprising host, pathogen, and environment—that must be solved to get a handle on any infectious disease outbreak.
"Definitively identifying Geomyces destructans as the cause, we now know that we have one piece of that triad that people can take and run with," said Castle, who was not involved with the study. "When we go out and start to look for [the fungus] in new areas, we don't have to be worried about looking for other causes [of WNS] at the same time." The discovery will also help speed efforts to streamline diagnostic tests to detect the fungus from soil or sediment samples, Castle added.
The researchers also demonstrated that transmission of the disease can occur bat-to-bat by showing that most members of a group of healthy bats came down with the disease when housed with infected individuals. Still, Blehart noted, human transportation of fungal spores is likely a more significant risk—one human carrying G. destructans spores to Australia (in which neither the fungus nor WNS has yet been detected) is more significant than bats spreading the disease from one cave to another within the US Northeast.
Indeed, Blehart, Castle, and other researchers agree that humans most likely helped spread G. destructans to North America from Europe, where the fungus infects but tends not to kill native bats. "Obviously it got over here from Europe," said Adams, "and I don't see any way it could have gotten here any other way than on a human."
Understanding the spread of the disease, which was first documented in a bat hibernating within a cave in Upstate New York, should also help wildlife managers get a handle on the fast spreading epidemic, which has now been documented as far north as Canada, west to Oklahoma, and is putting some populations at risk of extinction. Western wildlife managers and bat biologists, like Castle and Adams, are particularly concerned about the westward spread of WNS because the number of bat species in Western states is so much greater than in the East. "The biggest fear is that if it gets in the Western Rockies it could affect 30 additional species," Adams said. "Then it will also have a highway down into Mexico and down in to the New World tropics as well." If G. destructans makes its way into the tropics, Adams added, it could potentially affect "hundreds of species."
J.M. Lorch et al., "Experimental infection of bats with Geomyces destructans causes white-nose syndrome," Nature, doi:10.1038/nature10590, 2011.