Crawling through a narrow passage in a cave beneath a boreal forest near Fort Smith in Canada’s Northwest Territories in May, two biologists spotted a lifeless bat on a limestone ledge, its body covered in a spindly white fungus.

When they shone their headlamps to get a closer look, they saw another bat with the fungus on its forearms. “It hissed at us and crawled back into a crack,” Jesika Reimer says.

For Reimer, an ecologist specialising in bat populations in northern Canada and Alaska, there was little doubt. For 20 years, white-nose syndrome, a lethal disease caused by the fungus Pseudogymnoascus destructans (known as Pd), has swept west across North America, spreading from bat to bat and cave to cave, killing more than 6.7 million bats.

The fungus attacks their face, wings and arms while they are hibernating. They stir from rest and groom themselves, burning through the fat and energy reserves they need to survive the winter, and eventually die from dehydration or starvation. Smaller bats with lesser fat reserves are particularly at risk.

Jesika Reimer frees a bat from a net in Alaska’s Katmai national park. Photograph: Michael Code

Now it had reached Fort Smith, a town about 450 miles (725km) south of the Arctic Circle. “For 20 years, we’ve been tracking the spread of white-nose syndrome across the continent,” says Reimer’s colleague, Joanna Wilson, a biologist for the Northwest Territories government. “We always knew it would eventually get here.”

Even so, the biologists were shocked by what they saw. They had hoped to do a baseline survey of healthy, stable populations before the fungus arrived. In 2025, white-nose syndrome had been discovered in a cave in Fort McKay, Alberta, nearly 200 miles south. “We thought we’d have a year or two,” Reimer says.

A brown bat with white-nose syndrome in New York. The disease has moved north with a speed that has alarmed scientists in Canada. Photograph: Ryan von Linden/AP

Now the fungus has reached the northernmost-known hibernaculum, where bats shelter during the North American winter, it has raised urgent questions about its potential impact on bat populations in Canada and Alaska, Reimer says.

The caves near Fort Smith were first discovered in 2010, when a wildlife technician flying over the area noticed a distinctive karst feature below. A karst landscape is formed when bedrock such as limestone or dolomite dissolves, forming underground caves and sinkholes that provide an ideal habitat for wintering bats. The technician later hiked back on foot and confirmed it was, indeed, a hibernaculum. Two additional caves were found in the region.

The bat caves near Fort Smith, in Canada’s Northwest Territories. Photograph: Jesika Reimer

Until then, biologists had not known where the Northwest Territories’ hibernating bat species – including little brown bats (Myotis lucifugus), big brown bats (Eptesicus fuscus) and northern long-eared bats (Myotis septentrionalis) – were spending the winter.

They had located summer colonies, or roosts, which tipped them off that bats were sticking around through the winter months, but they did not know where or in what numbers. The 11-chambered cave shelters an estimated 3,000 bats, making it the largest hibernaculum in western Canada. It has been key to understanding the behaviour of northern bats.

In 2011, Reimer focused her postgraduate research on the cave to discover how bats survived at such an extreme northern latitude, where the summer months are cooler, shorter and the sun hardly sets. She found that northern bats had to adapt to survive, such as taking more risks than their southern counterparts, flying at dusk instead of in true darkness to forage for mosquitoes and moths, and eating more orb-weaver spiders – higher in energy content – to keep fuel levels up.

But no matter their adaptive abilities, smaller species of bats, such as little brown and northern long-eared myotis bats, are particularly susceptible to white-nose syndrome. Both are listed as species of special concern in the Northwest Territories. Since the fungus arrived in New York state in 2007 – thought to have been brought over from Europe on the shoes of a caver – it has killed 90% of little brown and 99% of northern long-eared myotis bats in the caves where it has spread.

Some little brown populations are slowly rebounding, Reimer points out. They are genetically adapting their metabolic rates to burn less fat and withstand infection.

Bats illuminated by scientists’ headlamps in a hibernaculum in Canada’s Northwest Territories. Photograph: Julie Thomas/Jesika Reimer

However, there is a less hopeful prognosis for the northern long-eared myotis, a species that weighs just 6-9g, and lacks the fat and energy reserves to outlast white-nose syndrome through the winter. “If it hits them as hard as it has elsewhere in their range, we can expect them to completely disappear from the ecosystem,” Reimer says.

In May, Reimer and Wilson led research efforts to capture bats outside the cave entrance using lightweight mist nets, documenting little browns and northern long-eared myotis with white fungus “riddling the wing membranes”, Reimer says, adding that there were clear signs of necrosis, when living tissue dies. They also used a UV light to scan for orange fluorescence on their wings, which indicates Pd.

They witnessed other unusual behaviours, too, including bats flying outside the cave in daylight. “That can be a sign of bats that are sick, or trying to recover,” Wilson says.

One of their colleagues discovered an infected bat, badly dehydrated, at a nearby lake. “It couldn’t fly,” Reimer says. When they examined its wings they found they were as “brittle as tissue paper”, she says. They gave the bat food and water, though they think it probably died later.

Signs of necrosis in a bat’s wing caused by white-nose fungus. Photograph: Jesika Reimer

While laboratory tests recently came back positive for white-nose syndrome, Reimer and Wilson were relieved that they did not see the same mass die-offs as documented in large caves in the south, including most recently in Alberta’s Rocky Mountains. They are not sure if it is because the fungus has only just arrived and has not yet become established, or if the cave’s sub-arctic latitude gives bats an adaptive edge.

One factor could be the temperature of caves. While the fungus thrives at 10C (50F), Reimer says, the northern caves are much cooler. “Some of the chambers drop below freezing throughout the winter,” she says.

And while Pd can still grow, it might not grow as quickly as it would in a warmer cave. That, says Wilson, gives researchers some “cautious hope”.

“We might see different outcomes,” she says.

And there is an additional reason for hope. Researchers are applying an experimental “probiotic cocktail”, developed by scientists at Ontario’s McMaster University from samples taken from the microbiomes on bats’ wings, to the surface of summer roosts. The substance gets on to their body and spreads from bat to bat in much the same way that Pd spreads.

It is thought that the probiotic – beneficial microbes that have anti-fungal properties – could improve the bats’ resilience to Pd infection by slowing its growth. Early field trials in British Columbia and Washington are yielding promising results: among Pd-infected bats, those with the highest amount of probiotic microbes had the least Pd.

“We see it as a treatment option that has a very good chance of helping and very little chance of harm,” Wilson says.

But further complicating the advance of white-nose syndrome is another big factor: wildfires. In 2023, large wildfires torched the trees surrounding the caves near Fort Smith. “It’s matchsticks. There is not a single piece of foliage to be found,” Reimer says.

The loss of habitat forces bats to fly longer distances to find food, summer roosts in old decayed trees, and mates during the autumn breeding season.

Jesika Reimer releases a bat after taking samples for her research. Scientists hope a probiotic could improve bats’ resilience to Pd infection. Photograph: Michael Code

Reimer also wonders if wildfires could have played a role in the rate at which the fungus spread, forcing bats from the Northwest Territories to move south and mix with Pd-infected bats – then bringing it back to the northernmost caves.

For now, there are more questions than answers, including what mitigation measures biologists will take – and if they will choose to mitigate at all, says Reimer. While the little brown bats may be able to bounce back on their own, interventions could give the northern long-eared myotis a “fighting chance” to survive and adapt, she adds.

Nothing more will be known until next spring, when Reimer and Wilson return to the caves to see how the bats have fared – and to search for more answers.

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