Are Bats Developing Resistance to White-Nose Syndrome?
From the Winter 2021 issue of "Microcosm."
Although the origin of SARS-CoV-2 has been hotly debated and remains under scientific review, it is hard to argue against the notion that the COVID-19 pandemic has cast a shadow over the bat name. Bats are proven reservoirs of many different microbes that can cause severe disease in humans, including, but not limited to, Bartonella spp., Leptospira sp., Salmonella spp., Escherichia coli, Rabies virus, Ebola virus, Influenza virus and several coronavirus species. However, they are also key contributors to global and environmental health. Bats are principal pollinators and seed dispersers. Without them, plants like bananas, mangoes, cashews, avocadoes, peaches and cloves would cease to exist, leaving animals at the bottom of the food chain vulnerable to starvation and lack of cover, and entire ecosystems would eventually deteriorate. Furthermore, bats feed on insects and help with the population control of mosquitoes, beetles, moths and leafhoppers, thus limiting the spread of mosquito-borne diseases and lessening the need for chemical pesticides in agriculture.
Unfortunately, these winged mammals are currently in the throes of a pandemic of their own, one that has killed more than 6 million North American bats in a little less than 2 decades. White-nose syndrome (WNS) is an especially lethal fungal disease caused by the ascomycete Pseudogymnoascus destructans. Data collected from 200 sites across 27 U.S. states and two Canadian provinces from 1995-2018 indicate that 3 bat species have been particularly devastated by the disease. Ninety percent decreases in population size have been observed in the decade since WNS emerged for the northern long-eared bat, little brown bat and tri-colored bat. All 3 species were listed as endangered in Canada under the Species at Risk Act in 2014. But studies are now suggesting that one of these endangered species, the little brown bat, may be developing resistance to the deadly fungus, and as it turns out, temperature regulation and metabolism during hibernation may be both the cause of and the solution to the problem.
How Does White-nose Syndrome Kill Bats?
P. destructans is an invasive, psychrophilic (cold-loving) fungus that grows optimally at temperatures ranging from 12.5-15.8°C and cannot grow in temperatures over 20°C. Active bats maintain body temperatures of 37-41°C and are therefore protected from infection. But hibernating bats reduce body temperatures in the winter to control metabolic rates and conserve energy, and that's when P. destructans strikes. Bats and human spelunkers carry the fungus from cave to cave, and it can survive without any hosts present for 10+ years, but when the temperature of an unsuspecting bat drops to levels that make it a hospitable host, P. destructans invades tissues of the ears, muzzles and wing membranes. The characteristic white fuzz that develops on the bat's bare skin is what gives WNS its name.
Infection causes skin irritation, tissue damage and dehydration, as well as behavioral activity suggestive of fever response, and these trigger WNS-infected bats to wake from hibernation prematurely when food sources are scarce. Combatting infection and repeated waking demand energy and deplete the bat's limited fat reserves, causing many to exhibit unusual behavior such as flying outside in the middle of winter in search of food and water. With little to no food available, victims of the disease eventually starve or freeze to death.
What's Different About White-nose Syndrome Survivors?
Bats in Eurasia, where the pathogen is thought to have originated, do not appear to be as severely affected by P. destructans as North American bats. In fact, scientists have demonstrated that Asian bats carry lower fungal loads than bats in the United States. The reason for this is not fully understood, but coevolution and ecological factors are likely both at play. For example, in the winter, less fungus has been found growing in Eurasian caves compared to North American ones. As a result, scientists have hypothesized that local bats are becoming infected less often, or later in the season, and becoming less sick from infection.
Now, a growing body of research indicates that North American little brown bat populations may be rebounding as well. In fact, population increases between 5-30% from previous pandemic lows have been observed at certain hibernation sites in New York. In an effort to determine whether survivors are developing resistance, scientists collected wing punch samples from survivors and victims of a WNS-induced mass mortality event in two little brown bat populations that are showing signs of recovery. Whole genome sequencing from 132 bats identified 63 unique single nucleotide polymorphisms (SNPs) that are more common in WNS survivors than in bats who died with the fungus. Scientists mapped these SNPs to reference genomes of bats and other animals, such as ground squirrels and mice, and identified one loci in a gene associated with the immune system. The remaining SNPs were located within genes linked to host response to WNS and changes in metabolism during hibernation. The results are supported by a 2019 study conducted in Michigan that concluded that survivors of WNS are better at fat storage than those that succumb to the disease. However, a 2020 study that pooled genetic information, as opposed to sequencing individual bat genomes, reported no population-wide signs of selection, leading some to hypothesize that different bat populations may be adapting differently.
What Can Be Done to Mitigate Further Ecological Losses of Bat Populations?
While bats do the heavy lifting, their human counterparts are continuing to investigate ways to help. Conservation strategies aim to slow human transport of P. destructans by closing hibernation caves to the public. Additional mitigation strategies include vaccine development, fungicide treatment, habitat improvement and food-supply increases prior to hibernation. But all of these methods are expensive, and none of them are particularly effective. Further research is needed to understand how the genotypic, phenotypic and behavioral changes of WNS survivors are related, but recent findings provide hope that little brown bats will have a future.