Its January, and an aluminium skiff glides slowly across the waters of an old, flooded railroad tunnel in northern Georgia. Its lone passenger is an atomiser, a machine commonly used to disperse air fresheners or mosquito repellents. Over the course of its half-hour journey through the 430-metre tunnel, the atomiser sprays an aerosolised cocktail of antifungal compounds that drift up to the craggy cave ceiling. The researchers who let loose these molecules are hoping the cocktail will reach and kill a deadly pathogen growing on the bats hibernating here. The research team, co-led by microbiologist Chris Cornelison of Kennesaw State University, is testing whether the compounds can slow the spread of white-nose syndrome (WNS), a fungal disease that has devastated bat populations in North America and shows no signs of slowing down. Such chemical treatments are an ever-present thread in the tapestry of the wildlife management and academic research response to WNS. But the use of any compound requires a careful weighing of its potential benefits against the probable drawbacks. Finding a novel way to strike down the fungal pathogen is only the first step. Ensuring that the method will not also harm the bats or disrupt delicate cave ecosystems greatly increases the complexity of the challenge that WNS presents. Yet in the face of this daunting task, scientists continue to search for solutions that will slow the spread of this deadly disease and give bats a fighting chance. The fungus that causes white-nose syndrome, Pseudogymnoascus destructans, forms mouldy lesions as it colonises bats skin, including on their ears, wingsand noses. The cold-loving fungus thrives only in winter, when bats hibernatea necessity for most of North Americas 47 bat speciesand their body temperatures drop. During this time, bats cluster together in hibernation sites like caves and old mines, allowing P. destructans to rapidly spread its infective spores through the population. As the pathogen burrows into bats skin, primarily that of their wings, the animals begin to arouse more often from their winter torpor. They also behave oddly, moving closer to cave or mine entrances and flying about in daylight, according to David Blehert of the U.S. Geological Surveys National Wildlife Health Centre. The animals use up precious energy stores, already depleted from their time in hibernation, and often starve or freeze to death before spring arrives, he says. Despite the best efforts of wildlife managers and researchers, WNS has swept through North American bat populations. Since winter 200607, when it first appeared in caves near Albany, NY, WNS has spread into 31 U.S. states and five Canadian provinces. Two years ago, P. destructans was detected for the first time in Washington state, a westward leap of more than 2,000 km from its previous frontier in the Great Plains. Today, caves in the northeastern U.S. that once hosted bat populations numbering in the hundreds of thousands are empty, or else house only a few hundred survivors. U.S. Fish & Wildlife Service scientists estimate that roughly 6 million bats died from WNS within the first five to six years of the outbreak. This decimation can have broad ecological consequences. Not only do the tiny, winged mammals play an important role in their own niche, they consume massive quantities of insects that eat crops and carry crop diseases. One study estimated that bat deaths from WNS could cost the agriculture industry between $4 billion and $53 billion per year (Science 2011, DOI: 10.1126/science.1201366). Over the past decade, researchers have managed to answer many questions about WNS. Where is it from? P. destructans is likely from Europe, where bats are somehow immune to the fungus, though how it made the Atlantic crossing is still unclear (mBio 2017, DOI: 10.1128/mBio.01941-17). How does it spread? Primarily through bat-to-bat contact, though spores may hitch a ride on humans shoes, clothing, or gear (J. Wildl. Dis. 2017, DOI: 10.7589/2016-09-206). Why does it kill bats? The fungus somehow elevates carbon dioxide concentrations in bats blood, increases their metabolism, and depletes their fat stores. The animals must eventually rouse from hibernation more frequently to exhale excess CO2 (BMC Physiol. 2014, DOI: 10.1186/s12899-014-0010-4). But one big question remains: How can we slow the spread of WNS and mitigate its devastating impacts on bats?
Challenges of a chemical approach
Initial attempts to slow the spread of the disease included spraying potent fungicides in bats hibernation sites and culling infected bat populations. But those efforts quickly demonstrated that the pathogen readily defied containment. The sheer number of bats hibernating in close quarters and migrating each summer gave the fungus an easy route to travel. And although fungicides can easily kill the delicate P. destructans, bats, too, are fragile creatures. Harsh chemicals or repeated handling during hibernation can stress bats at a vulnerable time. An optimal scenario, for example, would be to waft treatment-containing vapor over hibernating bats without physically disturbing them. However, researchers found out the hard way that bats abandon hibernation sites to escape any stimulus they find frightening or unpleasant.
Ecologists first detected white-nose syndrome on bats in New York state in 2006. Since then, the disease has spread throughout the eastern half of North America. Data are preliminary or provisional and last updated October 2017. Hazel Barton, a microbiologist at the University of Akron, had success inhibiting P. destructans growth in the lab using carvone, a terpenoid from spearmint oil extract. But when she moved the treatment to the field, her team found that bats hated it. They will do anything in their power to get away from spearmint oil, she says. To escape, the bats wriggled through tiny holes in the netted enclosure the researchers had arranged in the cave. Loud or clunky equipment will also disturb hibernating bats. Scientists doing fieldwork keep their headlamps pointed downward as often as possible and speak in whispers, but it is still enough to rouse the animals. Researchers must also consider the delicacy of the hibernation sites themselves when developing new treatments for WNS. Cave ecosystems are easily disturbed and can house other at-risk species. The presence of endangered arthropods in many U.S. caves means that any new treatment compounds, which already face regulations imposed by at least one of three federal agencies, must also meet U.S. Fish & Wildlife Service requirements to avoid harming the tiny creatures.
Given all the challenges of developing treatments for WNS, researchers have started putting new twists on old ideas. Rather than developing novel, naturally sourced antifungals from scratch, Kennesaw States Cornelison is testing Flavorzon 185B, which has already been approved by the U.S. Food & Drug Administration. It is a mix of 23 compounds based on a naturally produced suite of antimicrobial volatile organic compounds, with propanoic acid as the most abundant ingredient (Microbiology 2010, DOI: 10.1099/mic.0.032540-0). Nicknamed B23, the mixture has already been approved for use in horse bedding, so it was relatively straightforward to gain approval to test its efficacy in inhibiting P. destructans, Cornelison says. It checked off a lot of the boxes that we already needed. For the past two hibernation seasons, Cornelisons team has been dispensing B23 with the quietest, most efficient atomiser they could find. Three times each season, the researchers delivered a dose of B23 to the WNS-infected tunnel through a mist of fine droplets. They collaborate with U.S. Fish & Wildlife Service and the Georgia Department of Natural Resources to survey the caves bat population during the treatment. Other researchers are turning to the bats themselves for clues on fighting off the fungal disease. Several teams have investigated bats skin microbiomes to uncover whether any bacterial species can stymie P. destructans. After culturing bacteria from bat skin, a research group at the University of California, Santa Cruz, reported in 2015 that bacteria, primarily in the genus Pseudomonas, can inhibit P. destructans growth (PLOS One2015, DOI: 10.1371/journal.pone.0121329). Unfortunately, even though these bacteria are successful on a petri dish, they do not have a monopoly on their hosts wings, and any antifungal compounds they produce might not be present in high enough concentrations to be protective. Scientists are probing deeper in this direction by studying the skin microbiota of bats that have survived WNS, hoping to find other bacteria that may play a role in resilience to the disease. A team headed by researchers from the University of Montreal compared the skin microbiomes of noninfected little brown bats with those of WNS-surviving bats and found that the post-WNS infection microbiomes were enriched with types of bacteria that have some antifungal activity (Microbiome 2017, DOI: 10.1186/s40168-017-0334-y). The authors noted that if these bacteria play a protective role, encouraging the growth or transmission of those types of microbes might help bolster bat populations. To that end, some researchers in Canada began looking for a way to boost beneficial bat bacteria. Instead of developing a new treatment for WNS, however, Naowarat Cheeptham and her colleagues opted to search for a preventative measure. Cheeptham, a microbiologist at Thompson Rivers University, is based in British Columbia, which is preparing for the impending spread of WNS from Washington state. She and her collaborators are hoping to develop a probiotic powder to boost the presence of beneficial bacteria on bat populations that havent yet encountered WNS. By fostering a skin microbiome less hospitable to P. destructans in the summer months, such an application could tip the scales in the bats favour. The team is now determining which bacterial species would be most effective in such a probiotic and working out the best way to formulate and deliver the powder, Cheeptham says. There are a lot of challenges ahead of us, she says. We hope this will work, but if we dont do it, well never know. Though these new findings sound promising, researchers still dont have direct evidence of whether or how bacteria actually block white-nose infection in caves. Many strains are known to produce antifungal volatile organic compounds, such as decanal, 2-ethyl-1-hexanol, nonanal, benzothiazole, benzaldehyde, and N,N-dimethyloctylamine (Mycopathologia 2013, DOI: 10.1007/s11046-013-9716-2), that could be candidate chemical treatments. However, the time it takes to isolate individual compounds, determine their efficacy, ensure their safety for use in caves, and develop a bat-friendly delivery method continues to hamper this approach to combating the disease.
Shining a (UV) light on the problem
In addition to biological or chemical methods of controlling WNS, recent evidence indicates that ultraviolet light may also hold promise. In January, researchers led by Jonathan Palmer and Daniel Lindner of the U.S. Forest Service reported that P. destructans lacks a crucial enzyme needed to repair DNA damage caused by UV light (Nat. Commun. 2018, DOI: 10.1038/s41467-017-02441-z). Researchers at Bucknell University, led by biologist Ken Field, are now testing out how UV-C light (100280 nm), which kills the fungus very effectively, affects the course of WNS infections. His team brings naturally infected hibernating bats to the lab and holds their wings open under a UV lamp for varying lengths of time. The bats then complete their hibernation in an artificial, WNS-free chamber on campus. At the end of the experiment, Field says the group will try to determine how long the UV exposure needs to be to stunt fungal growth on the bats. The team will also determine whether the bats are able to repair any UV-induced DNA damage to their skin or eyes while hibernating. Even if P. destructans can be undone with a few seconds of UV-C exposure, deploying the treatment in the field wont be simple. Questions of how to power UV lamps, where to place them within hibernation sites, and whether bats would even tolerate them jumble together with concerns about how the light will affect other cave-dwelling organisms. This propagation of uncertainty is the same issue that researchers have been dealing with since the outbreak began: Discoveries beget new questions as the disease marches on.
Working on multiple fronts
As spring begins, bats will stir, rousing from their torpor. Wildlife biologists will soon know what toll WNS has taken this winter. Jeremy Coleman, the national coordinator for U.S. Fish & Wildlife Services WNS response program, has had a bats-eye view of the North American efforts ever since the outbreak began. Our approach all along has been do everythingbasic research on the fungus, pathology studies in the bats, and epidemiology on the disease spread with the idea that we might find some temporary fixes and maybe some long-term solutions, he says. Trying new approaches to study this fungal pathogen is critical, he says, not only to protect increasingly decimated bat populations, but also to build a knowledge bank in the event of other devastating fungal pathogens that may affect other animals or plants. Fungi are really difficult critters to manage and to kill, Coleman says. We have an opportunity here to work on this one, but there could also be a greater utility for tools that we develop for the next disease or the next issue or the next species thats being affected.
Chemical & Engineering News, 6 April 2018 ; http://pubs.acs.org/cen/news