The disease that has ravaged bat populations across North America since 2006 is caused by a fungus, scientists confirmed in a study published Thursday.

Since the first known outbreak in a colony of bats in upstate New York, white-nose syndrome (WNS) -- named after the discoloration on infected animals' snouts -- has spread 2,000 kilometres (1,200 miles) across the United States and into Canada, killing at least a million of the winged mammals.

WNS wipes out up to 97 percent of winter colonies of species that hibernate, including little brown bats, northern long-eared bats and the endangered Indiana bat, according to the United States Geological Survey (USGC).

Some scientists suspected that the culprit behind the disease was a fungus called Geomyces destructans.

But skeptics noted that bats in Europe have been colonised by the same fungus without any increase in deaths and argued that WNS was caused by some other agent.

Probing further, USGS scientists led by David Blehert undertook three lab experiments.

In the first, they exposed 29 healthy, hibernating little brown bats to G. destructans samples that had been harvested from a pure culture.

All the bats tested positive for the fungus by the end of the 102-day experiment, whereas those in a non-infected control group remained healthy.

Among the bats that died, all had lesions consistent with white-nose syndrome.

In a second experiment, healthy bats were housed together with hibernating bats naturally infected with the fungus, a situation that arises in nature and could account for its rapid spread.

Nearly 90 percent of the non-infected group developed WNS symptoms within the 102-day period, "demonstrating for the first time that WNS is transmissible," the researchers said.

Finally, to see if the disease can be spread through the air without physical contact, the scientists placed healthy and infected bats in separate but nearby meshed cages.

After 102 days, none of the animals exposed to possible airborne fungi from bats with WNS showed any signs of infection..

"Fungal pathogens have the unique capacity to drive host populations to extinction because of their ability to survive in host-free environments," the researchers said.

"Given the high mortality rate and speed at which WNS has spread, the disease has the potential to decimate North American bat populations and cause species extinctions similar to those documented for amphibians."

The researchers speculated that European bats had developed immunity to the fungus, which is why their populations have not been affected by it.

Bats are in important natural pesticide worth at least 3.7 billion dollars per year to farmers, a recent study calculated.