White-Nose Syndrome Threatens the Survival of Hibernating Bats in North America
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During the winter of 2006–2007, an affliction of unknown origin dubbed “white-nose syndrome” (WNS) began devastating colonies of hibernating bats in a small area around Albany, New York. 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 consistently spread more than 2,000 kilometers (1,200 mi) away from the original site, and have infected bats in at least half or more of states and provinces in the U.S. and Canada. Most species of bats that hibernate in eastern North America 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.
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.
History of White-nose Syndrome and Diseases in Hibernating Bats cont.
Following this decade of initial and contiguous WNS spread, a non-migratory bat with WNS was detected in the Pacific Northwest during spring of 2016, more than 2000 kilometers beyond all previously known occurrences. The sudden spread and widespread mortality associated with WNS are 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 lasting multiple years have never been previously documented in bats.
An Emerging Disease of Hibernating Bats
White-nose syndrome 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 Pseudogymnoascus [formerly 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, thrives in the darkness, low temperatures (5–10ºC; 40–50ºF), and high levels of humidity (>90%) characteristic of bat hibernacula. Unlike typical fungi, P. destructans does not 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.
White-nose syndrome was first documented in a cave that is visited by tens of thousands of tourists each year, and the disease has since spread outward from that site. The focal area of origin and subsequent distribution of affected sites, as well as experimental evidence, indicate that P. destructans could be an exotic species with the capacity to spread rapidly among populations of hibernating bats. Researchers in Europe have long noticed similar fungal growth on the faces, ears, and wings of hibernating bats in Europe, but observed no associated mass mortality. Work is currently underway to assess whether there is any connection between fungi seen on bats in North America and Europe. Recent efforts by researchers reveal that P. destructans occurs on hibernating bats in several countries of Europe, as well as Asia.
What Makes Bats Susceptible to White-Nose Syndrome?
The WNS fungus harms bats during a period of their lifecycle when they are extremely vulnerable—hibernation. 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 most 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. Unlike typical microbial pathogens that cause collapse of internal organ systems, the skin infection caused by P. destructans may act as a chronic disturbance during hibernation, and fungal-associated aberrant behaviors might 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 this winter fungal infection. The WNS fungus establishes itself in the skin tissues of bats when their body temperatures are lowered during hibernation (2–10ºC; 35–50ºF). Although life-threatening cutaneous fungal infections of this sort are rare in warm-blooded birds and mammals, they occur more frequently in “cold-blooded” animals (e.g., chytridiomycosis in amphibians, and crayfish plague). The cold-loving fungus seems to be infecting bats when they reduce their body temperatures during hibernation to levels characteristic of “cold-blooded” animals. Fungal infiltration of the wing membranes of bats may be particularly problematic.
Wing membranes represent about 85 percent of a bat’s total surface area and play a critical role in balancing complex physiological processes. Healthy wing membranes are vital to bats, as they help regulate body temperature, blood pressure, water balance, and gas exchange—not to mention the ability to fly and to feed. Although white-nose syndrome was named after the obvious sign of white noses on affected bats, bat wings may indeed be the most vulnerable point of infection. Because the newly identified fungus represents a potential biological invasion or emerging disease, with severe implications for hibernating bats in North America, it is important to focus on the history of its geographic distribution in the context of the distributions of affected species and those of federal concern that are potentially in harm’s way.
Threat Posed by White-Nose Syndrome to Bat Diversity and Abundance in North Ameica
Forty-five species of bats occur in the United States and Canada, and bats represent more than 10 percent of mammalian species diversity in the region. The map (above) shows bat species richness in the continental United States and Canada. Warmer colors represent higher species richness, cooler colors represent fewer species.
More than half of the species of insectivorous bats that occur in the U.S. rely on hibernation as a primary strategy for surviving the winter when insect prey is not available. The tables (right) list all 45 species of bats known to occur in the continental United States and Canada. Species in the upper list are those that in most areas rely on hibernation to survive the winter. Species in the lower list are those that generally do not rely on hibernation as a winter survival strategy, although 8 of those species (# 6-13) may also hibernate after migrating long distances. Those highlighted in bold text are species currently known to be affected by white-nose syndrome.
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 4 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.
The shaded red areas on the map below represent the overlapping distributions of 19 species of bats occurring in the U.S. that do not rely on hibernation as a primary strategy for surviving the winter. The blue shaded areas of the map below represent the overlapping distributions of 25 species of bats occurring in the U.S. that rely on hibernation to survive the winter, plus the distributions of several species of migratory tree bats that may also hibernate.
Among the 25+ species of bats that hibernate across North America, 4 species and subspecies are federally listed as endangered and an additional 13 are federal species of concern (former Category 2 candidates for listing under the U.S. Endangered Species Act). All four endangered species and subspecies of hibernating bats in the U.S., which rely on undisturbed caves or mines for successful hibernation, are at risk from white-nose syndrome. A fifth species, the northern long-eared bat (Myotis septentrionalis) was recently listed as threatened under the U.S. Endangered Species Act due to the adverse effects of WNS on its population. All listed hibernating bat species are currently within the affected area, and the remaining subspecies may be affected in the next few years, if not sooner.
The maps included on the ScienceBase page show the distribution of endangered species of hibernating bats (shaded areas) in relation to the expanding distribution of the fungus (Pseudogymnoascus destructans) and the disease white-nose syndrome (yellow counties). Endangered species include Ozark big-eared bats (Corynorhinus townsendii ingens), Virginia big-eared bats (C. t. virginianus), Indiana bats (M. sodalis), and gray bats (M. grisescens). For an updated map of sites affected by WNS see https://www.whitenosesyndrome.org/. Although the true potential for this fungus to spread is unknown, the possibility of it undermining the ubiquitous survival strategy of bats at higher latitudes has enormous implications. We are just beginning to appreciate the roles that bats play in North American ecosystems, and it is clear that threats like white-nose syndrome have the potential to influence ecosystem function in ways that we currently do not understand. 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 are conducting primary research and supporting the research and technical assistance needs of the U.S. Fish and Wildlife Service and other federal and state agencies as they respond to the developing situation. Collaborative efforts between USGS scientists and their research partners now aim to develop and use new technologies for studying bats and disease as WNS ‘goes underground’ in western North America. A particularly promising line of new USGS research involves trying to determine if and how bats naturally survive WNS. Earlier USGS studies indicate that certain habitats and behaviors may help bats survive WNS. Identifying those natural survival strategies will help enhance survival and prioritize limited resources toward benefitting populations of bats most likely to make it through this unprecedented disease crisis.
Maps created by Paul Cryan, USGS.