A lethal fungus devastating U.S. bat populations in the East and Midwest has crossed the Continental Divide for the first time, unexpectedly popping up in Washington—approximately 2000 kilometers farther west than previously seen.

The discovery of white-nose syndrome in a single, sickly little brown bat (Myotis lucifugus) found in mid-March by hikers at the edge of the Cascade Mountains, 50 kilometers east of Seattle, Washington, is confirmation of what scientists considered the inevitable spread of the disease across the continent. “This is a nightmare scenario come true,” says Jeremy Coleman, an ecologist and head of the U.S. Fish and Wildlife Service’s white-nose syndrome program in Hadley, Massachusetts. “This is the news we have been bracing for and warning about going back for the last 8 years.”

But the syndrome’s appearance in the far northwest corner of the country, announced Thursday by state and federal wildlife agencies, came as a surprise. In recent years, it had only reached as far west as Minnesota and Nebraska, after an orderly march across the continent from its start in upstate New York.

First identified in the United States in 2008 in caves near Albany, the disease is thought to have killed more than 6 million bats in a range of species in 28 states and five Canadian provinces. The fungus, Pseudogymnoascus destructans, thrives in cold, humid environments, and infects bats when they are hibernating during the winter. It severely damages the bats’ skin. The animals are thought to die because the malady makes them burn more energy, become dehydrated, or wake and leave their shelter when it’s too cold. The disease gets its name from a distinctive halo of white fuzz that can grow around an infected bat’s muzzle.

Wildlife managers have sought to stem the disease’s spread. In some places, caves where bats hibernate have been closed to the public, amid concerns that spores could hitchhike on spelunkers’ gear. Recreational cavers and researchers have been asked to take elaborate precautions to clean or quarantine their equipment. But complacency set in as time passed and the disease didn’t materialize in the West, Coleman says. "I think the expectation was we were just going to watch this slow spread through the Midwest and over Canada and then into the Rockies."

Now, with the case in Washington, state and federal scientists and wildlife managers are faced with a host of questions: How did the disease get there? How widespread is it? What does it mean for bats there and elsewhere?

At the U.S. Geological Survey’s (USGS’s) National Wildlife Health Center in Madison, scientists are sequencing the genome of the fungus from the Washington bat, to see if it’s a genetic match to other cases in the United States. The fungus behind the syndrome is endemic to Europe and Asia, so it’s possible the disease arrived in Washington from a different direction, says David Blehert, chief of the Wildlife Disease Diagnostic Laboratories in Madison. The infected bat was a western variant of the little brown bat, meaning it wasn’t a stowaway from further east.

Scientists at the USGS center, along with the Washington state Department of Fish and Wildlife, are also preparing to look for more evidence of the fungus where the bat was found. A check earlier this week failed to turn up more dead bats there, says Katie Haman, a wildlife veterinarian at the state agency in Olympia. A nighttime survey with acoustic devices also didn’t hear more little brown bats, suggesting they haven’t yet emerged from hibernation.

When they do become active, the plan is to catch hundreds of bats in the area and swab them for signs of how widespread the fungus is there, says Anne Ballmann, a wildlife epidemiologist at the USGS center in Madison who leads their effort to track the syndrome’s spread. They are also asking wildlife rehabilitation centers to report any cases of sick bats in the area. The Washington case was noticed when the hikers brought the bat to a rehabilitation center.

Even if it turns out not to be an isolated case, it could be some time before it’s clear how the disease will move in western North America. For example, although many eastern bats congregate in huge numbers to hibernate, bats in the West generally don’t. The differing fate of bats was illustrated in a recent study that found bats in China had lower infection levels than their North American counterparts, suggesting the Chinese bats are more resistant to the fungus. In the Pacific Northwest, “it can be potentially devastating to our bats,” Haman says. “Because the ecology and habits of the bats is so different, we can't predict how bad it can be. We just don’t know.”