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Serving Northern St. Louis County, Minnesota

White-nose syndrome taking hold in state

Soudan Mine showing highest mortality

Marshall Helmberger
Posted 3/29/17

REGIONAL—State and federal officials say the fungus that has nearly wiped out bat populations in the eastern United States over the past decade has taken a firm grip on Minnesota. That’s based on …

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White-nose syndrome taking hold in state

Soudan Mine showing highest mortality

Posted

REGIONAL—State and federal officials say the fungus that has nearly wiped out bat populations in the eastern United States over the past decade has taken a firm grip on Minnesota. That’s based on DNR bat surveys and on dead bats gathered by the public that show the presence of the illness—known as white-nosed syndrome— that’s associated with the fungus. The disease is named for the white fungal growth observed on infected bats. It is not known to pose a threat to humans, pets, livestock or other wildlife.

Hardest hit, to date, is the Soudan Mine, which researchers believe used to serve as the winter roost of an estimated 10,000 bats, mostly little brown and northern long-eared bats. The latest survey conducted within the mine suggests that as many as 73 percent of those bats are no longer there.

Meanwhile, growing evidence now points to a problem that has spread throughout much of the state, with the fungus now confirmed in six counties, including St. Louis, and suspected in another four, including Lake County in northeastern Minnesota.

The other counties where the fungus is confirmed include Becker, Dakota, Fillmore, Goodhue, and Washington, while the fungus is suspected, but not yet confirmed, in Hennepin, Pine, and Ramsey counties in addition to Lake County.

So far, the most evidence of significant bat mortality is limited to the Soudan Mine, where park staff counted nearly 3,000 dead bats during a systematic two-week search of the grounds surrounding the mine earlier this winter. One of the signs of white-nose syndrome is large numbers of hibernating bats leaving their winter roost early, where they typically succumb to cold temperatures or starvation.

DNR bat researcher Gerda Nordquist said that tally likely underestimates the death toll in Soudan from the disease. She noted that during milder weather, the bats often make it as far as the nearby forest, where they most likely die without being counted. “And the ravens and other scavengers are pretty quick to get on the dead bats,” she noted, which probably further reduced the number found by DNR staff.

During an extensive survey conducted in late February, Nordquist and fellow researchers documented a sharp decline in the number of bats sighted as compared to a 2013 survey, when the fungus that causes white-nose syndrome was first detected at the mine.

The bat mortality at Soudan is similar to what’s been seen at other large bat hibernacula in the eastern U.S., where the syndrome first took hold back in 2007. And Jonathan Reichard, white-nose syndrome coordinator for the U.S. Fish and Wildlife Service, said there’s reason to believe the bat deaths will continue at Soudan. He noted that bat deaths at many eastern sites reached 98-100 percent in many cases, before “bottoming out.” In some locations, said Reichard, bat populations stabilized and, in a few instances, researchers are seeing limited recovery in numbers of little brown bats. Most of the other species affected by the syndrome are continuing to experience declines, even where the fungus has been present for a decade.

There is evidence that bats are adjusting their hibernation locations in response to the fungus, although it’s unclear whether bats are benefitting from the change. In the Soudan Mine survey, researchers noted that many bats had relocated to colder portions of the mine, near the Alaska shaft, which are believed to be less conducive to the growth of the fungus. That same pattern was noticed at Mystery Cave, a DNR-managed site in southeastern Minnesota. Nordquist said the survey there found almost all of the bats had moved from deep within the cave to near entrances, where conditions are significantly cooler and bats are normally not found in winter. During a survey conducted there last week, Nordquist said bat numbers appeared unchanged from prior surveys, but she suspects that may be because the bats are now more easily accessible to survey counters, given their relocation.

Although the disease is transmitted primarily from bat to bat, people can inadvertently carry fungal spores to other caves on clothing and caving gear. For several years, public tours of Soudan Underground Mine and Mystery Cave have begun with a brief lesson on how to prevent the spread of WNS. Both before and after tours, visitors are required to walk across special mats designed to remove spores from footwear, and they are advised not to wear the same clothing, footwear or gear when visiting other caves or mines where bats may be present. Multiple washings in a standard washing machine will not provide sufficient decontamination.

Tours will continue at Soudan Underground Mine and Mystery Cave, where the DNR will continue to follow recommended national decontamination protocols to prevent human transport of fungal spores. The DNR urges owners of private caves to learn about WNS and take similar visitor precautions as outlined in the protocols.