On a summer evening three years ago my wife and I counted 75 little brown bats scrabbling out from behind four small shutters on our house in upstate New York and setting off for a night of insect foraging. A year later the number had swelled to 150; the moth and mosquito populations were becoming less bothersome than ever before. Then things took an abrupt turn for the worse: last year the numbers plummeted, and on a recent summer evening this year only six bats emerged.

The drop would come as no surprise to wildlife biologists in the Northeast. The house is just an hour's drive from ground zero of the worst disease outbreak in bat populations on record. First observed in Howe Caverns near Albany, N.Y., in early 2006, white nose syndrome has spread north to New Hampshire and Vermont and south to Virginia. At least a million bats in six species have already perished, and death rates at infected hibernacula range between 90 and 100 percent.

Many observers expect a wave of new outbreaks this year, even among previously uninfected bat species (so far the disease seems harmless to humans and other animals). Yet despite widespread concern and media attention, scientists are still trying to figure out exactly what is killing the creatures and are frantically searching for ways to stop it. And because bats are essential to the control of nocturnal flying insects, the outbreak could upset local ecologies, weaken the health of forests and even affect crop yields.

White nose syndrome (WNS) takes its name from a fungus that looks like a white, powdery substance on the muzzles, wings and ears of bats. The fungus, previously unknown to science, has been classified and named as Geomyces destructans. No one has proved it is the killer—it could be just an opportunistic invader taking advantage of some other infection—but many biologists think it is. It grows only between about 36 and 72 degrees Fahrenheit, a relatively cold range for a fungus but typical of the year-round temperatures in the depths of most U.S. caves. So far the only bats infected with the fungus rely on insects for food and on hibernation to survive the insect-free winter months.

According to Marvin Mori­arty of the U.S. Fish and Wild­­life Service, another piece of evidence that points to the fungus is that when it grows on the wings of the animals, it causes swelling and scarring. The wing membranes are essential for regulating physiological functions such as body temperature and blood pressure. Hibernating bats are already dangerously close to depleting their stored fat reserves before they emerge from torpor. If WNS interferes with their physiology, it could interrupt hibernation and lead the bats to use up more energy than they can afford. In short, they would starve. Supporting this theory is that many WNS victims are also emaciated, and bats from infected caves are unusually active in winter, perhaps in a futile attempt to find food.

To devise a plan to stop WNS, biologists want to know much more about the Geomyces fungus—its origins, distribution, mode of transmission and the like—as well as the bats’ reaction to it. Such data might, for instance, lead to a vaccine against WNS.

Funds for fighting WNS have been meager, however. A total of only $1.1 million, from government and private funds, has gone to scientific and control efforts since the disease appeared. Yet in testimony before a House subcommittee on June 4, biologist Thomas H. Kurz of Boston University stated that the funding needed to mount a realistic response is at least $17 million. This past May, 25 U.S. Senators and Representatives signed a letter to Interior Secretary Ken Salazar urging emergency funding for agencies with the expertise to “determine a cause and develop solutions to this crisis.”

Meanwhile the most visible response to WNS has been to declare caves and mines off-limits to visitors. But whether people are spreading the fungus around is unclear. According to Robert Zimmerman, a caver who has written extensively about the outbreak, the first places WNS appeared in Pennsylvania and West Virginia were “popular recreational caves.” Yet WNS has not been detected in other popular caving regions, notably those in Indiana, Tennessee, Alabama and Georgia. David Blehert of the National Wildlife Health Center in Madison, Wis., and his colleagues are working to culture the fungus from the floors and walls of caves, to determine whether clothing and equipment could spread the fungus.

But if the syndrome spreads primarily from bat to bat—as seems the case—closures will have little effect. Unless more answers appear soon, draconian measures such as killing all the bats in infected caves may be all that can keep the spread of WNS from rewriting the ecological rules.

A Bat Disease That’s Bad News for Humans, Too
People could soon feel the devastating effects of white nose syndrome (WNS) among bats. The most immediate change may be the number of mosquito bites people get this summer. According to Greg Turner of the Pennsylvania Game Commission, a bat may consume as much as its own weight in insects each night, including mosquitoes. Bat guru Merlin D. Tuttle, who founded Bat Conservation International, notes that bats are the primary predators of pests that “cost American farmers and foresters billions of dollars annually.” If WNS spreads to the American South and West, it could also lead to huge losses of crops pollinated by bats. As Turner points out, bats are major pollinators of plantains and avocados and are the sole pollinators of the agave plant; margarita cocktail lovers owe the tequila in their drink to the activities of bats.

Note: This article was originally printed with the title, "Night Stalker."