It's been six years since the dawn of white-nose syndrome - a bat-killing fungus that's sweeping west across North America - but the disease is so fast and stealthy that scientists are still struggling to keep up. They reached a major milestone this week, albeit a discouraging one: They calculated about how many bats have died so far.

White-nose syndrome has killed at least 5.7 million bats in the U.S. and Canada, according to a new report from the U.S. Fish and Wildlife Service, and possibly as many as 6.7 million. After emerging in a single New York batcave in 2006, the fungus has spread to 16 states and four Canadian provinces, leaving a trail of roughly 6 million dead bats in its wake. That's a big jump from the agency's 2009 estimate of 1 million bats killed. "White-nose syndrome has spread quickly through bat populations in eastern North America, and has caused significant mortality in many colonies," U.S. WNS coordinator Jeremy Coleman says in a statement. "Many bats were lost before we were able to establish pre-white-nose syndrome population estimates." WNS is so wily, in fact, that scientists only identified its cause three months ago. A previously unknown fungus called Geomyces destructans is behind the disease, which has a mortality rate as high as 100 percent in some batcaves, or "hibernacula." The fungus creates white fuzz on bats' noses and wings, and seems to wake them up from winter hibernation too early. The FWS describes it like this:

"While they are in the hibernacula, affected bats often have white fungus on their muzzles and other parts of their bodies. They may have low body fat. These bats often move to cold parts of the hibernacula, fly during the day and during cold winter weather when the insects they feed upon are not available, and exhibit other uncharacteristic behavior."

Scientists have had a hard time keeping tabs on WNS partly because it's hard to keep tabs on bats. They like to hibernate in treacherous caves, often on inaccessible walls and ceilings, and they tend to cluster in dense, uncountable masses. Federal biologists have long kept population counts for certain endangered species, like the Indiana bat, but formerly "common" species like the little brown bat were "historically not the primary focus of seasonal bat population counts," the FWS explains in a press release. And if monitoring the disease has been a challenge, trying to contain it has been a nightmare. Scientists aren't sure how G. destructans is spreading from cave to cave - especially since so few bats make it out of infected caves alive - but they suspect human cavers may be accidentally carrying around spores on their shoes, clothing or equipment. The FWS closed thousands of caves and former mines across 33 states in 2009, and the agency has worked with spelunking groups in hopes of reducing the chance that humans are helping WNS spread. Maybe that has slowed down the fungus, but it's hard to tell. In a press release Tuesday, Bat Conservation International director Nina Fascione warned WNS is on the verge of wiping out entire species. "We knew that white-nose syndrome has been taking a devastating toll on bats, and this confirms our worst fears," Fascione says of the FWS report. "Extinctions are a real and imminent threat across North America." Bat extinctions aren't just an academic issue - 1 million bats can eat nearly 700 tons of insects per year, including mosquitoes that carry human diseases and agricultural pests that damage human crops. Losing 6 million bats, therefore, has likely already hurt U.S. farmers. "If WNS continues to take such a huge toll, the environmental and economic costs will be enormous," Fascione says. Here's a map of where WNS has spread so far (click to enlarge): For more info about white-nose syndrome, check out the video and links below: