DULUTH, Minn.-The number of bats counted in the Soudan Underground Mine has dropped 70 percent due to white-nose syndrome, according to the annual survey of the state's largest bat wintering area.
Researchers have known since 2013 that the deadly fungus was present on some bats that spend their winter deep underground in the former iron ore mine near Tower. Last winter was the first time they had seen hundreds of dead bats outside the mine during winter months, a sure sign of white-nose syndrome.
This winter, the deaths have mounted to catastrophic levels.
"Last year we had maybe 1,000 dead bats on the surface. This year it's more than 2,000 and counting. The ravens are really enjoying it, so it's hard to get a good count of the dead bodies," said Jim Essig, manager at Lake Vermilion-Soudan Underground Mine State Park.
In the eastern U.S., bat wintering areas, call hibernacula, have seen white-nose syndrome losses ranging from 90 to 100 percent, usually peaking several years after the disease is first seen. Essig is hoping that the disease may already have peaked in the Soudan mine, "but we don't have any way to know that."
"It's bad, obviously. But maybe it's not quite as bad as expected," Essig said. "I'm hoping we can settle in at 70 percent and maybe the rest survive. But that's just me trying to be positive."
Researchers go to the mine each winter to count bats and monitor their health. This year they repeated a more extensive count last conducted in 2013, before white-nose syndrome began killing bats, with the total number of bats counted down 70 percent.
At the Soudan mine, both little brown bats and northern long-eared bats are dying from the fungus that's already killed millions of bats in eastern states.
Until now, as many as 15,000 bats spent their winters in the labyrinth of underground shafts and tunnels in the mine, by far the largest colony in Minnesota.
White-nose syndrome was first observed in North America in 2007 in a cave in upstate New York. The fungal disease spread fast, and has now has been found in 29 states and five Canadian provinces.
It's possible some species could become "functionally extinct" within a few years, according to the U.S. Fish and Wildlife Service, which has called the disease the worst wildlife health crisis in the nation's history.
The northern long-eared bat last year was formally placed on the endangered species list as threatened, offering federal protection for one of the species most affected by the disease. Minnesota has seven species of bats, four of which hibernate during the winter and are at greatest risk of contracting the disease.
It's not clear how the disease has spread so quickly, but it's believed that it is transmitted both between bats, and from one cave to another in the footwear or clothing of people who visit caves.
White-nose syndrome is named for the fuzzy white growth of fungus observed on the faces of infected bats. Infected bats show unusual behavior, such as flying during the day in summer or leaving caves during their usual winter hibernation, when no bugs are present for them to eat. A wildlife veterinarian at the University of Wisconsin confirmed the disease kills bats by causing their bodies to overheat, burning energy too quickly and at a time - in winter - when no insects are present to replace the lost calories and when it's far too cold for the mammals to survive outside.
"They freeze to death pretty fast here when they come out in winter," Essig said.
Bats can live more than 30 years in the wild but have a very low reproductive rate, which means it is very difficult for populations to rebuild after the disease hits. Even if some bats survive at Soudan, it would take decades to rebuild their numbers, Essig noted.
Experts say they are considering options to try to keep the disease out of specific caves, but once it enters it is impossible to eradicate.
The loss of bats, the only flying mammal, is considered troubling because bats play an important role in many ecosystems. A single bat can eat more than 1,000 mosquitoes in a single night. Bats are considered important agriculturally because of the huge number of insects they can keep away from crops, and in forested areas for reducing the number of tree-damaging bugs.
White-nose syndrome is not known to pose a threat to humans, pets, livestock or other wildlife.
Research continues to bring better understanding of white-nose syndrome, and scientists are beginning to see promising results that eventually could help treat affected bats. But so far nothing has been found to stop or even slow the bat disease that is spreading across North America.
In one Vermont cave where a quarter-million bats died a few years ago from white-nose syndrome, a few bats have returned. Scientists are trying to find out why they survived when others did not. They also are looking at why the disease doesn't kill bats in its native Europe, and they are looking at both natural bacteria and fungicides that may help repel the fungus.