Web edition: April 9, 2012
Since 2006, some 6 million to 7 million North American bats have succumbed to white-nose syndrome, a virulent fungal disease. That figure, issued in January by the U.S. Fish & Wildlife Service, at least sextupled the former estimate that biologists had been touting. But the sharp jump in the cumulative death toll isn’t the only disturbing new development. On April 2, scientists confirmed that white-nose fungus has apparently struck bats hibernating in two small Missouri caves.
The finding brings to 19 the number of states impacted by the fungus. It also marks the first confirmation that the pathogen is causing clinical disease west of the Mississippi.
The good news, at least for now: No bat deaths have been attributed to the disease in Missouri, says bat biologist Tony Elliott with the state’s Department of Conservation. But his team’s new announcement brings to three the number of infected caves within the state. It also puts wildlife managers on high alert that area bats may be in peril.
Potentially more troubling, North American bats are not the only ones in the cross-hairs. Earlier this year, researchers in the Czech Republic reported the first European case of clinical disease caused by the fungus responsible for white-nose syndrome. The live victim was a male greater mouse-eared bat that had been hibernating in a cave at a site known as Byčí skála.
Responding to the European case: “It’s not a surprise, but we don’t know what it means,” says Jeremy Coleman, a U.S. Fish & Wildlife Service biologist who serves as national white-nose-syndrome coordinator. It could be the animal was weakened and more vulnerable to the ubiquitous fungus than its hibernating kin. It could be a sign that some strains of the fungus in Europe are especially virulent. Or, he observes, it might simply signal that many Euro bats develop mild disease, only to recover — in sharp contrast to their cousins across the Atlantic.
The fungus responsible, a relatively newly identified species known as Geomyces destructans, has been racing through colonies of hibernating North American bats since at least the winter of 2005. Last year, data emerged indicating the same fungus inhabits caves and other sites where bats hibernate in Europe — and probably has been part of their ecosystems for hundreds of years, if not millennia.
This has led ecologists to suspect that the disease striking American bats traces to a recent European émigré.
Until now, the fungus attached itself to European bats but caused no apparent harm. The likely reason: Either the European fungus is weaker than the American strain, or — and more likely — European bats co-evolved with the fungus and gradually became resistant to it. (That's certainly one interpretation of a study released April 9 on hibernating bats experimentally exposed to each strain of the fungus: It found the European strain was more virulent, which would be one response of a fungus that evolved to successfully continue to infect heavily resistant bats.)
What’s new in the Czech study, explains pathologist Carol Meteyer of the U.S. Geological Survey’s National Wildlife Health Center in Madison, Wisc., is the confirmation of tissue damage characteristic of clinical white-nose sydrome: skin being digested by the pathogen as the fungi’s fibrous segments enter a bat’s wing and begin replacing its cells.
The big concern: Might the Czech data point to a crack in European bats’ armor against the fungi?
On April 6, the U.S. Fish and Wildlife Service issued seven new grants averaging $500,000 each to research teams studying white-nose syndrome. “It’s our top priority to understand the bats’ susceptibility and resilience,” Coleman says. Such research projects are important, he says, to establish whether nascent signs of recovery in a few early-hit North American hibernating sites are anomalous, are signs that some bats are developing immunity to the fungus — or merely represent stragglers that the fungus has yet to find.
Virus hit Missouri in 2010
Two years ago, Missouri biologists surveying caves and other sites where animals hibernate saw signs suggesting the presence of white-nose fungus on resting bats. They collected little bits of the fungus and sent the material to the National Wildlife Health Center where one of Meteyer's colleauges conducted a genetic-fingerprinting analysis. It tested positive for Geomyces destructans.
But fungus on a bat's fur does not itself constitute disease. For that, Meteyer explains, microscopic analyses of skin tissue must confirm that the pathogen "is eroding it — actually eating away the skin of the bat." Back in 2010, her center lacked the tissues for such analyses.
In the mean time, Missouri stepped up its surveys of bat-hibernating sites. This March it found signs of white-nose infected animals in two small caves — one hosting fewer than 10 bats and the other with around 60. Elliott says they collected a little brown bat from one cave and tricolored bats from a second. After euthanizing, the animals were shipped to Meteyer where she confirmed disease was present in both species of bats.
She also cautions that diagnosing the disease can be challenging. Her lab makes definitive assessments of white-nose syndrome, and lately some specimens have arrived that at least initially had a surface coating of a white-nose-like fungus. But it was a benign imposter, she notes. And the rub for those scouting signs of the bat pandemic's spread: "You cannot distinguish those [benign fungi] from white-nose syndrome just by looking at the bat in the cave."
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