White fungus obliterating American bats
In 2006, an unknown fungus was photographed on a bat in a cave in upstate New York. In 2007, the condition was called “white nose syndrome” due to the furry white deposit seen on the nose and wings, and it killed thousands of bats. The widening circle of destruction has now reached Tennessee, North Carolina, and Canada from the Maritimes to Ontario, and it’s expected to continue expanding.
Deadly, exotic, and easily transported, the fungus, now named Geomyces destructans, has killed as many as 1 million bats in the eastern United States. The high death rate among six species of insect-eating bats in the Northeast has raised questions about their survival.
Bats are the only mammals that really fly, making them inherently cool. They fly at twilight and night, making them inherently mysterious. Add in their biodiversity — second only to rodents among the mammals — and their use of sonar to locate prey, and you have a fascinating order of animals.
For controlling agricultural insects, bats are worth at least $3 billion a year to U.S. agriculture, according to a 2011 study from Boston University. “People often ask why we should care about bats,” said study co-author Paul Cryan, a research scientist with the U.S. Geological Survey in Fort Collins, Colo. “This analysis suggests that bats are saving us big bucks by gobbling up insects that eat or damage our crops. It is obviously beneficial that insectivorous bats are patrolling the skies at night above our fields and forests—these bats deserve help.”
As conservation officials scramble to respond to white nose, they are enacting quarantines to prevent people – cavers, bat-lovers and scientists alike – from transporting the fungus between caves. Last year, for example, the National Wildlife Refuge System halted public access to all caves and mines on its refuges, and set protocols to prevent scientists from spreading the infection.
In May, 2011, the Fish and Wildlife Service rolled out a national plan for confronting and controlling white nose syndrome.
But bats can do plenty of transportation on their own. Even non-migratory bats may fly 200 miles between their hibernation site and their summer range, says David Blehert, a microbiologist at the U.S. Geological Survey National Wildlife Health Center in Madison, Wis., and a leader of white nose studies. “They can move large distances, across state lines, so there is potential for significant disease spread based on bat-to-bat interactions.”
What is the white nose syndrome situation now? Why is it so deadly? What bright ideas are afoot to preserve insect-eating bats, and what is the likely end game?
In the short time since white nose syndrome appeared in 2006, scientists have pinpointed a fungus called G. destructans as the killer. But how does G. destructans do its work? One clue comes from the fact that it only kills during hibernation, when bats live in mines and caves at a rather chilly 7°C. “The fungus only grows in the cold, and when insectivorous bats hibernate in a temperate region, they drop their core body temperature to the ambient level,” says Blehert.
(The fungus is not likely to attack fruit-eating bats, says Blehert, because they do not have long periods of “torpor,” the slow-metabolism hibernation state that is conducive to the white-nose fungus.)
A low body temperature allows the bats to survive winter without eating, but it could also curtail the immune system, Blehert says. “Studies of bat immunology are in their infancy, but based on what is known about the physiology of other hibernating mammals, especially the 13-lined ground squirrel it’s likely that the immune system becomes suppressed, and that leaves them particularly vulnerable” to the fungus.
How does the fungus kill? It apparently does not enter systemic circulation, as internal organs are not damaged. All mammals awaken from hibernation occasionally, but Craig Willis of the University of Manitoba has speculated that infected bats have more waking hours, causing them to run out of energy during a period when they neither eat nor drink.
Blehert and his colleagues favor a second explanation: dehydration. Despite the “white nose” name, Blehert says, the most significant infection occurs on the wings. “The wings of a bat have eight times as much skin as the trunk; it’s a massive, very delicate and exposed membrane” with a single layer of epidermis surrounding a thin layer of connective tissue and some muscles and glands. “The fungus selectively invades the wing skin, and destroys everything in its path,” Blehert says.
Beyond their role in flight, bat wings are also needed to regulate temperature, fluids and electrolytes. “The wings may be the Achilles heel that exposes them to such significant infection,” Blehert says.
Indeed, an emerging disease that is devastating amphibians, the chytrid fungus, also affects the skin, and is thought to kill by causing an electrolyte imbalance. “The amphibian’s skin is very important for the balance of water and electrolytes, which has been the basis for our hypothesis about why white nose syndrome is so deadly. There was a paper1 in 2009 that demonstrated that a superficial chytrid infection causes an ion imbalance in frogs, causing a disruption of the potassium gradient that causes the heart to stop. A superficial fungal infection causes a cardiac arrest! This is a very different concept than getting athlete’s foot and having an itchy foot.”
Stopping the wave of death
As dead bats pile up in caves, what can be done to stop the spread of G. destructans? The first step, trying to slow dispersal, is already under way in affected states, with restrictions on cave entry, and new protocols for disinfecting equipment and people who have a legitimate reason to visit hibernation spots.
The fungus does respond to common anti-fungal agents, according to a 2011 study, which found, unexpectedly, that the meds worked at the low cave temperatures that the fungus prefers. “The challenge is, how could you use pharmaceuticals to manage a disease in free-ranging wildlife?” says Blehert. “They don’t go to the doctor, and they inhabit environments that are likely contaminated with fungus. Say you could treat bats and cure them of the infection. If you can’t remediate their hibernation sites, they will become reinfected when they re-enter the cave.”
The authors of the anti-fungal study did raise the possibility of using meds to decontaminate caves, but this process is not being done, Blehert says. “Going into a cave with a general fungicide would be like dropping a nuclear bomb on a city. Caves are full of bacteria, fungi, invertebrates and vertebrates that may only exist in that unique ecosystem, and getting rid of such an important group of organisms [fungi] could risk significant unintended consequences.”
Willis has proposed using little heaters, since bats seem to fare better in warmer regions of caves, perhaps because that sustains immune function. Small heaters are being tested as bat refuges in some New York State caves, says Lisa Warnecke, a post-doctoral fellow at Manitoba.
Lessons from Europe
G. destructans is an “emerging exotic disease,” and to investigate such diseases, scientists always want to know how the pathogen interacts with hosts in its land of origin, which seems to be Europe:
Although the fungus been found in at least five bat species in Europe, die-offs have not been seen there, suggesting that something is different about how the pathogen, host and environment interact. Pathogens and hosts co-evolve through time in a complex dance:
In the lab in Manitoba, Willis and Warnecke are studying how long little brown bats are awake during hibernation, whether the fungus is a necessary and sufficient cause of death, and if the North American or European strains of fungus have different effects on the bats. “If both isolates show the same severity for North American bats, that may mean that bats in Europe have co-evolved with the fungus and are resistant to it,” says Warnecke. “On the other hand, if the European isolate does not cause trouble for North American bats, then the fungus in North America is a mutant that has gotten really aggressive.”
White nose syndrome has killed a million bats in the East. How can we stop the destruction?
Other factors could explain the lack of disease in Europe, says Blehert. “European bats are larger, which may provide them with more of a buffer against a physical insult like a fungal infection.” The little brown bat, the preeminent victim of white nose, weighs about 6 grams – about the weight of two pennies, Blehert says.
European bats also tend to hibernate in small groups. “They don’t have those 100,000-plus hibernacula like we see in the United States. With fewer animals, the disease transmission dynamic is likely to be reduced, with less amplification of the fungus, and lower rates of bat-to-bat transmission.”
Scientist: “The fungus selectively invades the bat’s wing skin, and destroys everything in its path.”
In the long run, Blehert says, American bats may evolve some resistance. “In general, the population decline in caves and mines comes to about 78 percent, but the bats have not disappeared. We would expect something that gets into population to cause high mortality and a steep drop-off in population. Then, with fewer animals around, disease transmission could moderate.”
Although the regional extinction of the brown bat has been predicted to occur 16 years from now, “our bats may ultimately develop population dynamics more like Europe, with fewer animals and moderated disease transmission and progression,” Blehert says.
Evolution, in other words, could select for animals that, for behavioral or immune reasons, are less susceptible to white-nose.
But letting the situation play out without trying to help the bats, Blehert says, amounts to a high-stakes gamble with one of the wonders of the night sky.
- Pathogenesis of Chytridiomycosis, a Cause of Catastrophic Amphibian Declines, Jamie Voyles et al, Science 23 October 2009: 582-585. [DOI:10.1126/science.1176765]
2 White-Nose Syndrome Fungus (Geomyces destructans) in Bat, France, Sébastien J. ↩
- White-Nose Syndrome Fungus (Geomyces destructans) in Bat, France, Sébastien J. Puechmaille et al, Emerg Infect Dis. 2010 February; 16(2): 290–293.
doi: 10.3201/eid1602.091391. ↩
- White-Nose Syndrome Fungus (Geomyces destructans) in Bats, Europe, Gudrun Wibbelt et al, Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 16, No. 8, August 2010. ↩
- National Wildlife Health Center: white-nosed syndrome. ↩
- USGS research. ↩
- White-nose news ↩
- USFWS’ captive breeding project. ↩
- Chiroptera: the bat order. ↩
- Bat Conservation International. ↩
- Podcasts and videos on WNS. ↩
- White-nose in Europe. ↩
- No mass mortality in Europe. ↩
- Chytrid fungus infecting amphibians. ↩
- Origin of frog fungus. ↩