By David Quammen
On the outskirts of Madison, Wisconsin, stands a low brick structure equipped with ventilation scrubbers and surrounded by a tall chain-link fence: the Tight Isolation Building of the U.S. Geological Survey's National Wildlife Health Center (NWHC), a federal research facility devoted to combating wildlife diseases. Inside, a cinder block corridor circuits the Animal Isolation Wing, passing a series of well-sealed experiment rooms, each visible through a thick window. One room is furnished with sawdust and burrowlike pipes to approximate the habitat for prairie dogs involved in a vaccine trial against Yersinia pestis, the organism that causes plague. In another room zebra finches in birdcages are playing a role in research toward a vaccine for West Nile virus. Two rooms are darkened, for the comfort of hibernating bats. The first contains normal animals of the species Myotis lucifugus, commonly called little brown bats. They are the controls. The second dark room houses little browns exposed to Geomyces destructans, a filamentous white fungus of unknown origin that first appeared among North American bats in 2006. In just four years, it has hit hibernating bat populations in New York, Vermont, and a growing list of other states and Canadian provinces more lethally than Yersinia pestis hit the peasants of medieval France.
David S. Blehert, a microbiologist at NWHC, leads the laboratory study of this nefarious fungus. He enters the second dark room wearing Tyvek coveralls, rubber boots, latex gloves, a red-filtered headlamp, and a respirator. Moving quietly to avoid rousing the animals, he approaches a large glass-fronted cabinet in which sits a small, screened cage of bats. The cabinet is a florist's refrigerator, adopted by Blehert because hibernating bats, like cut lilies, do best at low temperatures and high humidity. Blehert peers into the cooler, checking the bats for evidence of fungal growth around their muzzles or on their wings. White fuzz on the snout, which looks like rime on the beard of a skier, is a signal that the bat may be infected; it's also the source of the label "white-nose syndrome" for this affliction.
No sign of change, Blehert tells me back in the locker room. No mortalities so far, and no visible fungus. But the experiment is still in an early stage.
How does this fungus kill the bats? "That we don't know," he says. "It is, I believe, the first disease ever characterized specifically targeting a hibernating animal." So its mode of lethality may be different from anything science has ever seen. And that's only one of the unknowns.
The fungus itself seems to be new to North America. Its presence was first documented—but not yet recognized—in a photograph taken in February 2006 at Howes Cave, west of Albany, New York. A year later, people began to report something peculiar: little brown bats flying outside nearby caves during daylight in the midst of winter. A little brown bat is a tiny creature, smaller than a human thumb, and dependent on its two grams of stored fat to keep it alive through the cold season. Hibernation is essential to making its energy resources last; a single arousal can cost it a month's worth of fat. When a crew from New York's Department of Environmental Conservation made their routine annual inspection of Hailes Cave, another hibernaculum nearby, they found thousands of dead bats, scattered all over the cave, in various stages of deliquescence and decay. "It was carnage," according to Al Hicks, a mammal specialist with the department.
Since then, the problem has spread quickly and far. Biologists estimate that a million or more animals were lost in three years, with populations at some sites eliminated. Six species have the disease, one of which had been declared endangered long before white-nose syndrome: the Indiana bat (Myotis sodalis). Three others are at very high risk, including the gray bat (Myotis grisescens), also endangered. Great progress was made within recent decades in restoring gray bat populations. "We've put enormous effort in this," says Merlin Tuttle, founder of Bat Conservation International. "It could now be unraveled in just a few years."
It's hard to foresee where Geomyces destructans might stop, short of infecting every population of hibernating bats in North America. Harder still is to say what, if anything, can be done to mitigate the destruction.
Hibernation—that's a crucial piece of the problem. Fungi generally don't cause severe disease in warm-blooded creatures (nobody dies of athlete's foot) because high body temperatures aren't conducive to runaway fungal growth. Hibernation, on the other hand, entails lowering of body temperature along with other parameters of metabolism, such as breathing rate and heart rate. Of the 45 bat species resident in the United States and Canada, about two dozen hibernate. They congregate in caves, mine shafts, and even buildings, each hibernaculum chosen according to species-specific requirements of temperature range and humidity. Little brown bats prefer temperatures between 40° and 45°F and humidity around 90 percent. Those conditions are also optimal for Geomyces destructans, as David Blehert has discovered while working to grow it in his lab.
But a fungus needs nutrition as well as a comfortable environment. It takes its food from other creatures. Ordinarily, the immune system of any mammal will work to fight off a fungal parasite. Not necessarily, though, if the mammal is hibernating. Work done in the lab of Tom Kunz, a bat researcher at Boston University, suggests that a side effect of hibernation—when a bat dials down its metabolism—might be suppression of its immune responses. Marianne Moore, a biologist in Kunz's lab, wonders whether that immune suppression, amid the cool temperatures, is what allows Geomyces destructans to bloom so aggressively on wintering bats. (Humans aren't susceptible.) This newest addition to the Geomyces genus seems to have found its way to the one group of mammals least capable of defending against it.
Found its way—from where? No one knows. The same fungus has been seen on bats in Europe, but there it hasn't caused any noticeable disruptions or deaths. In other words, the fungus is present but not the syndrome. White-nose syndrome means not just fungus-frosted snout fur but also corrosive white lesions on the wings of the bat and untimely arousal from hibernation, possibly because the white stuff is irritating, stifling, or itchy. The wing lesions impair flight; the untimely arousal costs bats their fat reserves, exposing them to starvation or freezing, whether or not they emerge from the cave in a desperate, futile search for food. At what point, and why, does an annoying fungal infection develop into full-blown white-nose syndrome? Again, nobody knows.
Bats don't fly across the Atlantic, so if Geomyces destructans reached Howes Cave from Europe, it was probably carried there by a human—a tourist with dirty shoes, maybe, or a caver in fungus-flecked coveralls. Seen in that light, it's just the latest in a long list of devastating invasive species. A century ago the culprit was another fungus, Cryphonectria parasitica, better known as chestnut blight. Before it arrived, American hardwood forests were full of tall, stately chestnuts, but by 1940 those trees were virtually gone.
"This is the chestnut blight of bats," Jim Kennedy says, as we drive toward Hubbard's Cave, a gray bat hibernaculum in central Tennessee. Kennedy is a biologist and a caver, employed by Bat Conservation International. His job involves teaching other biologists and cavers how to census bat populations in hibernacula with minimal disruption and, in the process, to chart the spread of white-nose syndrome. He thinks like an ecologist, concerned not just for bats but for the ecosystems in which they participate—living caves, forests, farmlands. By one estimate, the million bats lost so far to white-nose syndrome would be consuming about 700 tons of insects a year. "We may see big, big changes," Kennedy adds. "There's no predicting."
Beyond the unsolved riddles of the disease itself is another: What might America look like without any hibernating bats?
Hubbard's Cave, which the Nature Conservancy owns, is naturally concealed at the end of a mountain road. A stream descends through a wooded gulch, then cascades into a sinkhole, along the bottom edge of which are three cave entrances opening deep within the cavitied limestone. All are blocked with huge steel gates that allow bats to fly in and out but exclude unauthorized human visitors. We climb down an aluminum ladder into the sinkhole. Then our host from the Nature Conservancy unlocks a panel at the bottom of one gate, and eight of us—bat counters and others—wiggle through. Every item with us, including my waterproof notebook, will be bagged in plastic when we exit and later disinfected. If the fungus lives in this cave, Jim Kennedy and his colleagues won't carry it into the next.
We explore the several passageways amid thousands of roosting gray bats, not one of which—so far as anyone can see—has a whitened nose. There is no ragout of dead bats on the floor. We have arrived before the fungus. But Kennedy isn't sanguine. "It's not if it gets here. It's when it gets here. We're bracing ourselves for that," he says. "This cave alone has half a million bats."
While Kennedy and the others work, I linger before an astounding sight. On the tan limestone wall of one chamber, lit dimly by the ricochet of our headlamps, hangs a single thick mass of gray fur. It's a cozy, inert aggregate of living bat bodies, clinging two-deep, three-deep, and cheek by jowl with one another, their little clawed feet hooked to the porous vertical rock. They form a solid, irregular patch, as big as a living room carpet. This single patch, I'm told, might encompass 300,000 bats. It resembles a buffalo robe. It resembles a huge, dark amoeba. It puts me in mind of a giant Rorschach blot, testing our visions of the future.
© 2010 National Geographic Society.
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