Landscape structure and ecology influence the spread of a bat fungal disease

Cooling of bat hibernacula to mitigate white-nose syndrome

White-nose syndrome (WNS) is a fungal disease that has caused precipitous declines in several North American bat species, creating an urgent need for conservation. We examined how microclimates and other characteristics of hibernacula have affected bat populations following WNS-associated declines and evaluated whether cooling of warm, little-used hibernacula could benefit bats. During the period following mass mortality (2013-2020), we conducted 191 winter surveys of 25 unmanipulated hibernacula and 6 manipulated hibernacula across Pennsylvania (USA). We joined these data with additional datasets on historical (pre-WNS) bat counts and on the spatial distribution of underground sites. We used generalized linear mixed models and model selection to identify factors affecting bat populations. Winter counts of Myotis lucifugus were higher and increased over time in colder hibernacula (those with midwinter temperatures of 3-6 °C) compared with warmer (7-11 °C) hibernacula. Counts of Eptesicus fuscus, Myotis leibii, and Myotis septentrionalis were likewise higher in colder hibernacula (temperature effects = -0.73 [SE 0.15], -0.51 [0.18], and -0.97 [0.28], respectively). Populations of M. lucifugus and M. septentrionalis increased most over time in hibernacula surrounded by more nearby sites, whereas Eptesicus fuscus counts remained high where they had been high before WNS onset (pre-WNS high count effect = 0.59 [0.22]). Winter counts of M. leibii were higher in hibernacula with high vapor pressure deficits (VPDs) (particularly over 0.1 kPa) compared with sites with lower VPDs (VPD effect = 15.3 [4.6]). Counts of M. lucifugus and E. fuscus also appeared higher where VPD was higher. In contrast, Perimyotis subflavus counts increased over time in relatively warm hibernacula and were unaffected by VPD. Where we manipulated hibernacula, we achieved cooling of on average 2.1 °C. At manipulated hibernacula, counts of M. lucifugus and P. subflavus increased over time (years since manipulation effect = 0.70 [0.28] and 0.51 [0.15], respectively). Further, there were more E. fuscus where cooling was greatest (temperature difference effect = -0.46 [SE 0.11]), and there was some evidence there were more P. subflavus in hibernacula sections that remained warm after manipulation. These data show bats are responding effectively to WNS through habitat selection. In M. lucifugus, M. septentrionalis, and possibly P. subflavus, this response is ongoing, with bats increasingly aggregating at suitable hibernacula, whereas E. fuscus remain in previously favored sites. Our results suggest that cooling warm sites receiving little use by bats is a viable strategy for combating WNS.

Host traits and environment interact to determine persistence of bat populations impacted by white-nose syndrome

Emerging infectious diseases have resulted in severe population declines across diverse taxa. In some instances, despite attributes associated with high extinction risk, disease emergence and host declines are followed by host stabilisation for unknown reasons. While host, pathogen, and the environment are recognised as important factors that interact to determine host-pathogen coexistence, they are often considered independently. Here, we use a translocation experiment to disentangle the role of host traits and environmental conditions in driving the persistence of remnant bat populations a decade after they declined 70-99% due to white-nose syndrome and subsequently stabilised. While survival was significantly higher than during the initial epidemic within all sites, protection from severe disease only existed within a narrow environmental space, suggesting host traits conducive to surviving disease are highly environmentally dependent. Ultimately, population persistence following pathogen invasion is the product of host-pathogen interactions that vary across a patchwork of environments.

Population genetics as a tool to elucidate pathogen reservoirs: Lessons from Pseudogymnoascus destructans, the causative agent of White-Nose disease in bats

Emerging infectious diseases pose a major threat to human, animal, and plant health. The risk of species-extinctions increases when pathogens can survive in the absence of the host. Environmental reservoirs can facilitate this. However, identifying such reservoirs and modes of infection is often highly challenging. In this study, we investigated the presence and nature of an environmental reservoir for the ascomycete fungus Pseudogymnoascus destructans, the causative agent of White-Nose disease. Using 18 microsatellite markers, we determined the genotypic differentiation between 1497 P. destructans isolates collected from nine closely situated underground sites where bats hibernate (i.e., hibernacula) in Northeastern Germany. This approach was unique in that it ensured that every isolate and resulting multilocus genotype was not only present, but also viable and therefore theoretically capable of infecting a bat. The distinct distribution of multilocus genotypes across hibernacula demonstrates that each hibernaculum has an essentially unique fungal population. This would be expected if bats become infected in their hibernaculum (i.e., the site they spend winter in to hibernate) rather than in other sites visited before they start hibernating. In one hibernaculum, both the walls and the hibernating bats were sampled at regular intervals over five consecutive winter seasons (1062 isolates), revealing higher genotypic richness on walls compared to bats and a stable frequency of multilocus genotypes over multiple winters. This clearly implicates hibernacula walls as the main environmental reservoir of the pathogen, from which bats become reinfected annually during the autumn.

Disease or drought: environmental fluctuations release zebra from a potential pathogen-triggered ecological trap

When a transmission hotspot for an environmentally persistent pathogen establishes in otherwise high-quality habitat, the disease may exert a strong impact on a host population. However, fluctuating environmental conditions lead to heterogeneity in habitat quality and animal habitat preference, which may interrupt the overlap between selected and risky habitats. We evaluated spatio-temporal patterns in anthrax mortalities in a plains zebra (Equus quagga) population in Etosha National Park, Namibia, incorporating remote-sensing and host telemetry data. A higher proportion of anthrax mortalities of herbivores was detected in open habitats than in other habitat types. Resource selection functions showed that the zebra population shifted habitat selection in response to changes in rainfall and vegetation productivity. Average to high rainfall years supported larger anthrax outbreaks, with animals congregating in preferred open habitats, while a severe drought forced animals into otherwise less preferred habitats, leading to few anthrax mortalities. Thus, the timing of anthrax outbreaks was congruent with preference for open plains habitats and a corresponding increase in pathogen exposure. Given shifts in habitat preference, the overlap in high-quality habitat and high-risk habitat is intermittent, reducing the adverse consequences for the population.

Heterothermy and antifungal responses in bats

Hibernation, a period where bats have suppressed immunity and low body temperatures, provides the psychrophilic fungus Pseudogymnoascus destructans the opportunity to colonise bat skin, leading to severe disease in susceptible species. Innate immunity, which requires less energy and may remain more active during torpor, can control infections with local inflammation in some bat species that are resistant to infection. If infection is not controlled before emergence from hibernation, ineffective adaptive immune mechanisms are activated, including incomplete Th1, ineffective Th2, and variable Th17 responses. The Th17 and neutrophil responses, normally beneficial antifungal mechanisms, appear to be sources of immunopathology for susceptible bat species, because they are hyperactivated after return to homeothermy. Non-susceptible species show both well-balanced and suppressed immune responses both during and after hibernation.

Ten-year projection of white-nose syndrome disease dynamics at the southern leading-edge of infection in North America

Predicting the emergence and spread of infectious diseases is critical for the effective conservation of biodiversity. White-nose syndrome (WNS), an emerging infectious disease of bats, has resulted in high mortality in eastern North America. Because the fungal causative agent Pseudogymnoascus destructans is constrained by temperature and humidity, spread dynamics may vary by geography. Environmental conditions in the southern part of the continent are different than the northeast, where disease dynamics are typically studied, making it difficult to predict how the disease will manifest. Herein, we modelled WNS pathogen spread in Texas based on cave densities and average dispersal distances of hosts, projecting these results out to 10 years. We parameterized a predictive model of WNS epidemiology and its effects on bat populations with observed cave environmental data. Our model suggests that bat populations in northern Texas will be more affected by WNS mortality than southern Texas. As such, we recommend prioritizing the preservation of large overwintering colonies of bats in north Texas through management actions. Our model illustrates that infectious disease spread and infectious disease severity can become uncoupled over a gradient of environmental variation and highlight the importance of understanding host, pathogen and environmental conditions across a breadth of environments.

Landscape Genetic Connectivity and Evidence for Recombination in the North American Population of the White-Nose Syndrome Pathogen, Pseudogymnoascus destructans

White-Nose Syndrome is an ongoing fungal epizootic caused by epidermal infections of the fungus, Pseudogymnoascus destructans (P. destructans), affecting hibernating bat species in North America. Emerging early in 2006 in New York State, infections of P. destructans have spread to 38 US States and seven Canadian Provinces. Since then, clonal isolates of P. destructans have accumulated genotypic and phenotypic variations in North America. Using microsatellite and single nucleotide polymorphism markers, we investigated the population structure and genetic relationships among P. destructans isolates from diverse regions in North America to understand its pattern of spread, and to test hypotheses about factors that contribute to transmission. We found limited support for genetic isolation of P. destructans populations by geographic distance, and instead identified evidence for gene flow among geographic regions. Interestingly, allelic association tests revealed evidence for recombination in the North American P. destructans population. Our landscape genetic analyses revealed that the population structure of P. destructans in North America was significantly influenced by anthropogenic impacts on the landscape. Our results have important implications for understanding the mechanism(s) of P. destructans spread.

Ecology and impacts of white-nose syndrome on bats

The recent introduction of Pseudogymnoascus destructans (the fungal pathogen that causes white-nose syndrome in bats) from Eurasia to North America has resulted in the collapse of North American bat populations and restructured species communities. The long evolutionary history between P. destructans and bats in Eurasia makes understanding host life history essential to uncovering the ecology of P. destructans. In this Review, we combine information on pathogen and host biology to understand the patterns of P. destructans spread, seasonal transmission ecology, the pathogenesis of white-nose syndrome and the cross-scale impact from individual hosts to ecosystems. Collectively, this research highlights how early pathogen detection and quantification of host impacts has accelerated the understanding of this newly emerging infectious disease.

Mobility and infectiousness in the spatial spread of an emerging fungal pathogen

Emerging infectious diseases can have devastating effects on host communities, causing population collapse and species extinctions. The timing of novel pathogen arrival into naïve species communities can have consequential effects that shape the trajectory of epidemics through populations. Pathogen introductions are often presumed to occur when hosts are highly mobile. However, spread patterns can be influenced by a multitude of other factors including host body condition and infectiousness. White-nose syndrome (WNS) is a seasonal emerging infectious disease of bats, which is caused by the fungal pathogen Pseudogymnoascus destructans. Within-site transmission of P. destructans primarily occurs over winter; however, the influence of bat mobility and infectiousness on the seasonal timing of pathogen spread to new populations is unknown. We combined data on host population dynamics and pathogen transmission from 22 bat communities to investigate the timing of pathogen arrival and the consequences of varying pathogen arrival times on disease impacts. We found that midwinter arrival of the fungus predominated spread patterns, suggesting that bats were most likely to spread P. destructans when they are highly infectious, but have reduced mobility. In communities where P. destructans was detected in early winter, one species suffered higher fungal burdens and experienced more severe declines than at sites where the pathogen was detected later in the winter, suggesting that the timing of pathogen introduction had consequential effects for some bat communities. We also found evidence of source-sink population dynamics over winter, suggesting some movement among sites occurs during hibernation, even though bats at northern latitudes were thought to be fairly immobile during this period. Winter emergence behaviour symptomatic of white-nose syndrome may further exacerbate these winter bat movements to uninfected areas. Our results suggest that low infectiousness during host migration may have reduced the rate of expansion of this deadly pathogen, and that elevated infectiousness during winter plays a key role in seasonal transmission. Furthermore, our results highlight the importance of both accurate estimation of the timing of pathogen spread and the consequences of varying arrival times to prevent and mitigate the effects of infectious diseases.

Continued preference for suboptimal habitat reduces bat survival with white-nose syndrome

Habitat alteration can influence suitability, creating ecological traps where habitat preference and fitness are mismatched. Despite their importance, ecological traps are notoriously difficult to identify and their impact on host–pathogen dynamics remains largely unexplored. Here we assess individual bat survival and habitat preferences in the midwestern United States before, during, and after the invasion of the fungal pathogen that causes white-nose syndrome. Despite strong selection pressures, most hosts continued to select habitats where disease severity was highest and survival was lowest, causing continued population declines. However, some individuals used refugia where survival was higher. Over time, a higher proportion of the total population used refugia than before pathogen arrival. Our results demonstrate that host preferences for habitats with high disease-induced mortality can create ecological traps that threaten populations, even in the presence of accessible refugia.

Temperature-dependent host–pathogen interactions may lead species to shift their thermal preferences under pathogen pressure. However, here the authors show that bats have not altered their microclimate preferences due to temperature-mediated mortality from white-nose syndrome, finding instead a sustained preference for warmer sites with high mortality.

The role of social structure and dynamics in the maintenance of endemic disease

Social interactions are required for the direct transmission of infectious diseases. Consequently, the social network structure of populations plays a key role in shaping infectious disease dynamics. A huge research effort has examined how specific social network structures make populations more (or less) vulnerable to damaging epidemics. However, it can be just as important to understand how social networks can contribute to endemic disease dynamics, in which pathogens are maintained at stable levels for prolonged periods of time. Hosts that can maintain endemic disease may serve as keystone hosts for multi-host pathogens within an ecological community, and also have greater potential to act as key wildlife reservoirs of agricultural and zoonotic diseases. Here, we examine combinations of social and demographic processes that can foster endemic disease in hosts. We synthesise theoretical and empirical work to demonstrate the importance of both social structure and social dynamics in maintaining endemic disease. We also highlight the importance of distinguishing between the local and global persistence of infection and reveal how different social processes drive variation in the scale at which infectious diseases appear endemic. Our synthesis provides a framework by which to understand how sociality contributes to the long-term maintenance of infectious disease in wildlife hosts and provides a set of tools to unpick the social and demographic mechanisms involved in any given host-pathogen system.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00265-021-03055-8.

Population Connectivity Predicts Vulnerability to White-Nose Syndrome in the Chilean Myotis (Myotis chiloensis) - A Genomics Approach

Despite its peculiar distribution, the biology of the southernmost bat species in the world, the Chilean myotis (Myotis chiloensis), has garnered little attention so far. The species has a north-south distribution of c. 2800 km, mostly on the eastern side of the Andes mountain range. Use of extended torpor occurs in the southernmost portion of the range, putting the species at risk of bat white-nose syndrome, a fungal disease responsible for massive population declines in North American bats. Here, we examined how geographic distance and topology would be reflected in the population structure of M. chiloensis along the majority of its range using a double digestion RAD-seq method. We sampled 66 individuals across the species range and discovered pronounced isolation-by-distance. Furthermore, and surprisingly, we found higher degrees of heterozygosity in the southernmost populations compared to the north. A coalescence analysis revealed that our populations may still not have reached secondary contact after the Last Glacial Maximum. As for the potential spread of pathogens, such as the fungus causing WNS, connectivity among populations was noticeably low, especially between the southern hibernatory populations in the Magallanes and Tierra del Fuego, and more northerly populations. This suggests the probability of geographic spread of the disease from the north through bat-to-bat contact to susceptible populations is low. The study presents a rare case of defined population structure in a bat species and warrants further research on the underlying factors contributing to this. See the graphical abstract here. https://doi.org/10.25387/g3.12173385

Genome-Wide Changes in Genetic Diversity in a Population of Myotis lucifugus Affected by White-Nose Syndrome

Novel pathogens can cause massive declines in populations, and even extirpation of hosts. But disease can also act as a selective pressure on survivors, driving the evolution of resistance or tolerance. Bat white-nose syndrome (WNS) is a rapidly spreading wildlife disease in North America. The fungus causing the disease invades skin tissues of hibernating bats, resulting in disruption of hibernation behavior, premature energy depletion, and subsequent death. We used whole-genome sequencing to investigate changes in allele frequencies within a population of Myotis lucifugus in eastern North America to search for genetic resistance to WNS. Our results show low F_ST values within the population across time, i.e., prior to WNS (Pre-WNS) compared to the population that has survived WNS (Post-WNS). However, when dividing the population with a geographical cut-off between the states of Pennsylvania and New York, a sharp increase in values on scaffold GL429776 is evident in the Post-WNS samples. Genes present in the diverged area are associated with thermoregulation and promotion of brown fat production. Thus, although WNS may not have subjected the entire M. lucifugus population to selective pressure, it may have selected for specific alleles in Pennsylvania through decreased gene flow within the population. However, the persistence of remnant sub-populations in the aftermath of WNS is likely due to multiple factors in bat life history.

Resistance is futile: RNA-sequencing reveals differing responses to bat fungal pathogen in Nearctic Myotis lucifugus and Palearctic Myotis myotis

Abstract

Resistance and tolerance allow organisms to cope with potentially life-threatening pathogens. Recently introduced pathogens initially induce resistance responses, but natural selection favors the development of tolerance, allowing for a commensal relationship to evolve. Mycosis by Pseudogymnoascus destructans, causing white-nose syndrome (WNS) in Nearctic hibernating bats, has resulted in population declines since 2006. The pathogen, which spread from Europe, has infected species of Palearctic Myotis for a longer period. We compared ecologically relevant responses to the fungal infection in the susceptible Nearctic M. lucifugus and less susceptible Palearctic M. myotis, to uncover factors contributing to survival differences in the two species. Samples were collected from euthermic bats during arousal from hibernation, a naturally occurring phenomenon, during which transcriptional responses are activated. We compared the whole-transcriptome responses in wild bats infected with P. destructans hibernating in their natural habitat. Our results show dramatically different local transcriptional responses to the pathogen between uninfected and infected samples from the two species. Whereas we found 1526 significantly upregulated or downregulated transcripts in infected M. lucifugus, only one transcript was downregulated in M. myotis. The upregulated response pathways in M. lucifugus include immune cell activation and migration, and inflammatory pathways, indicative of an unsuccessful attempt to resist the infection. In contrast, M. myotis appears to tolerate P. destructans infection by not activating a transcriptional response. These host-microbe interactions determine pathology, contributing to WNS susceptibility, or commensalism, promoting tolerance to fungal colonization during hibernation that favors survival.

Graphic abstract

Electronic supplementary material

The online version of this article (10.1007/s00442-019-04499-6) contains supplementary material, which is available to authorized users.

Winter habitats of bats in Texas

Few studies have described winter microclimate selection by bats in the southern United States. This is of particular importance as the cold-adapted fungus, Pseudogymnoascus destructans, which causes the fatal bat disease white-nose syndrome (WNS), continues to spread into southern United States. To better understand the suitability of winter bat habitats for the growth of P. destructans in this region, we collected roost temperature and vapor pressure deficit from 97 hibernacula in six ecoregions in Texas during winter 2016-17 and 2017-18. We also measured skin temperature of Rafinesque's big-eared bats (Corynorhinus townsendii), Townsend's big-eared bats (C. townsendii), big-brown bats (Eptesicus fuscus), southeastern myotis (Myotis austroriparius), cave myotis (M. velifer), tri-colored bats (Perimyotis subflavus), and Mexican free-tailed bats (Tadarida brasiliensis) during hibernation to study their use of torpor in these habitats. We found that temperatures within hibernacula were strongly correlated with external air temperatures and were often within the optimal range of temperatures for P. destructans growth. Hibernacula and skin temperatures differed among species, with Rafinesque's big-eared bats, southeastern myotis, and Mexican free-tailed bats occupying warmer microclimates and having higher torpid skin temperatures. For species that were broadly distributed throughout Texas, hibernacula and skin temperatures differed within species by ecoregion; Tri-colored bats and cave myotis in colder, northern regions occupied colder microclimates within hibernacula and exhibited colder skin temperatures, than individuals of the same species in warmer, southern regions. These data illustrate the variability in microclimates used as hibernacula by bats in Texas and suggest similar variation in susceptibility to WNS in the state. Thus, monitoring microclimates at winter roosts may help predict where WNS may develop, and where management efforts would be most effective.