Bd is a generalist fungal pathogen that lives in the keratinized skin of amphibians and spreads through aquatic zoospores. It has spread from its natural range in Asia to every continent inhabited by amphibians and is considered one of the pathogens contributing to global amphibian declines. Its effects differ among species and populations, and not every infected population undergoes an obvious decline.

Accurate detection is therefore important for establishing where Bd occurs and for comparing infected and apparently uninfected sites. A negative result from an insensitive method can classify a genuinely infected pond as uninfected and weaken subsequent analyses of population trends. The newt component of the study covered eight ponds south of Oslo that had been sampled for population monitoring since 2013.

Environmental DNA testing for Bd began in 2017, but results from the same ponds sometimes varied with the sample type used. Meaningful within-year comparisons were possible in 2018, 2023 and 2024, when researchers collected compatible samples during the same field seasons. Great crested newts and smooth newts were trapped along pond shorelines, and all captured animals were released after sampling.

A separate toad comparison used six fresh roadkill carcasses collected in April 2022 at a crossing in Nittedal, where the local population had previously been reported to have an 80% Bd prevalence. For pond-water sampling, researchers collected 15 subsamples of 0.2 litres at locations one to four metres apart and combined them into one mixed sample. This approach was intended to represent a pond more closely because environmental DNA can be unevenly distributed.

In 2018, 0.5 litres of mixed water was filtered through a 0.45-micrometre cellulose nitrate membrane. In later years, mixed water was filtered through 2.0-micrometre glass-fibre filters, which could process approximately 1 to 1.5 litres. During three May sampling periods in 2024, duplicate sets of both filter types were tested.

Field negative controls and technical controls did not amplify Bd DNA, providing no evidence of contamination in those samples. The 2024 filter comparison produced 94% positive qPCR results for the 0.45-micrometre filters and 77% for the 2.0-micrometre filters, although the difference was not statistically significant (t = 0.49, p = 0.62). The amount of DNA placed into the reaction nevertheless affected results for the finer filters: 5 nanograms of input produced a signal 1.3 cycles earlier than 1 nanogram (t = 2.52, p = 0.013).

Pond 8 provided a different pattern. There, the 2.0-micrometre filter yielded signals 2.04 cycles earlier at 1 nanogram of input (t = 3.53, p < 0.01) and 1.80 cycles earlier at 5 nanograms (t = 2.46, p = 0.017). Those values indicated higher Bd DNA concentrations in the larger-pore filter when detection succeeded, even though the finer filter had produced more positive reactions overall.

Detection in open water also varied among the three spring sampling dates in 2024. Ponds 7 and 8 had their highest Bd concentrations at the first visit, but each subsequent visit detected Bd in one additional pond, and the concentration in pond 6 increased over the two-week period. Ponds 1 and 2 produced an occasional positive water result at one sampling time.

These observations do not establish why detection changed: the three dates provided insufficient temporal resolution for statistical separation of seasonal patterns from other variation, and the study lacked continuous pond-specific temperature data. Longer seasonal sampling would be needed to determine how temperature and other temporal factors relate to Bd DNA in pond water. Amphibian-based samples often proved more informative than mixed pond water.

Among great crested newts, the proportion of positive swabs ranged from 50% under a strict threshold of at least three positive droplets to 90% when samples with one or two droplets were included. For smooth newts, all five ponds yielded at least one or two positive droplets, but only one pond met the stricter threshold. By comparison, the 0.45-micrometre mixed-water filters produced two unequivocally positive ponds, ponds 7 and 8, while pond 4 was weakly positive.

These comparisons were limited to the five ponds sampled with both methods in 2018, and the source material does not report a seasonal or host-dependent release mechanism for the observed differences. The 2023 comparisons included mixed pond water, bathwater from captured newts and collective swab samples from three ponds. Mixed water identified ponds 6, 7 and 8 as positive, although pond 7 produced only one positive reaction among six qPCRs using 1 nanogram of DNA input.

Pond 6 was detected for the first time, with five of six reactions positive. Across the five sample types examined from ponds 6, 7 and 8, extracted DNA concentrations ranged from 33.94 nanograms in a great crested newt swab sample to 2,456 nanograms in smooth newt bathwater. Concentrations of extracted DNA did not predict whether Bd was detected in the qPCR reactions, indicating that a larger total DNA yield did not by itself provide a detection advantage in these comparisons.

The toad experiment compared filtered bathwater, soft sandpaper, webbed skin and soft swabs. Each carcass was placed in 0.5 litres of pond water for 10 to 15 minutes, after which the bathwater was filtered. All six toads were positive for Bd, with at least 52% of their qPCR reactions positive.

Bathwater, soft sandpaper and webbed-skin samples all tested positive in every individual when 5 nanograms of DNA were used. At 1 nanogram of input, only one of 18 reactions from bathwater and soft sandpaper was negative, whereas seven of 18 webbed-skin reactions were negative. Soft swabs performed less consistently: dry swabs gave positive signals in 9.5% of reactions at 1 nanogram and 50% at 5 nanograms, while swabs stored in buffer were positive in 3.5% and 48.2% of reactions, respectively.

Bathwater produced the highest average Bd signal in the toad samples, with mean qPCR cycle values of 32.2 using 5 nanograms of DNA and 34.3 using 1 nanogram. At the higher input, bathwater differed significantly from soft sandpaper, webbed skin and combined soft swabs, with p values of 0.027, 0.022 and less than 0.01. Bathwater and soft sandpaper did not differ significantly at 1 nanogram (p = 0.3).

The results do not establish a precise mechanism for the poor performance of toad swabs. The animals had recently emerged from hibernation, and their swabs contained brownish debris, whereas the newt swabs came from wet animals captured directly from ponds. Soft sandpaper was tested only on the larger toads, not on the more delicate newts, so its performance in newt populations remains uncertain.

On the basis of these comparisons, the researchers identified filtering amphibian bathwater as the most economical method for determining the Bd status of a pond: capture local animals, hold them in water for about 10 to 15 minutes and filter the water. Where that procedure is not feasible, the toad results indicate that soft sandpaper can provide a straightforward alternative, although it was not tested on newts. Several swabs or sandpaper samples can also be pooled for a site-level screen, after which resources can be directed to more detailed prevalence work at confirmed positive ponds.

If only open water is sampled, the findings support using multiple filter pore sizes and testing replicated DNA inputs to reduce the risk of missing weak or inhibited signals. The monitoring data did not provide evidence of a clear short-term population decline that could confidently be attributed to Bd. Catch per unit effort from 2013 to 2024 showed smooth newt numbers increasing in most ponds during the final five years, while great crested newts were scarce and intermittently recorded.

Pond 8 maintained a relatively stable great crested newt population. The researchers did not statistically compare population sizes between infected and uninfected ponds because the study included only eight ponds, infection classifications were uncertain, and amphibian abundance varied naturally. The results therefore concern the reliability of detecting Bd in wild amphibians, not proof of population-level effects, and they reinforce the need for accurate infection surveys before infected and apparently uninfected sites are compared in ecological models.

Subject of Research: Comparison of field sampling methods for detecting Batrachochytrium dendrobatidis DNA in wild amphibians

Article Title: Comparing sampling protocols and results to better detect the amphibian fungal parasite Batrachochytrium dendrobatidis in the wild

Article References: Taugbøl, A. (2025). Comparing sampling protocols and results to better detect the amphibian fungal parasite Batrachochytrium dendrobatidis in the wild. NeoBiota, 104, 73-93. https://doi.org/10.3897/neobiota.104.160230

Image Credits: AI Generated

DOI: 10.3897/neobiota.104.160230

Keywords: chytrid fungus, Batrachochytrium dendrobatidis, amphibian disease, environmental DNA, field sampling, newts, common toads, wildlife surveillance, false negatives, Norway, Comparing, sampling

Amphibians are among the most threatened vertebrate groups worldwide, with population declines driven by climate change, habitat degradation and the spread of emerging infectious diseases. Bd, the causative agent of chytridiomycosis, was first described in 1999 as the agent behind massive amphibian die-offs, but its presence in Mexico dates back to the 1970s. The pathogen is now widespread, particularly in the central and southern regions of the country, and has been reported to infect at least 50 amphibian species in Mexico. Hylidae and Plethodontidae are among the most affected families, with the genera Hyla, Ambystoma and Pseudoeurycea ranking as the most impacted, their species concentrated across central and southern Mexico.

Mexico is a global hotspot of amphibian diversity and endemism, hosting approximately 3.69 percent of the world’s anuran species. The country’s most diverse amphibian family, Plethodontidae, comprises 140 species that primarily inhabit high-elevation forests in the Trans-Mexican Volcanic Belt, the Sierra Madre Oriental and the montane regions of the Sierra Madre del Sur, the Balsas Depression and Oaxaca. Hylidae is the second most diverse family with approximately 101 species, while Ambystomatidae, though lower in species richness with 12 to 18 species depending on the taxonomic framework, includes 16 endemic species, making Mexico the centre of diversity for the group.

The research team selected 13 amphibian species based on endemism, conservation status and ecological relevance within the montane regions of central Mexico. The selection included three Hyla species (H. euphorbiacea, H. eximia and H. plicata), three Ambystoma species (A. altamirani, A. lermaense and A. velasci) and seven Pseudoeurycea species (P. altamontana, P. gadovii, P. juarezi, P. lineola, P. longicauda, P. smithi and P. werleri). All are endemic to Mexico, and most inhabit high-elevation cloud forests or temperate montane ecosystems that are particularly susceptible to climate change, habitat loss and emerging diseases. The species span a broad spectrum of IUCN Red List categories: four are listed as Least Concern, one as Vulnerable, seven as Endangered and one, P. smithi, as Critically Endangered.

Notably, most Pseudoeurycea species included in the study are extremely data deficient. For several of them, only presence records are available, with almost no information on their population sizes, ecology, reproductive behaviour or physiological tolerances. This scarcity of data results in their exclusion from many broad-scale conservation initiatives, despite the likelihood that they are among the most vulnerable to environmental disturbance and chronic infections. By including this taxonomically and ecologically diverse group, ranging from broadly distributed species to microendemics with only a handful of records, the study aimed to provide a more integrative evaluation of the potential impacts of Bd across a gradient of conservation statuses and research gaps.

Methodologically, the researchers compiled occurrence records from online databases including GBIF, iNaturalist and CONABIO, while Bd records were obtained from a previous study and its associated Dryad data repository, covering the period from 2000 to 2022. Records were filtered to remove coordinate errors, duplicates and points outside known geographic ranges, then spatially rarefied to retain only one occurrence per square kilometre, an approach shown to reduce model overfitting. Nineteen bioclimatic variables at roughly one kilometre resolution were obtained from WorldClim, along with elevation and land-cover variables derived from satellite imagery. A Pearson correlation analysis eliminated highly correlated variables, retaining seven final land-cover layers: percentage cover of agriculture, arid vegetation, grassland, fir forest, pine forest, oak forest and cloud forest.

Two distinct modelling methodologies were employed to facilitate comparative analysis. The first used the maximum entropy algorithm implemented through the ENMeval package in R, testing regularisation multipliers and feature class combinations, with 100,000 background points randomly generated to characterise the environmental background. A block method partitioned occurrence data geographically based on latitude and longitude, ensuring that training and evaluation datasets were separated across space. The second approach constructed an ensemble model with the biomod2 package, combining ten algorithms including generalised linear models, random forest, Maxent and XGBOOST, each run ten times with different selections of training and testing datasets. Model performance was assessed using the area under the curve of receiver operating characteristic plots, partial ROC analyses and the true skill statistic.

The results showed that the maximum temperature in the warmest month, the minimum temperature in the coldest month and precipitation in the wettest and driest months were the most important environmental variables for the amphibian species. For most Ambystoma species, the optimal habitat suitability fell between roughly minus two and ten degrees Celsius for the coldest month, while A. lermaense presented an optimum temperature range of 15 to 25 degrees. For Bd, the maximum temperature in the warmest month was positive within a range of approximately 15 to 35 degrees, and the minimum temperature of the coldest month was a negative variable. The optimal temperature range for Bd was found to lie between 10 and 25 degrees, aligning with observations from European studies. Elevation was a positive variable for all species, and pine, oak and cloud forest percentages were positive for nearly all species.

According to the tenth percentile training presence threshold, Bd had the greatest potential distribution among all species studied, covering 267,557 square kilometres, while the most restricted species, P. altamontana, covered just 9,982 square kilometres. Niche overlap analysis revealed that Bd and A. altamirani showed relatively higher overlap values (Schoener’s D = 0.423; Hellinger’s I = 0.572), suggesting a greater degree of environmental similarity compared to other species pairs. In contrast, A. lermaense exhibited the lowest overlap with Bd (D = 0.038; I = 0.178). Barplot analysis indicated that P. juarezi, P. longicauda and P. altamontana exhibited the highest percentage of spatial overlap with Bd, all exceeding 70 percent of their distribution ranges, while A. velasci and H. eximia had the lowest overlap, below 40 percent.

By integrating spatial exposure information with IUCN Red List categories, the researchers developed a composite vulnerability index that ranked P. altamontana, P. longicauda and P. juarezi as the most vulnerable species. A principal component analysis of vulnerability patterns revealed distinct species clusters, with the first two components accounting for 92.3 and 7.7 percent of the variance respectively. Species richness maps showed that amphibian diversity peaks in the central highlands of Mexico, particularly in regions overlapping with the Trans-Mexican Volcanic Belt, where richness values reach up to nine species per pixel, and the overlay analysis demonstrated that Bd suitability was notably high in these same regions. The authors emphasise the urgency of implementing targeted conservation actions, including long-term monitoring, early detection of infections, identification of resistant or tolerant populations and regulation of ecotourism in vulnerable regions. They also highlight the potential importance of range break zones, such as Oaxaca and the Altiplano Sur, as natural barriers that could limit Bd spread, and note that community-based conservation initiatives and voluntary reserves managed by indigenous people offer an opportunity to expand protected areas and implement habitat restoration programmes.

Subject of Research: Ecological niche overlap between the chytrid fungus Batrachochytrium dendrobatidis and endemic amphibians in Mexico's Central-Southern Highlands

Article Title: Mapping Risk Zones: niche overlap between Batrachochytrium dendrobatidis and endemic amphibians in Mexico’s Central-Southern Highlands

Article References: Heredia-Bobadilla, R. L., Manjarrez, J., Gómez-Benítez, A., Reyes-Velázquez, E. A., Carmen-Cristobal, J. M., Sánchez-Lara, A. J., Martínez-Alva, G., Bolom-Huet, R., & Sunny, A. (2026). Mapping Risk Zones: niche overlap between Batrachochytrium dendrobatidis and endemic amphibians in Mexico’s Central-Southern Highlands. Nature Conservation, 63, 299-335. https://doi.org/10.3897/natureconservation.63.163964

Image Credits: AI Generated

DOI: 10.3897/natureconservation.63.163964

Keywords: Batrachochytrium dendrobatidis, chytridiomycosis, amphibian conservation, ecological niche modelling, Mexico, Trans-Mexican Volcanic Belt, Sierra Madre del Sur, Pseudoeurycea, Ambystoma, Hyla, niche overlap, biodiversity hotspots

The culprit, Batrachochytrium dendrobatidis, is a microscopic fungal pathogen that causes chytridiomycosis, a disease that has become synonymous with catastrophic amphibian loss worldwide. Believed to have originated in Asia, the fungus likely swept across the globe during the 1970s and 1980s, yet scientists did not formally describe it until 1999. By that time, hundreds of amphibian species had already been pushed to the brink of extinction or beyond, and because so many of those declines occurred before anyone knew the pathogen existed, establishing responsibility has been extraordinarily difficult. Jack Boyette, an NZCBI graduate fellow and doctoral candidate at Pennsylvania State University who led the study, described the problem bluntly: after almost 50 years, researchers still are not confident what caused many historical amphibian die-offs, making these declines some of the most enduring cold cases in wildlife epidemiology.

The breakthrough came from the foresight of a collector who could never have imagined how his work would be used. William Ronald Heyer, a former curator of amphibians and reptiles at the National Museum of Natural History, conducted five field campaigns at Boracéia: two in the 1970s, before the sudden amphibian declines, and three in the 1980s, during the aftermath of the die-offs. That timing created a rare natural experiment, allowing researchers to trace the pathogen’s arrival across the critical window when the community collapsed. Boyette and his colleagues sampled the preserved specimens to determine when the chytrid fungus first appeared at the site and how infection prevalence changed through time, effectively reconstructing an outbreak that unfolded before the pathogen had even been named.

The results were stark. Specimens collected prior to 1979 showed no evidence of chytrid infection whatsoever, while more than half of the specimens from the 1980s tested positive. The regional invasion of the pathogen coincided exactly with the unexplained amphibian disappearances, providing the most direct evidence yet that the fungus, rather than environmental factors alone, drove the losses. While the researchers acknowledge that climate or extreme weather could have played some role, the temporal alignment between the pathogen’s arrival and the die-offs implicates chytrid as the primary force behind the catastrophe.

Perhaps the most surprising finding was the pattern of infection itself. The species that suffered the most dramatic declines showed no detectable infections in the museum specimens, while the species that never declined were heavily infected. This apparent paradox, the researchers hypothesize, reflects natural variation in disease tolerance across the amphibian community. The species that did not decline may have naturally tolerated the fungus, becoming infected without suffering mass mortality, which is why their carcasses were readily collected and preserved. By contrast, the species that declined most dramatically may have been so susceptible that infected frogs died before Heyer could collect them, leaving few infected individuals in the historical record. The dead, in other words, were largely absent from the sample because they died too fast to be caught.

These patterns suggest the chytrid fungus spread rapidly through the amphibian community at Boracéia, illuminating how quickly an emerging disease can sweep through a population of naive hosts that has never encountered the pathogen before. Michael Campana, a genomicist at NZCBI who participated in the study, emphasized that museum collections are a game changer for researchers seeking to reconstruct historical disease impacts. Collectors could not have anticipated how important their specimens would become to modern conservation challenges, he noted, but without them, scientists would never be able to detect pathogens that went unnoticed for decades or reconstruct disease outbreaks that would otherwise remain mysteries.

The team did not rely solely on historical material. Between 2022 and 2024, they returned to Boracéia six times alongside researchers from the University of São Paulo to sample modern amphibian populations. Of the 17 species represented in Heyer’s collection, only 12 could still be found at the site. The fate of the other five remains uncertain, though their continued absence raises concerns that they may have been lost locally, a sobering reminder that the historical die-off may have left permanent scars on the community.

Yet the modern story at Boracéia is ultimately one of resilience. Most amphibian species appear to have recovered since the early 1980s, and while the chytrid fungus still infects frogs at the site, the current infection patterns suggest the evolution of widespread pathogen tolerance among the species that historically declined. That tolerance likely promoted their persistence and recovery in the decades since the collapses, demonstrating that host populations can adapt to a pathogen that once devastated them. Understanding how that adaptation occurred is now a central question for the researchers.

The implications extend well beyond a single field station in São Paulo. Reconstructing historical disease dynamics may help researchers anticipate future disease outcomes, a capability that has become urgently relevant as another fungal pathogen, Batrachochytrium salamandrivorans, threatens salamander populations in Europe and could pose a serious risk to salamanders in North America. The researchers caution that the site-specific nature of the study means the results cannot be extrapolated across the broader Atlantic Forest, but they argue that the work solves the mystery of Boracéia’s disappearing frogs and provides a blueprint for other researchers to conduct similar disease retrospectives using museum collections worldwide.

The investigation is far from over. The team is now using DNA extracted from the museum samples to investigate the genomic mechanisms that shaped historical amphibian responses to chytrid, while other ongoing projects aim to fill gaps in historical specimen records using ancient environmental DNA. Together, these efforts promise a more detailed picture of chytrid emergence, spread and impact over the past century, deepening understanding of the pathogen’s historical toll on biodiversity and helping scientists anticipate its future moves in a world where fungal pathogens remain a major and growing threat to wildlife.

Subject of Research: Museum specimen analysis links the chytrid fungus to historical amphibian declines at Brazil's Boracéia Biological Station

Article Title: Museum specimens reveal pathogen behind decades-old mystery of disappearing frogs

Article References: Museum specimens reveal pathogen behind decades-old mystery of disappearing frogs. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: amphibian declines, chytrid fungus, Batrachochytrium dendrobatidis, museum specimens, Atlantic Forest, Brazil, conservation genomics, wildlife epidemiology, Boracéia Biological Station, Smithsonian, disease tolerance, Molecular Ecology

The research, published in the Journal for Nature Conservation, builds on a deceptively simple ecological insight. Just as human influenza surges in winter and recedes in summer, chytrid severity fluctuates with temperature and humidity. The fungus thrives in cool, moist conditions and falters when environments become too hot or too dry for its zoospores to survive and infect amphibian skin. By combining previously developed models that tracked where the disease occurs and when it reaches peak intensity, the research team constructed new chytrid suitability maps for Panama. These maps identify climatic refugia, places that are environmentally hostile to the pathogen yet perfectly hospitable to the frogs themselves. In effect, the scientists have produced a ranking system for potential release sites, allowing conservationists to direct scarce reintroduction resources toward locations where the odds of survival are highest.

The urgency of this work is rooted in painful experience. Conservationists have previously released captive-bred harlequin toads back into Panamanian forests, only to watch many of the animals succumb to the chytrid fungus that persists in soils, waterways and the skin of resistant carrier species. Brian Gratwicke, a conservation biologist at the Smithsonian’s National Zoo and Conservation Biology Institute and senior author of the study, explained that these repeated losses underscored the need for a smarter siting strategy. By mapping climatic refuges, he noted, the team finally has a practical tool to rank candidate release sites and select places where released animals have the best chance of persisting. The approach transforms what has largely been a gamble into a data-driven decision, grounded in the seasonal dynamics of a pathogen that can be modeled with the same rigor applied to crop pests or human disease vectors.

The analytical core of the study lies in comparing where harlequin toads still survive against the modeled intensity of chytrid infection across seasons. The results were striking. Most Atelopus species in Panama were found to persist only at sites where more than 80 percent of the seasons were unsuitable for medium and high-intensity infections. In other words, surviving populations cling to the very few corners of the landscape where the fungus cannot reliably build to lethal levels. This pattern provides strong indirect evidence that the disease, rather than habitat loss alone, is the dominant filter determining where these amphibians can endure. It also validates the underlying models: if the suitability maps were meaningless, surviving populations should be distributed randomly with respect to predicted disease pressure, and they are not.

One species stands out as a remarkable exception. The variable harlequin toad, Atelopus varius, was the only species found at sites classified as suitable for the disease. Scientists interpret this as consistent with growing evidence that the species may have evolved some degree of resistance to chytrid. Such evolutionary responses have been documented elsewhere in the amphibian world, where populations that survived initial epizootics sometimes carry genetic variants that blunt the fungus’s effects. If A. varius truly possesses enhanced tolerance, it could serve as both a biological benchmark for understanding resistance and a candidate for reintroduction programs that do not require the strictest disease-free conditions. For the many Atelopus species that lack this advantage, however, the refugia maps remain the most promising blueprint.

Beyond guiding reintroductions, the maps serve a second, equally valuable purpose: locating wild populations that may have quietly persisted in places scientists have not yet searched. Carrie Lewis, a doctoral candidate at George Mason University and lead author of the publication, emphasized that the ability to identify climate refugia is critical for finding populations that might have disappeared from most of their former range. These surviving pockets could act as genetic reservoirs, supplying founders to bolster assurance colonies under human care and providing stock to augment dwindling wild populations. In conservation genetics, maintaining diversity is paramount; small, isolated refugial populations risk inbreeding, and infusing new genes from rediscovered survivors could mean the difference between a population that limps along and one that recovers robustly.

The study also confronts an uncomfortable complication: climate change is redrawing the map in real time. The models forecast areas that, while currently suitable for the fungus, may become unsuitable in the coming years as temperatures rise and rainfall patterns shift. These emerging refuges could expand the portfolio of potential release sites, offering new opportunities that did not exist a decade ago. Yet the same climatic forces carry a threat. Many harlequin toads are adapted to cool, mountainous habitats, and warming may render those high-elevation refuges inhospitable to the frogs even as they become inhospitable to the fungus. The interplay between pathogen suitability and amphibian habitat suitability is therefore not a simple overlap but a moving target that conservation planners must track continuously, updating models as conditions change.

The research is already moving from theory to practice. Roberto Ibáñez, a staff scientist at the Smithsonian Tropical Research Institute and co-author of the study, reported that guided by the climate refugia and habitat-suitability maps, the team has begun surveying potential sites for habitat quality and preparing for release trials. These field experiments will allow scientists to test the climate refugia hypothesis directly, comparing survival rates of released frogs at sites predicted to be disease refuges against those at more chytrid-suitable locations. Such experimental validation is essential; models are hypotheses, and only on-the-ground outcomes with tagged, monitored animals can confirm that the mapped refuges deliver the protection they promise. Success would establish a template transferable to other chytrid-affected landscapes across the neotropics.

The institutional machinery behind this effort reflects decades of investment. The Panama Amphibian Rescue and Conservation Project, founded in 2009, unites Zoo New England, Cheyenne Mountain Zoo, the Smithsonian’s National Zoo and Conservation Biology Institute and the Smithsonian Tropical Research Institute in a coordinated mission to build captive populations of species at risk, develop methods to reduce the impact of chytrid, and return imperiled species to the wild. The new research was supported in part by the Bezos Earth Fund, signaling growing philanthropic recognition that amphibian conservation requires both ex situ insurance colonies and in situ solutions. The modeling study itself employed computational simulation, a method well suited to integrating climate data, disease dynamics and species distributions across complex tropical terrain.

For a genus that has become an emblem of the global amphibian crisis, the Panama maps represent something rare: actionable hope. Harlequin toads, with their jewel-like colors and their dependence on clean, flowing streams, are sentinels of ecosystem health, and their decline has rippled through the food webs of montane forests. If release trials in the mapped refuges succeed, captive-bred frogs descended from rescued founders could once again patrol Panamanian streams, and the framework could be extended to other countries and other pathogens. The study does not promise a cure for chytrid, which remains ubiquitous across the region. What it offers instead is precision, the knowledge of exactly where the odds tilt in the frogs’ favor, and a disciplined strategy for spending that advantage before it disappears in a warming, shifting world.

Subject of Research: Mapping climatic refugia of the chytrid fungus to guide reintroduction of harlequin toads in Panama

Article Title: Climate refuges may be key to restoring imperiled amphibians

Article References: Climate refuges may be key to restoring imperiled amphibians. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: amphibian chytrid fungus, harlequin toads, Atelopus, climate refugia, Panama, reintroduction, conservation biology, Smithsonian, Batrachochytrium dendrobatidis, captive breeding, disease ecology, climate change

The experimental design was unusually thorough for a wildlife disease study. The team, led by Brian Gratwicke of the Smithsonian’s National Zoo and Conservation Biology Institute and Owen Osborne of Bangor University, divided toads into five treatment groups. One group of twenty animals was infected with Bd, then cleared using itraconazole, a standard antifungal used in amphibian rescue programs, and then infected again. A second group of twenty received a mock infection, underwent the same itraconazole treatment, and was then exposed to Bd for the first time. Two smaller groups of ten animals each served as controls that experienced infection or mock infection without re-exposure, and a final group of ten remained untreated throughout. This structure allowed the researchers to disentangle the effects of prior infection, antifungal treatment, and re-infection on survival, pathogen load, immune gene expression, and the communities of bacteria and fungi living on the skin.

The survival results were unambiguous and troubling. Toads that had been previously infected and cleared developed detectable Bd infections faster and died sooner after re-infection than naïve animals that had never encountered the fungus. In other words, the infection-clearance-infection protocol, which mirrors the approach used in some captive breeding and reintroduction programs, did not improve survival. It actively worsened outcomes. This finding stands in sharp contrast to earlier studies in other amphibian species, in which surviving a Bd infection and being cleared of it sometimes produced acquired resistance or reduced susceptibility upon re-exposure. For Atelopus glyphus, one of the most Bd-susceptible species known, no such protective effect emerged.

To understand the mechanism behind this heightened vulnerability, the researchers turned to transcriptomics, sequencing RNA from the toads’ skin to measure the activity of immune-related genes. Their analysis revealed that even after Bd had been cleared from previously infected animals, the fungus left a lasting imprint on the immune system. Animals in the Bd-mock group, which had been infected and cleared but never re-exposed, showed downregulation of immune genes that normally respond to Bd compared with animals that were re-infected. This sustained immunosuppression, the authors suggest, likely reduced the effectiveness of the immune response when the pathogen returned, explaining the accelerated infection dynamics and earlier mortality in the previously infected group.

When the researchers examined what happened upon re-infection in more detail, they found that as fungal loads climbed, Bd itself became the dominant force shaping both immune gene expression and bacterial community composition, regardless of whether an animal had prior infection history. Previously infected animals did not mount any novel immune response upon re-exposure. Instead, their gene expression patterns resembled those of naïve infected animals, only more pronounced. This suggests that the initial infection produced no long-lasting immunological memory that could be mobilized against the pathogen a second time, and that the immune system was simply responding more intensely to a heavier fungal burden.

The skin microbiome added another layer to the story. Prior infection and itraconazole treatment shifted the toads’ bacterial and fungal skin communities into a new state, one enriched in bacteria that are putatively capable of inhibiting Bd. On its face, this might seem like a beneficial change, a kind of microbial vaccination induced by the pathogen itself. Yet the enriched anti-Bd bacteria clearly did not translate into protection, since previously infected animals died faster. The researchers noted that observed differences in bacterial communities were closely tied to host immune gene expression, indicating that the skin microbiome and the immune system are tightly coupled, and that microbial shifts alone cannot compensate for a compromised immune response.

Intriguingly, the fungal component of the skin microbiome told a different story. While the initial infection and treatment period reshaped fungal communities, those communities did not shift again in response to re-infection. The authors concluded that the fungal community changes were most likely a side effect of itraconazole treatment rather than a host- or pathogen-driven response, a reminder that antifungal therapy itself can perturb the delicate microbial ecosystems on amphibian skin. This has practical implications for rescue programs that routinely use itraconazole to clear infections before releasing animals back into the wild.

The broader significance of the study lies in its demonstration that disease outcomes following pathogen clearance are species-specific and cannot be generalized across amphibian diversity. Bd has caused the steepest disease-driven biodiversity loss ever recorded, and harlequin toads of the genus Atelopus have been among its most catastrophic victims, with many Central and South American species vanishing or collapsing to remnant populations. Captive assurance colonies, such as those maintained by the Panama Amphibian Rescue and Conservation Project, a collaboration involving the Smithsonian Tropical Research Institute, Zoo New England, the Cheyenne Mountain Zoo and other partners, represent a lifeline for these species. The new results suggest that simply clearing Bd from captive animals before release may not only fail to protect them but could actively sensitize them to the pathogen awaiting them in their native streams.

For conservation practitioners, the message is that reintroduction strategies must account for the immunological legacy of prior infection. The integrative approach used here, pairing survival data with gene expression and microbiome sequencing, offers a template for evaluating whether any intervention intended to reduce susceptibility actually works before animals are committed to the wild. The researchers emphasize that strategies to improve survival of wildlife impacted by emerging infectious disease remain urgently needed for species of conservation concern, but that such strategies must be tested rigorously in the species they are meant to help. In the case of Atelopus glyphus, the hope that infection history could serve as a shield has been replaced by a more nuanced and challenging reality: for some of the world’s most endangered amphibians, the scars left by chytrid fungus run deeper than the infection itself.

Subject of Research: Effects of prior Batrachochytrium dendrobatidis infection on immunity, skin microbiome and re-infection mortality in harlequin toads

Article Title: Prior infection of harlequin toads by Batrachochytrium dendrobatidis alters their immune-microbial state and increases mortality upon re-infection

Article References: Gratwicke, B., Osborne, O. G., Hartmann, A., Ávila, N., Baitchman, E., Togna, G. D., Figueroa, Y., Guerrel, J., Illueca, E., Linhoff, L., Madison, J. D., Ellison, A., Grayfer, L., Ibáñez, R., & Muletz-Wolz, C. R. (2026). Prior infection of harlequin toads by Batrachochytrium dendrobatidis alters their immune-microbial state and increases mortality upon re-infection. BMC Genomics. https://doi.org/10.1186/s12864-026-13310-9

Image Credits: AI Generated

DOI: 10.1186/s12864-026-13310-9

Keywords: Batrachochytrium dendrobatidis, harlequin toads, Atelopus glyphus, chytridiomycosis, amphibian conservation, immune gene expression, skin microbiome, itraconazole, re-infection, immunosuppression, wildlife disease, BMC Genomics

Bd is arguably the most destructive pathogen ever documented in wild vertebrates. The chytrid fungus has decimated amphibian populations on every continent where amphibians live, driving scores of species toward extinction and reshaping entire ecosystems in its wake. Because Bd is an aquatic organism that infects its hosts through the skin, researchers and wildlife managers have generally focused their attention on the water bodies where amphibians congregate to breed, treating those brief aquatic gatherings as the principal arenas of transmission. For the Yosemite toad, a species that spends the vast majority of its life on land, this reasoning suggested a comforting conclusion: the toads should be at comparatively low risk, vulnerable only during the short window of the mating season when they enter the water.

The new study, published in Functional Ecology by UCLA ecologist Dave Daversa and colleagues, demolishes that assumption. When the researchers surveyed toads emerging from their winter hibernation burrows, they found previously healthy animals carrying heavy fungal loads. The infections, in other words, were not being acquired in the breeding pools at all. They were building up underground, during the coldest and most hostile months of the Sierra Nevada year, in the abandoned rodent burrows that the toads use as winter refuges. The results of the surveillance study, Daversa said, were completely unexpected, sharply contrasting with the prevailing view that the high-risk season for Bd infection occurs when amphibians are breeding in water.

The Yosemite toad, Anaxyrus canorus, is a small and distinctive resident of the high country, living almost exclusively within Yosemite National Park and neighboring Sequoia and Kings Canyon national parks. The toads measure only about one to three inches long, and they walk rather than hop, an unusual gait that suits their slow, deliberate lifestyle in meadow habitats. Females are larger than males and wear bumpy skin mottled in attractive shades of green, while males are less colorful. The species breeds in small pools formed by snowmelt, but for much of the year the toads keep to land, sheltering from predators and extreme temperatures in burrows abandoned by rodents. When the brutal High Sierra winter arrives, they hibernate in these underground chambers, sometimes buried beneath many feet of snow.

That lifestyle has not shielded the species from decline. Climate change is rendering the toads’ mountain habitats drier and hotter, and fewer tadpoles are surviving to adulthood. Infection with Bd, which is fatal to the toads, is widespread across their range. The combined pressures led the federal government to list the Yosemite toad as threatened under the Endangered Species Act in 2014, and California classifies it as a highest-priority species of special concern. In an effort to bolster wild populations, toads have been reared at the San Francisco Zoo and returned to Yosemite, but until now it was not understood how the fungus might be undermining those reintroduction efforts.

To find out, a team of researchers from UCLA and Yosemite National Park conducted an intensive surveillance program from 2021 through 2023, designed to determine how Bd prevalence and severity varied across seasons and life stages. The researchers compared toads before and after cohorts entered winter dormancy, capturing metamorphs — juvenile toads that had recently completed metamorphosis — from high-altitude locations near Tioga Pass. They took skin swabs from the animals immediately after they emerged from hibernation in May, again during the summer, and once more before hibernation in September. The following spring, during early snowmelt, they returned and sampled the same individual toads as they emerged from hibernation. A parallel survey sampled toads at every phase of the life cycle, producing a total of 1,800 samples collected over three years at six sites.

Each swab was screened for Bd using established molecular techniques, including polymerase chain reaction tests of the same kind that became familiar worldwide during the COVID-19 pandemic. Daversa noted that the pandemic made it easier to explain to hikers he encountered along the trails what he was doing as he swabbed toads. The laboratory results, however, were anything but routine. The data unambiguously identified the winter months of underground dormancy as the key season of rapid Bd proliferation, and the effect was particularly pronounced in metamorphs, the youngest and most vulnerable life stage. Toads that had tested clean or carried only trace infections in the autumn emerged from their burrows in spring bearing strong infections acquired on land.

The finding stunned the research community because it contradicted nearly everything scientists believed they knew about the pathogen’s biology. Bd was thought to grow best under more moderate conditions, and the toads were assumed to be at low risk precisely because they spend so little time in water. The idea that the fungus could not merely survive but actively multiply at the super-low temperatures of a Sierra winter — either above ground or inside the burrows — came as a genuine surprise. UCLA conservation biologist and study co-author Brad Shaffer said it certainly appears that the Bd fungus has the ability to move between individuals, adding that its capacity to increase in number on land at such low temperatures is a real surprise.

Several important questions remain unresolved. The researchers cannot yet say for certain that toads are contracting new infections inside the burrows themselves. An alternative explanation is that the animals enter hibernation carrying very light, undetectable infections, and that for reasons still unknown each individual then develops a heavy fungal load over the winter. Studying the burrows directly during hibernation is extraordinarily difficult because the chambers lie buried beneath deep snow for months at a time. UCLA wildlife disease ecologist and co-author Jamie Lloyd-Smith framed the puzzle in terms of how pathogens persist in host populations with extremely seasonal life histories, observing that this fungus is not supposed to grow happily at such low temperatures. One hypothesis the team speculates about involves the host’s own defenses: the immune state of a hibernating toad may drop even lower than the fungus’s ability to grow, allowing infections to run unchecked while the animal lies dormant and cold-stunned.

Whatever the precise mechanism proves to be, the discovery has already changed conservation practice on the ground. Reintroducing captive-reared toads at the metamorphic stage, or younger, would be cheaper and easier for the program based at the San Francisco Zoo. But because the youngest toads are precisely the ones most at risk of devastating overwinter Bd infections, managers now release animals only after several years of growth, when they have reached a size and maturity at which the winter fungal surge poses less of a threat. Follow-up studies are planned to pin down exactly how the fungus persists and spreads in the burrows, and whether the hibernating toads’ suppressed immune systems hold the key. For a species clinging to survival in one of America’s most beloved national parks, the finding offers something rare: a genuine strategic insight into an enemy that, until now, had been hiding in the toads’ own homes.

Subject of Research: Winter proliferation of the amphibian chytrid fungus Batrachochytrium dendrobatidis in the hibernation burrows of Yosemite toads

Article Title: Yosemite toad killer may be hiding in their own homes

Article References: Yosemite toad killer may be hiding in their own homes. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Yosemite toad, Batrachochytrium dendrobatidis, chytrid fungus, amphibian disease, hibernation, burrows, conservation biology, Endangered Species Act, captive rearing, reintroduction, Sierra Nevada, UCLA