White nose syndrome is a fungal disease that has killed over 90% of some North American bat species since it was first detected in 2006. Caused by a cold-loving fungus that invades bats’ skin during winter hibernation, it disrupts their ability to survive the months-long fast between fall and spring. The disease has now been confirmed in 42 U.S. states and 9 Canadian provinces, making it one of the most devastating wildlife diseases in modern history.
What Causes It
The culprit is a fungus called Pseudogymnoascus destructans. It thrives in cold, dark environments and can only grow at temperatures below about 68°F (20°C), which makes the caves and mines where bats hibernate an ideal breeding ground. The fungus is believed to have been introduced to North America from Europe or Asia, where it exists but doesn’t cause significant die-offs. European bats appear to have evolved some tolerance over centuries of exposure. North American bats had no such advantage.
The first evidence of the disease appeared in a photograph of a hibernating bat taken during the winter of 2005–2006 in a cave near Albany, New York. From there, it spread rapidly across the eastern United States and has since moved westward, with confirmed cases now reaching states like Washington, Oregon, Idaho, Montana, and Wyoming.
How the Fungus Kills Bats
Bats enter hibernation with a carefully rationed supply of body fat that has to last the entire winter. Their metabolism slows to a crawl, and they may only wake briefly a handful of times over several months. The fungus disrupts this delicate energy balance. USGS researchers found that infected bats use twice as much energy as healthy bats during hibernation, burning through their fat reserves far too quickly.
The fungus grows on exposed skin, particularly the muzzle and wings. Wing membranes are especially vulnerable because bats rely on them for more than flight. Wings help regulate water loss, blood pressure, and gas exchange. When the fungus damages this tissue, it creates a cascade of problems: dissolved carbon dioxide builds up in the blood, causing dangerous pH imbalances throughout the body. Potassium levels spike, which can interfere with normal heart function. Infected bats wake from hibernation far more often than they should, burning calories they can’t replace because there are no insects to eat in winter.
By the end of the season, infected bats have dramatically less fat and proportionally more lean tissue compared to healthy bats. Many simply starve. Others die from the physiological imbalances before they ever get the chance to fly again.
Visible Signs of the Disease
The name comes from the most obvious symptom: a white, powdery fungal growth visible on infected bats’ muzzles and wings. But behavioral changes can also signal an outbreak. Bats flying outdoors during the day in freezing winter temperatures is abnormal and a potential red flag. So is finding bats clustered near the entrances of caves or mines rather than deep inside, or discovering dead and dying bats on the ground during winter months. Healthy hibernating bats are tucked away in stable, cold spots deep within their hibernation sites, not near openings where temperatures fluctuate.
Which Species Are Hardest Hit
Three species have lost more than 90% of their populations in fewer than 10 years: the northern long-eared bat, the little brown bat, and the tri-colored bat. The northern long-eared bat was listed as endangered under the Endangered Species Act largely because of white nose syndrome. Little brown bats were once among the most common bat species in North America. That is no longer the case.
Not every bat species is equally vulnerable. Species that hibernate in large, tightly packed colonies in caves tend to suffer the worst losses, while tree-roosting bats or species that hibernate in smaller groups face lower exposure. But the fungus persists in cave environments even when bats aren’t present, meaning recolonization of contaminated sites carries ongoing risk.
The Economic Toll
Bats are voracious insect eaters. A single little brown bat can consume thousands of insects per night, including agricultural pests like moths, beetles, and mosquitoes. This free pest control has been valued at billions of dollars annually for U.S. farmers. One economic analysis estimated that between 2006 and 2017, the decline in bat populations from white nose syndrome resulted in $26.9 billion in combined agricultural losses and increased pesticide costs for affected communities. When bats disappear, farmers lose a natural defense against crop damage and must rely more heavily on chemical alternatives.
How Far It Has Spread
As of early 2026, white nose syndrome has been confirmed in 42 U.S. states and 9 Canadian provinces. The initial spread was concentrated in the Northeast and Appalachian region, but the fungus has now reached nearly every corner of the continent. Western states including Colorado, New Mexico, Oregon, and Washington all have confirmed cases. In Canada, the disease stretches from Nova Scotia and Newfoundland in the east to Alberta and Saskatchewan in the west.
The fungus can spread bat to bat through direct contact, but it also persists on cave surfaces, soil, and equipment. Humans visiting contaminated caves can inadvertently carry fungal spores to new locations on clothing or gear, which is why many agencies have closed caves or implemented decontamination protocols for cavers.
Vaccine Development and Treatment Efforts
Scientists at the USGS National Wildlife Health Center have developed an oral vaccine that can be given to wild bats. The vaccine uses a modified raccoon poxvirus to deliver protection, and researchers are also exploring a topical version that bats would ingest through their natural grooming behavior, making large-scale application more practical.
Field trials began in 2019 and have since treated more than 5,000 bats across seven states, including little brown bats and several other vulnerable species. The results are encouraging: vaccinated bats show lower fungal loads, less severe wing damage, and higher survival rates the following year compared to unvaccinated bats. Researchers have also found that timing matters. Female bats respond better to vaccination at their summer maternity roosts, while males are more responsive during the fall and early winter when they’re most active.
One important caveat is that the vaccine appears most effective when administered before or immediately after the fungus arrives in a new area. For populations that have already been devastated, vaccination alone may not be enough to rebuild numbers. This makes western bat populations, where the disease is still relatively new, a high priority for intervention.