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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.

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