tri-colored bat

A tri-colored bat shows symptoms of white-nose syndrome at Fern Cave in Alabama. (Photo: Darwin Brock/USFWS)

White-nose syndrome, a fungal disease that has killed 6 million American bats since 2006, was confirmed in two new U.S. states this week. Its arrival in Michigan and Wisconsin pushes the national total to 25 states, which means half the country is now plagued by this runaway wildlife epidemic.

"We face the loss of multiple bat species and the benefits they provide to our ecosystems and our people," Erin Crain of Wisconsin's Department of Natural Resources said in a statement Thursday.

White-nose syndrome (WNS) has spread wildly since its debut in a single New York bat cave eight years ago. It first showed up west of the Mississippi River in 2012, and was confirmed as far south as Georgia and South Carolina last year. It has now been documented in 25 states as well as five Canadian provinces, and suspected cases already stretch into the Great Plains.

Only a few infected bats have been verified so far in Michigan and Wisconsin — five and two, respectively — but given the disease's knack for rapid expansion, experts are bracing for much worse. "Even though we've known this disease was coming, it is a disappointing day," said Dan O'Brien, a wildlife veterinarian with Michigan's Department of Natural Resources, in another statement issued Thursday. "We will now shift gears and try to stop the spread of this serious disease."

white-nose syndrome map

WNS is named for a telltale white fuzz that grows on the noses, wings and tails of infected bats. Its source was initially a mystery, but scientists now say it's caused by Pseudogymnoascus destructans, a previously unknown species of cave-dwelling fungus. Spores were most likely introduced to North America from Europe, where native bats are apparently immune to similar cave fungi. Unwitting spelunkers may have carried those spores across the Atlantic on their shoes, clothes and equipment, inadvertently helping the fungus invade a continent full of defenseless bats.

P. destructans doesn't affect people or other animals, and although it has a mortality rate near 100 percent in bat colonies, it doesn't even seem to kill bats directly. Instead it wakes them up from hibernation prematurely, leaving them to search fruitlessly for insects in the dead of winter. Many infected bats are found with empty stomachs and low fat stores, suggesting they starved to death.

Both Michigan and Wisconsin have measures in place to thwart WNS, such as requiring people to disinfect their shoes and equipment before entering caves or banning human entry to some caves entirely. More than 80 percent of Michigan's largest bat hibernacula are now closed to the public, and Wisconsin also prohibits any equipment in caves that visitors have already used outside the state.

Experts say these efforts may have helped delay the arrival of WNS, but the disease is notoriously resourceful. Enough bats may survive its effects to spread spores to new caves, for example, and a recent study found that once P. destructans is established, it will likely survive even if all the bats die.

"It can basically live on any complex carbon source, which encompasses insects, undigested insect parts in guano, wood, dead fungi and cave fish," that study's lead author said at the time. "We looked at all the different nitrogen sources and found that basically it can grow on all of them."

northern long-eared bat

A northern long-eared bat in Illinois displays visible symptoms of white-nose syndrome. (Photo: U.S. Fish and Wildlife Service)

Aside from trying to prevent people from spreading spores, scientists are also looking into ways to disinfect caves, alter their temperature and humidity, or even treat individual bats. No practical solutions have emerged so far, though, and rehab efforts are complicated by the fact that bats are also known to carry rabies, an unrelated disease that can infect humans and other mammals.

"At this point, there is no effective treatment for WNS and no practical way to deliver the treatment to millions of affected bats even if treatment existed," O'Brien says. "Rehabilitation of bats is prohibited in Michigan because of the potential for the exposure of humans to rabies. The best thing the public can do when they find a dying or dead bat is to leave it alone and keep children, livestock and pets away."

The disease only seems to threaten hibernating bats, but that includes more than half of the 45 native U.S. species. Several already-endangered species like gray bats and Indiana bats are now further endangered by WNS, and the U.S. Fish and Wildlife Service reportedly expects to add another species — the northern long-eared bat — to the national endangered species list this fall.

Losing millions more bats, not to mention entire species, would be an ecological and economic disaster. Bats are key predators of insects, including disease carriers like mosquitoes and agricultural pests, so a healthy bat population improves crop yields and reduces the need for pesticides. A 2011 study found bats save the U.S. economy between $3.7 billion and $53 billion every year.

There are still rays of hope, though. Scientists are studying relatives of P. destructans in hopes of finding an Achilles' heel, as well as certain bacteria or other "bio-control agents" that may eventually help increase bats' chances for survival. And if all else fails, experts hope enclaves of hibernating bats can endure in parts of the Deep South where winters are shorter and milder.

Russell McLendon is science editor at MNN. Follow him on Twitter and Google+.

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Russell McLendon ( @russmclendon ) writes about humans and other wildlife.

Deadly bat epidemic spreads, now in half of U.S. states
The bat-killing fungal infection known as white-nose syndrome has been found in two new U.S. states.