WASHINGTON — A deadly fungal disease that has killed millions of bats in North America is likely caused by a pathogen that came from Europe, international researchers said on Monday.

White-nose syndrome (WNS), caused by a pathogen Geomyces destructans, was first observed to be killing bats in a cave in upstate New York in 2006, and has since killed some 6.7 million bats in the United States and Canada.

But the epidemic has not been seen in Europe, leading researchers to wonder if the disease may have been introduced from abroad by tourists who unwittingly brought the fungus into US bat caves.

A research team decided to test this idea by exposing a population of little brown bats from the Canadian province of Manitoba, where WNS has not afflicted the population, to strains originating from the United States and Germany.

One group was exposed to the US strain of the fungus, while another group was exposed to the German type.

The European type was first seen to start killing bats after 71 days, while the North American type began killing bats on day 88, according to the study published in the Proceedings of the National Academy of Sciences.

"The results of this experiment are really quite strong evidence of that invasive pathogen idea," said lead author Craig Willis, an associate professor of biology at the University of Winnipeg, in an interview with AFP.

"If anything the European version was a little bit nastier."

European bats have likely evolved some resistance to the pathogen over time, Willis explained. Recent research has shown that some hibernating bats in Europe are infected with the pathogen but that it does not kill them.

North American bats have not evolved that same level of protection and are more vulnerable to the disease, which causes them to wake frequently during hibernation and subsequently waste needed body fat reserves.

The disease gets its name from the light-colored fungus that clusters around the bats' ears, nose and wings.

In some ways, the finding is good news because it suggests that European bats are not at risk from a North American strain of the disease, though a study next year testing the US-Canada strain on European bats should provide more data.

However, the disease continues to wipe out bat populations -- so far afflicting four Canadian provinces and 16 US states -- with no end in sight.

Willis said researchers think the fungus was likely introduced by someone from abroad going into caves in New York state, because the disease was first documented in 2006 an area called Howe's Caverns that receives tens of thousands of visitors each year.

The outbreak of WNS has since spread 2,000 kilometers (1,200 miles) across the United States and into Canada.

"We know the fungus can survive and persist in the environment on climbing equipment and on boots and shoes and those types of things, so it is possible that someone tracked it into this cave," he said.

"We will probably never know for sure how it arrived but we know where it did arrive."

Now that scientists believe they are dealing with an invasive disease, efforts to control it may focus more intensely on how and when it spreads.

Bats tend to die from the disease during or after their hibernation period, but it is unclear if they spread the fungus as they move from cave to cave in the fall, or if survivors spread it when they emerge in the spring, or if it lives on during the summer only to turn lethal in the cold months.

"Managing the agents that spread the disease is the critical priority for bats and that is really hard to do because they travel so far and they are totally uncooperative -- they won't do what you tell them and they are really cryptic and hard to study," said Willis.

Bats are valuable to the economy because they provide natural pest control for forests and farms, with some research showing they are worth as much as $3.7 billion dollars per year to farmers.

The syndrome is particularly lethal for winter colonies of species that hibernate, including little brown bats, northern long-eared bats and the endangered Indiana bat, and wipes out up to 97 percent of colonies affected according to the United States Geological Survey.