The very popular Ape Cave draws some 150,000 hikers in the summer season alone. It’s a challenge to screen all of them for the fungus that has already killed 6 million small-eared bats in 28 states and five Canadian provinces.

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Long before white-nose syndrome reached the Pacific Northwest, the U.S. Forest Service had warned visitors to Ape Cave at Mount St. Helens of the disease. Now that it’s been found in Washington state, the message has new gravity.

Situated within a couple of hours’ drive from two major metropolitan areas that are home to two international airports, Mount St. Helens National Volcanic Monument is among the Northwest’s most popular tourist attractions. One of the most alluring features of the monument is the Ape Cave, a nearly 2.5-mile-long lava tube on the south side of the volcano.

The cave is open all year, but the Forest Service estimates that between Memorial Day and Labor Day alone, 150,000 unguided hikers from around the world walk through the lightless, undulating black volcanic rock formation. But its popularity brings peril for native bat populations. Whether they know it or not, the cave’s human visitors could be carrying Pseudogymnoascus destructans, or Pd, a fungus responsible for causing white-nose syndrome and killing at least 6 million small-eared bats in 28 states and five Canadian provinces.

“We’ve always kind of looked at Ape Cave especially as being an Achilles’ heel,” said Mitch Wainwright, a wildlife biologist working in the south district of the Gifford Pinchot National Forest where Mount St. Helens is located. “During the screening process, some rangers definitely had people from Europe come through where this fungus is endemic, and other people that have been in caves in white-nose affected areas.”

White-nose syndrome is cause by a fungus that thrives in cold environments, like caves, where bats like to hibernate. It’s named for the white fungus that grows on the muzzles and wings of infected bats. The fungus is endemic to Europe and Asia, and it was first discovered about a decade ago in New York state.

In 2011, the disease’s confirmed western front stood in Illinois, Kentucky and Tennessee, but officials at Mount St. Helens took a proactive approach and started educating visitors about the disease, its transmission and preaching the gospel of clothing and caving equipment decontamination.

Then on March 11, two hikers found a little brown bat infected with the fungus on a hiking trail in North Bend, about 30 miles east of Seattle.

“It’s definitely stressful,” Wainwright said of the discovery. “It really heightens the need for the public to be aware of what (the disease) is and how it’s spread and to be familiar with decontamination protocol.”

“From my standpoint … we are trying to do more education outreach,” said Sue Ripp, spokeswoman for the Gifford Pinchot. “We’ve been increasing our awareness over the past how many years … this is a good thing, and who knows what it’ll lead to.”

Officials have warning literature posted on the Mount St. Helens website and at the Ape Cave’s openings. During the height of tourist season, forest rangers screen and educate visitors who may be carrying the fungus, but they can’t screen everyone. Plus, the cave is open year-round, but the screeners are only seasonal.

Ape Cave is free from the fungus, and the Forest Service wants to keep it that way. In February, Wainwright sent samples from the Ape Cave floor and 71 bats in hibernation caves to researchers with the U.S. Geological Survey in Wisconsin. All samples showed no white-nose fungus.

Bats don’t sleep in Ape Cave — Wainwright is unsure why, but speculates it might be due to the human visitors — but they do visit it frequently.

Also, big-eared varieties of bats, several of which live near Mount St. Helens, don’t seem to be susceptible to the disease.

“They may have the fungus but don’t develop the syndrome,” Wainwright said.

In the U.S., the fungus appears to affect only little-eared varieties of bats, but when one member a the colony is infected, around 90 percent of the other bats are dead within two years.

An advantage for Western bat populations is that they don’t hibernate together by the thousands like their relatives back east. A large hibernation colony in Washington might include up to 400 bats. Bats in the St. Helens and Mount Adams area also have a wide swath of habitat in the forms of caves and lava tubes bats can disperse into. Known hibernation caves around the volcano are closed for winter.

For those reasons, “it may not spread as rapidly (in the west), as it did back east — or at least not right away,” Wainwright said, noting biologists don’t know where the area’s little brown bats hibernate.

Still, the Forest Service wants to maintain the area’s fungus-free status, but that goal could conflict with the Mount St. Helens’ increasing popularity. So many people are visiting the monument that it’s starting to impact the quality of the visitor experience. In response, the Forest Service is reconsidering how it manages Ape Cave visitation and how it conducts screenings for white-nose syndrome.

The talks are ongoing, but the Forest Service might extend the screening season and increase education efforts.

“It’s on the radar,” Ripp said. “Those discussions are already occurring.”