Photograph by Stephen Alvarez, National Geographic

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White fungus covers the muzzles of infected little brown bats hibernating in a West Virginia cave. The illness is responsible for dramatic declines in several bat species, including the little brown bat.

Photograph by Stephen Alvarez, National Geographic

How Killer Fungus Burns Up Bats From the Inside

Hibernating bats with white-nose syndrome expend twice as much energy as healthy ones, a new study says.

A deadly invasive fungus has been wiping out hibernating bats for years in the U.S., eating away at their wings and muzzles. But it's been unclear exactly how the interloper kills its victims, making efforts to save infected bats take a bit of guesswork.

Now, a recent study has cracked the mystery. White-nose syndrome—so called because researchers first observed the white fungus growing on bat noses in Albany, New York—causes severe dehydration and tampers with a bat's body chemistry. This discovery, which confirms previous studies, could point the way to possible treatments or strategies to minimize the disease's impact on North America's bats. (See "Deadly Bat Fungus Spreading in U.S.")

The fungus has spread along eastern North America into Canada and down to South Carolina and west as far as Missouri. Some populations in Pennsylvania have declined as much as 99 percent, and seven hibernating bat species in the U.S. and Canada have confirmed cases.

In the lab, the study authors discovered that infected little brown bats (Myotis lucifugus) burn twice the energy during hibernation as uninfected individuals do. Sick bats also suffer from relatively acidic blood, which disrupts enzyme function and causes dehydration. The researchers published their results in December in the journal BMC Physiology.

These internal changes occurred in the early stages of infection, which surprised lead study author Michelle Verant, a veterinarian getting her doctorate at the University of Wisconsin-Madison. Obvious signs of the disease—such as daytime flying, an increase in how often the bats wake up from hibernation, and visible growth of the fungus on the wings and muzzle—occur later.

"Finding these potentially life-threatening changes even before we see signs suggests [white-nose syndrome] is a more chronic disease," she said. The fungus acts as an energy-suck all winter, draining a bat's life away and causing the abnormal behavior in later stages that exacerbate the animal's condition. (See "Bats May Be Wiped Out by Fungus in U.S. Northeast.")

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Holes and lesions in the wing of this dead little brown bat are courtesy of the fungus that causes white-nose syndrome.

Déjà Vu

The illness affects only hibernating bats. The fungus prefers cold, humid environments around 40° to 50°F (4° to 10°C)—exactly the kinds of conditions found in hibernation caves. Some sites have 99 to 100 percent humidity, said Greg Turner, a wildlife biologist with the Pennsylvania Game Commission.

If a sick bat survives the winter, the animal can recover if its wings aren't too shredded from the fungus and if it has enough energy to search for food, said Turner, who was not involved in the study. Turner and colleagues have also been able to rehabilitate sick bats emerging from hibernation by providing water and food.

However, surviving the syndrome one winter is no guarantee that the same individual won't get sick again the following winter, said study author Verant. Bats don't build up immunity to the fungus, so they can fall ill over and over again.

The record-holder for reinfection was a bat that got sick every year for five years, she said.

An Inkling of Hope

Fortunately, there are signs that bat populations exposed to white-nose syndrome for several years are starting to adapt, said Turner. Population declines in some areas of New York and Pennsylvania aren't as horrendous as in previous years. And the wildlife biologist has evidence that some adult survivors are packing on more body fat before they hibernate.

When the fungus arrives in a new area—spread either by humans or the arrival of infected bats—massive die-offs occur among certain species for about two years, Turner explained. Not every species is as susceptible as others—the little brown bat, unfortunately, is one of the hardest hit thanks to its preference for cold, humid caves. (Read "Twilight for Bats" in National Geographic magazine.)

The death rate starts to slow down after about two years and seems to stabilize around year six, Turner said. Whether those relatively stable population numbers are due to increased survival or to new bats arriving in the area remains to be seen.

However, "we're very hopeful that we've hit rock bottom and we're starting to see a trend upward," Turner said. "We're analyzing that data as fast as we can."

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