Wisconsin’s bats are seeing a slight resurgence after White-Nose Syndrome depleted populations across the country. The Wisconsin DNR found that bat populations in caves had begun to increase after being devastated when WNS came to Wisconsin in 2014.
WNS is caused by the fungus, “Psuedogymnoascus destuctans,” that infects the bats’ wings and noses, giving it its name. It was originally discovered in the U.S. in 2006 among bats in a cave outside Albany, New York. It has now spread throughout the U.S. and Canada, reaching 45 of the 48 contiguous states and 10 of 13 Canadian provinces and territories.
Professor of Forest and Wildlife Ecology in the College of Agricultural and Life Sciences, Anna Pidgeon, said the fungus was likely spread from cave explorers in Europe who brought it to the United States.
“The fungus exists in Eurasia, but the bats in Eurasia do not react to [the fungus] in the way that bats in North America do,” Pidgeon said. “The suspicion is that people who explore caves, spelunkers, in Europe brought it over on their equipment or shoes to North America.”
Pidgeon said the reason for the fast spread is because bats live in multiple caves during their lives and hibernate very close to each other and the fungus spreads between them.
WNS affects bats that hibernate during the winter, Pidgeon said. In Wisconsin, that means the tricolored bat, northern long-eared bat, big brown bat and little brown bat.
Migratory bats are not known to be affected by the fungus. These bats often roost in trees and spaced apart from each other, limiting the chance for spread.
Professor and Extension Wildlife Specialist in CALS, David Drake, said the main danger WNS poses to bats is waking them from their hibernation.
“Bats are true hibernators, so they’re into a deep torpor,” Drake said. “When they wake up it costs them a lot of energy for them to go from torpor to an active level.”
This waking uses up the energy reserves stored in fat that the bats pack on during the summer and fall before hibernating for the winter. After they wake up, the bats are often disoriented, Drake said. If the bats leave the cave, they move from the controlled climate of the cave and into the cold of winter. The fungus ends up killing them through exhaustion, hypothermia from leaving the cave or dehydration.
The Center for Biological Diversity says damage to bats’ wing tissue can cause bats to wake but it is not known exactly what causes this.
Pidgeon said cave conditions actually favor the fungus’s growth, preferring colder temperatures, which makes things even worse for cave-hibernating bats.
“50 degrees and slightly humid environment of the cave is exactly the conditions that the fungus likes,” Pidgeon said.
Researchers from the U.S. Geological Survey’s National Wildlife Health Center partnered with the Wisconsin DNR to field test a WNS vaccine. The study lasted from 2019 to 2022 and showed that vaccinated bats had lower fungal loads than bats given a placebo.
Researchers additionally observed that both vaccinated and unvaccinated bats seemed to improve, both with lower loads of the fungus and emerging later from their winter Wisconsin caves. The observation has borne out in other bat populations in the eastern U.S. that were affected by WNS earlier in its spread.
Drake said there are concerns about trying to kill fungus in caves preemptively because of the fragility of the cave ecosystem.
“Cave ecosystems are so fragile, that if you start pulling on this thread, what else are you going to unravel that may create bigger problems in the cave system,” Drake said.
But, the populations of western bats could still be affected as the fungus moves west and unexposed bats have to grapple with it. California detected the WNS in four new counties in the past year.
WNS is a danger not only to bats, but also to ecological balance. Killing large numbers of bats has a long-term effect due to bats’ slow-reproducing nature. Pidgeon said this event threatens the long term stability of bat populations.
“Bats have one or two pups at a time, but it depends on species, so mostly one,” Pidgeon said. “It can take decades and decades to rebuild the population, if that’s even possible.”
Bats play key ecological roles, from eating insects to pollination. A study published in Science found that counties that had bat die-offs had an increase in insecticide use by farmers. This increased insecticide use was then linked to an 8% increase in infant mortality.
The increase in populations may be a hopeful sign for Wisconsin’s bats because it may signal a true change in the bats’ fight against WNS. There is new hope for North America’s bats, but the fight to protect them will continue.
