Over the past several years, a disease has spread through hibernating bats in the northeast of North America, placing a number of species at risk of extinction. The disorder, called white nose syndrome, appears to be caused by a fungus that spreads rapidly within bat colonies, killing off most of the infected animals. The fungus in question, Geomyces destructans, is also found in Europe, where there is no sign of white nose syndrome. Now, researchers have shown the European fungus is even more virulent than the native North American one, provided that the victims hail from North America.

White nose syndrome was first spotted in New York in 2006. Since then, it has spread rapidly, wiping out the majority of bats in the areas where it appears. The disease is so severe that a number of species may be at risk of local extinction in parts of their historic range. The fungus attacks the wings, ears, and face of the bats, burrowing into the skin and killing them before they can complete hibernation.

The fungus in question, G. destructans, isn't necessarily lethal to bats, however—it's been found living on them in Europe. This suggests two possibilities for the spread of white nose syndrome in North America: the fungus or the bats. Either the fungus has always been present here but a strain evolved virulence due to a novel mutation, or the fungus was only recently introduced from Europe, so the local bats were unprepared to deal with it.

To figure out which was the case, an international team of researchers obtained samples of the fungus from North American and European bats. They introduced it to a population of North America's little brown bats (Myotis lucifugus), kept in conditions that induce hibernation. A control population of uninfected bats was also followed through hibernation.

It wasn't the fungus. Both European and North American strains set off the usual suite of symptoms in the infected animals. In fact, the European strain was even more virulent, taking its first victim 17 days before the North American version killed a bat. It also wiped out that arm of the experiment 13 days earlier than the North American strain did.

Given their round-the-clock monitoring of the bats, the authors were also able to find evidence that supports the leading ideas about why the disease is fatal. The control group of bats only roused from hibernation every 20-30 days, and spent the rest of the time inactive. Although the infected groups showed identical behavior for the first 30 days of hibernation, they began to wake more often, with some of the infected bats rousing out of hibernation several times a week shortly before their death.

Why is this fatal? The little brown bat needs to be able to survive about six months in hibernation using only its stored fat reserves. The repeated wake-ups burn through these reserves much more quickly, leaving the bats starving long before the winter's over. The authors confirmed the infected bats had eliminated their fat reserves by the time they died.

What the study didn't tell us is what's waking the bats up. In fact, it eliminated two ideas that other researchers had suggested. One of these suggestions was that the periods where bats exited their hibernation were needed in order to raise the body temperature as part of an immune response; the other was that the fungus caused skin irritation that woke the bats. Both of these, however, would probably lead to extended periods of elevated activity. But the infected bats kept their periods of elevated metabolism very brief, and simply took them more often.

While the new finding is somewhat hopeful (in that it suggests the bats and fungus may be evolving a greater degree of tolerance), infections are still clearly lethal. At this point, the fungus is very widespread within the population. Bats won't make control efforts any easier, given how widely they travel while foraging. So although the study helps us understand the disease much better, it won't necessarily help us keep the bat population alive.

PNAS, 2012. DOI: 10.1073/pnas.1200374109 (About DOIs).