Researchers have found a clue in the mysterious die-off of bats that has struck the Northeast -- a new fungus that so far seems to be present only in bats and in caves where the die-off has occurred.
"The fungus is in some way involved in causing the bats to starve to death," said biologist Thomas Tomasi of Missouri State University in Springfield. "They are burning up too many calories, at a rate faster than they can sustain."
Bat experts are not yet sure, however, whether the fungus is the cause of the widespread deaths or is simply an opportunistic microorganism infecting animals that have already been weakened by some unknown threat.
"Whether it is the primary cause or not, we still have to find out whether it is newly introduced or if there are other factors that need to be addressed," said biologist Merlin Tuttle, founder and president of Bat Conservation International.
The disease, which bears many similarities to the colony collapse disorder that has decimated honeybee colonies across the country, first appeared in a cave near Albany, N.Y., in the winter of 2006. It has since spread to at least three other states in the region.
The most obvious symptom is the presence of a visible halo of white fungus around the faces of afflicted animals -- hence the common name, white-nose syndrome. The affected animals become severely emaciated, often emerging from their hibernation caves in the dead of winter in a futile search for food.
In some bat caves, more than 90% of the inhabitants died last winter. Overall populations have declined about 75% in the affected areas.
"I have been studying bats for 40-plus years, and this is unparalleled in the history of what I know about bats," said biologist Thomas H. Kunz, director of the Center for Ecology and Conservation Biology at Boston University.
Although bats can seem scary, a significant die-off could have severe economic and health consequences.
Bats represent about a quarter of all mammalian species and are voracious eaters of insects that attack crops and carry diseases. A single bat can eat more than 100% of its body weight in bugs each night.
In Texas, Tuttle said, free-tailed bats eat 200 tons of insects a night. Studies have shown that bats in the state consume large quantities of corn ear and armyworm moths that migrate from the south each spring.
The new study, reported Thursday in the online version of the journal Science, at least identifies what is producing the white nose.
Mycologist David S. Blehert of the U.S. Geological Survey's National Wildlife Health Center in Madison, Wis., and his colleagues identified the fungus as a previously unrecognized strain of the group Geomyces.
Geomyces are fungi that live in soil, water and air and are capable of growing at the low temperatures found in winter caves. Researchers had difficulties isolating it because when the dead animals warmed up in the laboratory, the fungus disappeared. It could only be grown if kept cold.
When Blehert's team studied bats microscopically, they found that the fungus invaded the skin by way of hair shafts, sebaceous glands and sweat glands. Once it gets into the wing, it erodes epidermal and connective tissues, affecting heat dissipation, water control and gas exchange.
"We found that this fungus had colonized the skin of 90% of the bats we analyzed from all the states affected by white-nose syndrome," Blehert said.
One hint that the fungus is the cause of the disease is that DNA sequences of fungus from all of the caves are identical, indicating that it is a newly introduced organism. If it were a latent, opportunistic species that attacks bats only after they are weakened by some other environmental factor, researchers would expect to see slightly different DNA profiles from different locations because they had been introduced at different times, Kunz said.
If it does cause the syndrome, that would be "pretty unusual," said bat biologist DeeAnn M. Reeder of Bucknell University in Lewisburg, Pa. "I don't know of any other fungus that infects mammals that is lethal."
But if it is the cause, she added, the fact that it can grow in only very cold conditions "gives us hope that it won't spread to warmer parts of the country where bats don't hibernate."
Meanwhile, researchers are still looking at a variety of other potential causes for the die-offs.
Some anecdotal evidence suggests that there has been a sharp reduction in the number of insect pests in the region after the introduction of powerful new pesticides. If that is the case, the bats may not be able to build up enough fat to last them through the winter, weakening them and making them more susceptible to the fungus.
Another possibility is that the pesticide itself is affecting the bats, either by direct exposure or through eating contaminated food.
Pesticides and fungi are also being considered as possible causes of colony collapse disorder in bees. And Tomasi notes that a different fungus is attacking frogs across the country, suggesting the possibility of some kind of environmental factor affecting a variety of species.
"These are all canaries in the mine that we need to pay attention to," Kunz said.