The arrows point to unusual white noses in a cluster of bats in a New York cave during the winter in 2006. The white is apparently caused by a fungus and may be related to an unusual number of bat deaths. (Photo by Nancy Heaslip)

The Disappearing Bats

Fungus is devastating the nocturnal insectivores and endangering agriculture's free pest control

First the honeybees disappeared; now it's bats. With good reason, farmers are worried.

“On a given night bats can eat up anywhere from one-half to their entire body weight in insects,” says bat authority Thomas Kunzy, Boston University William Fairfield Warren Distinguished Professor and director of BU’s Center for Ecology and Conservation Biology (CECB). “That converts into about 630 tons of insects that those million bats would eat. If we don’t have [the bats], those insects are still going to be out there. Many of them are crop pests, they’re garden pests. Without the bats, it means that farmers, in order to maintain their economic advantage, will throw pesticides [on their crops].”

Over the past several years, several species of American bats have come under attack from an invasive fungus responsible for a virus known as white-nose syndrome that has left bat caves littered with bodies.

Biologists estimate that 6.7 million bats have died as a result of white-nose syndrome in the past five years, according to Dee Ann Reeder, assistant professor of biology at Bucknell University and principle investigator at the Reeder Lab, which researches diseases affecting bats.

Speaking to Noelle Swan of Seedstock, Reeder underscored the huge implications of this loss, pointing out that "for every million bats that have died, 692 tons of insects are not being eaten every summer."

Bats perform a multibillion-dollar service for the agricultural industry. So far, the areas hit hardest by the fungus, mainly in the northeastern United States, have not been major agricultural regions. As the epidemic spreads further toward the Midwest and the breadbasket states, biologists and farmers are starting to worry.

Geomyces destructans, the fungus that causes white-nose syndrome, originates in the caves of Europe where bats are immune to the disease. Somehow, likely on the heels of a trans-Atlantic tourist, the fungus hitched a ride to upstate New York during the winter of 2006-2007, where bat populations had no natural defenses against the fungus, which penetrates into the bat's wing fibers during hibernation. Colonies of hibernating bats were reduced 80-97 percent at the affected caves and mines that were surveyed. Since then, white-nose syndrome or its causative agent have been detected more than 2,000 kilometers (1,200 miles) away from the original site, and has infected bats in at least 19 U.S. states and 4 Canadian provinces. Most species of bats that hibernate in the region are now known to be affected; little brown bats (Myotis lucifugus), northern long-eared bats (M. septentrionalis), and federally listed (endangered) Indiana bats (M. sodalis) have been hit particularly hard. The sudden and widespread mortality associated with white-nose syndrome is unprecedented in hibernating bats, which differ from most other small mammals in that their survival strategy is to live life in the slow lane: their life history adaptations include high rates of survival and low fecundity, resulting in low potential for population growth. Most of the affected species are long lived (5-15 years or more) and have only one offspring per year. Subsequently, bat numbers do not fluctuate widely over time, and populations of bats affected by white-nose syndrome will not recover quickly. Epizootic disease outbreaks have never been previously documented in hibernating bats.

U.S. Fish and Wildlife Service endangered species biologist Susi von Oettingen talks about white-nose syndrome in bats and investigates a hibernaculum in an abandoned mine and the area around it. Filmed in 2008, a year after WINS was indentified in North American bats.

White-nose syndrome (WNS) was named for the visible presence of a white fungus around the muzzles, ears, and wing membranes of affected bats. Based upon what is known about typical fungal pathogens of typical mammals, this fungal growth was initially thought to be a secondary infection of bats with compromised immune systems. However, bats are anything but "typical" mammals (see below). Since then, a previously unreported species of cold-loving fungus (Geomyces destructans) has been identified as a consistent pathogen among affected animals and sites. This fungus, now proven to be the causal agent of WNS (Lorch, et.al, 2011), thrives in the darkness, low temperatures (5-10ºC; 40-50ºF), and high levels of humidity (>90%) characteristic of bat hibernacula. Unlike typical fungi, Geomyces destructans cannot grow above 20°C (68ºF), and therefore appears to be exquisitely adapted to persist in caves and mines and to colonize the skin of hibernating bats. A consistent pattern of fungal skin penetration has been observed in more than 90 percent of bats from the WNS-affected region that were submitted for disease investigation.

Pathologic findings thus far indicate that bats affected by white-nose syndrome are infected by G. destructans, and many appear to prematurely run out of the stored body fat that they rely on for winter survival. Species of bats occurring at the higher latitudes of the world rely on insects for food, which disappear from those temperate zones during winter. Most species of temperate zone bats survive the winter by building up fat reserves during autumn and then going to cold places to hibernate and wait out the winter insect shortage. During hibernation, a bat slows down its metabolism so that its body temperature remains just a few degrees above air temperature. This strategy allows a bat to consume very little fat over winter. Bats could easily last several months in this deep state of torpor, but they need to warm their bodies up a few times each winter and arouse from hibernation so that they can drink, urinate, mate, relocate, and probably induce their immune systems to catch up. These natural arousals from hibernation consume a lot of energy, and about 90 percent of a hibernating bat's winter fat is burned to fuel natural arousals. If anything increases the frequency or duration of such arousals during winter, the energy balance of a hibernating bat can quickly tip toward starvation. Chronic disturbance of hibernating bats is known to cause abnormal arousal patterns, and can result in high rates of winter mortality. For example, certain inappropriate research methods (e.g., poorly applied wing bands and frequent winter visitations) directed toward hibernating bats in the 1950s and 1960s caused chronic disturbance that led to high mortality and population declines in several U.S. bat species (Ellison 2008). Unlike typical microbial pathogens that cause collapse of internal organ systems, theskin infection caused by G. destructans may act as a chronic disturbance during hibernation, and fungal-associated aberrant behaviors likely cause bats to consume critical body reserves too quickly during winter. In addition to disrupting hibernation cycles and prematurely expending energy reserves, it is likely that affected bats suffer other serious physiological problems (e.g., dehydration) associated with the fungus infecting the proportionally huge skin surfaces of their wings during hibernation. Eventually, most of the affected bats starve to death.

Since its arrival, the epidemic has continued to spread into the south and towards the Midwest. According to the U.S. Fish and Wildlife Service, biologists have confirmed cases in Kentucky, Alabama, and Missouri. In areas frequented by both people and bats, biologists have taken particular precautions to reduce the likelihood of humans aiding in the further spread. Some caves have been closed to the public; others have instituted protocols to remove spores from visitors shoes and gear after leaving the cave. However, there are no tools to keep the bats themselves from spreading the virus, says Reeder. Her lab has been examining differences between species, asking questions such as why is the little brown bat more susceptible than the big brown bat? Does the bat's size afford some level of protection?

In 2006, a few hundred bats were found dead in hibernating caves in the (northeastern) state of New York. The event barely registered for some scientists. By the following winter, the death toll had risen to a few thousand bats, sparking concern among some experts. Now a deadly fungus has claimed some 6.7 million bats. The dramatic reduction in the bat population and its potential extinction could have extensive health, economic and environmental effects. Producer Zulima Palacio has the story. Carol Pearson narrates.

The emergence and spread of a pathogenic fungus that infects hibernating bats has the potential to undermine the basic survival strategy of more than half the bat species in the U.S. and all species of bats that occur in the higher latitudes of North America. With the exception of four species of migratory tree bats, the other 18 bat species that occur above 40ºN in North America (roughly a line running from the top of California across Nebraska to Virginia) hibernate to survive the winter.

Since white-nose syndrome emerged during the winter of 2006-2007, a diverse group of scientists, resource managers, and conservation groups have worked diligently to establish its cause. Efforts are now being directed toward developing solutions to the WNS crisis and minimizing its impact on populations of hibernating bats in North America.

USGS scientists at the National Wildlife Health Center and Fort Collins Science Center are supporting the research needs of the U.S. Fish and Wildlife Service and other federal and state agencies as they respond to the developing situation.

Others have been conducting drug treatment trials. "Trouble is that a number of things kill this fungus in a petrie dish, but we don't know what they do in a hibernating animal," says Reeder, adding, "It's really hard to envision how you would treat in the field." She says that she has been working with Marcy Souza, assistant professor of veterinary medicine at the University of Tennessee, Knoxville, who developed implants to slowly release a drug similar to lamisil, a common treatment for finger and toenail fungus. Again, administration would be a challenge.

This coming year, Reeder says that her lab will focus on understanding the survivors in an effort to answer some key questions, such as: Did they hibernate in a different section of the cave that for some reason that was less hospitable for the fungus? Are they somehow physiologically equipped differently to resist the fungus? Is that trait heritable? Will natural selection be able to kick in before entire populations are wiped out?

Complicating this research is the lackadaisical attitude towards WNS in Europe. The sources of European bats’ immunity to WNS is simply not much on the radar of European bat biologists, whom Reeder laments as being “more hands off. Experiments that would be really useful to do probably won't happen."

Federal wildlife officials are currently weighing the possibility of attempting to maintain a captive population of bats as an insurance measure should wild populations be lost beyond recovery, says Reeder. However, she adds that most bats do not fare well in captivity.

Without any means to control the virus, the agricultural industry may need to assess how to respond to the loss of the pest control provided by bats. That free service equals over $3 billion a year, according to recent study on the potential agricultural impact of bat decline conducted by researchers at Boston University, The University of Tennessee, Knoxville, and the University of Pretoria, South Africa. Should farmers lose this free service, they will be faced with a sobering choice: accept increased crop loss due to an uptick in pests, or find some other means of controlling insects.

Sources: Seedstock, June 20, 2012

U.S. Geological Survey, (USGS)