Elevated Corticosterone Levels and Changes in Amphibian Behavior Are Associated with Batrachochytrium dendrobatidis (Bd) Infection and Bd Lineage

No sex-dependent mortality in an amphibian upon infection with the chytrid fungus, Batrachochytrium dendrobatidis

One of the major factors driving the currently ongoing biodiversity crisis is the anthropogenic spread of infectious diseases. Diseases can have conspicuous consequences, such as mass mortality events, but may also exert covert but similarly severe effects, such as sex ratio distortion via sex-biased mortality. Chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd) is among the most important threats to amphibian biodiversity. Yet, whether Bd infection can skew sex ratios in amphibians is currently unknown, although such a hidden effect may cause the already dwindling amphibian populations to collapse. To investigate this possibility, we collected common toad (Bufo bufo) tadpoles from a natural habitat in Hungary and continuously treated them until metamorphosis with sterile Bd culture medium (control), or a liquid culture of a Hungarian or a Spanish Bd isolate. Bd prevalence was high in animals that died during the experiment but was almost zero in individuals that survived until the end of the experiment. Both Bd treatments significantly reduced survival after metamorphosis, but we did not observe sex-dependent mortality in either treatment. However, a small number of genotypically female individuals developed male phenotype (testes) in the Spanish Bd isolate treatment. Therefore, future research is needed to ascertain if larval Bd infection can affect sex ratio in common toads through female-to-male sex reversal.

Stress response of fire salamander larvae differs between habitat types

The larvae of the European fire salamander (Salamandra salamandra) can inhabit two different habitats: streams and ponds. Streams are characterized by lower predation risks and higher food availability. Thus, ponds are considered a less suitable habitat. To investigate the differential impacts of these two habitats on larval physiology, we measured the stress response of larvae. After successfully validating the measure of water-borne corticosterone release rates in fire salamander larvae, we measured the baseline and stress-induced corticosterone of 64 larvae from ponds and streams in the field. We found that larvae in ponds have a higher baseline and stress-induced corticosterone levels. Additionally, we performed a reciprocal transplant experiment (RTE) and tested whether larvae can adapt their stress responses to changing habitats. After two weeks, we did not find an increase in corticosterone levels when comparing stress-induced corticosterone values with baseline corticosterone values in larvae transferred into ponds, irrespective of their habitat of origin. However, larvae transferred into streams still exhibited an increase in the stress-induced corticosterone response in comparison with the baseline values. These results show that non-invasive hormone measurements can provide information on the habitat quality and potential adaptation and thus emphasize the potential for its use in conservation efforts.

Estrogen contamination increases vulnerability of amphibians to the deadly chytrid fungus

Aquatic contaminants and infectious diseases are among the major drivers of global amphibian declines. However, the interaction of these factors is poorly explored and could better explain the amphibian crisis. We exposed males and females of the Brazilian Cururu Toad, Rhinella icterica, to an environmentally relevant concentration of the estrogen 17-alpha-ethinylestradiol (an emerging contaminant) and to the chytrid infection (Batrachochytrium dendrobatidis), in their combined and isolated forms, and the ecotoxicity was determined by multiple biomarkers: cutaneous, hematological, cardiac, hepatic, and gonadal analysis. Our results showed that Cururu toads had many physiological alterations in response to the chytrid infection, including the appearance of cutaneous Langerhans's cells, increased blood leukocytes, increased heart contraction force and tachycardia, increased hepatic melanomacrophage cells, which in turn led to gonadal atrophy. The estrogen, in turn, increased the susceptibility of the toads to the chytrid infection (higher Bd loads) and maximized the deleterious effects of the pathogen: reducing leukocytes, decreasing the contraction force, and causing greater tachycardia, increasing hepatic melanomacrophage cells, and leading to greater gonadal atrophy, which were more extreme in females. The exposure to estrogen also revealed important toxicodynamic pathways of this toxicant, as shown by the immunosuppression of exposed animals, and the induction of the first stages of feminization in males, which corroborates that the synthetic estrogen acts as an endocrine disruptor. Such an intricate relationship is unprecedented and reinforces the importance of studying the serious consequences that multiple environmental stressors can cause to aquatic populations.

Batrachochytrium dendrobatidis strain affects transcriptomic response in liver but not skin in latitudinal populations of the common toad (Bufo bufo)

Batrachochytrium dendrobatidis (Bd) is a fungal pathogen that has decimated amphibian populations worldwide for several decades. We examined the changes in gene expression in response to Bd infection in two populations of the common toad, Bufo bufo, in a laboratory experiment. We collected B. bufo eggs in southern and northern Sweden, and infected the laboratory-raised metamorphs with two strains of the global panzoonotic lineage Bd-GPL. Differential expression analysis showed significant differences between infected and control individuals in both liver and skin. The skin samples showed no discernible differences in gene expression between the two strains used, while liver samples were differentiated by strain, with one of the strains eliciting no immune response from infected toads. Immune system genes were overexpressed in skin samples from surviving infected individuals, while in liver samples the pattern was more diffuse. Splitting samples by population revealed a stronger immune response in northern individuals. Differences in transcriptional regulation between populations are particularly relevant to study in Swedish amphibians, which may have experienced varying exposure to Bd. Earlier exposure to this pathogen and subsequent adaptation or selection pressure may contribute to the survival of some populations over others, while standing genetic diversity in different populations may also affect the infection outcome.

Exploring water-borne corticosterone collection as a non-invasive tool in amphibian conservation physiology: benefits, limitations and future perspectives

Global change exposes wildlife to a variety of environmental stressors and is affecting biodiversity worldwide, with amphibian population declines being at the forefront of the global biodiversity crisis. The use of non-invasive methods to determine the physiological state in response to environmental stressors is therefore an important advance in the field of conservation physiology. The glucocorticoid hormone corticosterone (CORT) is one useful biomarker to assess physiological stress in amphibians, and sampling water-borne (WB) CORT is a novel, non-invasive collection technique. Here, we tested whether WB CORT can serve as a valid proxy of organismal levels of CORT in larvae of the common frog (Rana temporaria). We evaluated the association between tissue and WB CORT levels sampled from the same individuals across ontogenetic stages, ranging from newly hatched larvae to froglets at 10 days after metamorphosis. We also investigated how both tissue and WB CORT change throughout ontogeny. We found that WB CORT is a valid method in pro-metamorphic larvae as values for both methods were highly correlated. In contrast, there was no correlation between tissue and WB CORT in newly hatched, pre-metamorphic larvae, metamorphs or post-metamorphic froglets probably due to ontogenetic changes in respiratory and skin morphology and physiology affecting the transdermal CORT release. Both collection methods consistently revealed a non-linear pattern of ontogenetic change in CORT with a peak at metamorphic climax. Thus, our results indicate that WB CORT sampling is a promising, non-invasive conservation tool for studies on late-stage amphibian larvae. However, we suggest considering that different contexts might affect the reliability of WB CORT and consequently urge future studies to validate this method whenever it is used in new approaches. We conclude proposing some recommendations and perspectives on the use of WB CORT that will aid in broadening its application as a non-invasive tool in amphibian conservation physiology.

The adaptive microbiome hypothesis and immune interactions in amphibian mucus

The microbiome is known to provide benefits to hosts, including extension of immune function. Amphibians are a powerful immunological model for examining mucosal defenses because of an accessible epithelial mucosome throughout their developmental trajectory, their responsiveness to experimental treatments, and direct interactions with emerging infectious pathogens. We review amphibian skin mucus components and describe the adaptive microbiome as a novel process of disease resilience where competitive microbial interactions couple with host immune responses to select for functions beneficial to the host. We demonstrate microbiome diversity, specificity of function, and mechanisms for memory characteristic of an adaptive immune response. At a time when industrialization has been linked to losses in microbiota important for host health, applications of microbial therapies such as probiotics may contribute to immunotherapeutics and to conservation efforts for species currently threatened by emerging diseases.

Effects of exogenous elevation of corticosterone on immunity and the skin microbiome of eastern newts (Notophthalmus viridescens)

The amphibian chytrid fungus, Batrachochytrium salamandrivorans (Bsal) threatens salamander biodiversity. The factors underlying Bsal susceptibility may include glucocorticoid hormones (GCs). The effects of GCs on immunity and disease susceptibility are well studied in mammals, but less is known in other groups, including salamanders. We used Notophthalmus viridescens (eastern newts) to test the hypothesis that GCs modulate salamander immunity. We first determined the dose required to elevate corticosterone (CORT; primary GC in amphibians) to physiologically relevant levels. We then measured immunity (neutrophil lymphocyte ratios, plasma bacterial killing ability (BKA), skin microbiome, splenocytes, melanomacrophage centres (MMCs)) and overall health in newts following treatment with CORT or an oil vehicle control. Treatments were repeated for a short (two treatments over 5 days) or long (18 treatments over 26 days) time period. Contrary to our predictions, most immune and health parameters were similar for CORT and oil-treated newts. Surprisingly, differences in BKA, skin microbiome and MMCs were observed between newts subjected to short- and long-term treatments, regardless of treatment type (CORT, oil vehicle). Taken together, CORT does not appear to be a major factor contributing to immunity in eastern newts, although more studies examining additional immune factors are necessary. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Introduction to the special issue Amphibian immunity: stress, disease and ecoimmunology

Amphibian populations have been declining worldwide, with global climate changes and infectious diseases being among the primary causes of this scenario. Infectious diseases are among the primary drivers of amphibian declines, including ranavirosis and chytridiomycosis, which have gained more attention lately. While some amphibian populations are led to extinction, others are disease-resistant. Although the host's immune system plays a major role in disease resistance, little is known about the immune mechanisms underlying amphibian disease resistance and host-pathogen interactions. As ectotherms, amphibians are directly subjected to changes in temperature and rainfall, which modulate stress-related physiology, including immunity and pathogen physiology associated with diseases. In this sense, the contexts of stress, disease and ecoimmunology are essential for a better understanding of amphibian immunity. This issue brings details about the ontogeny of the amphibian immune system, including crucial aspects of innate and adaptive immunity and how ontogeny can influence amphibian disease resistance. In addition, the papers in the issue demonstrate an integrated view of the amphibian immune system associated with the influence of stress on immune-endocrine interactions. The collective body of research presented herein can provide valuable insights into the mechanisms underlying disease outcomes in natural populations, particularly in the context of changing environmental conditions. These findings may ultimately enhance our ability to forecast effective conservation strategies for amphibian populations. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Does the Glucocorticoid Stress Response Make Toads More Toxic? An Experimental Study on the Regulation of Bufadienolide Toxin Synthesis

Chemical defense is a crucial component of fitness in many organisms, yet the physiological regulation of defensive toxin synthesis is poorly understood, especially in vertebrates. Bufadienolides, the main defensive compounds of toads, are toxic to many predators and other natural enemies, and their synthesis can be upregulated by stressors, including predation risk, high conspecific density, and pollutants. Thus, higher toxin content may be the consequence of a general endocrine stress response in toads. Therefore, we hypothesized that bufadienolide synthesis may be stimulated by elevated levels of corticosterone (CORT), the main glucocorticoid hormone of amphibians, or by upstream regulators that stimulate CORT production. To test these alternatives, we treated common toad tadpoles with exogenous CORT (exoCORT) or metyrapone (MTP, a CORT-synthesis inhibitor that stimulates upstream regulators of CORT by negative feedback) in the presence or absence of predation cues for 2 or 6 days, and subsequently measured their CORT release rates and bufadienolide content. We found that CORT release rates were elevated by exoCORT, and to a lesser extent also by MTP, regardless of treatment length. Bufadienolide content was significantly decreased by treatment with exoCORT for 6 days but was unaffected by exposure to exoCORT for 2 days or to MTP for either 6 or 2 days. The presence or absence of predation cues affected neither CORT release rate nor bufadienolide content. Our results suggest that changes in bufadienolide synthesis in response to environmental challenges are not driven by CORT but may rather be regulated by upstream hormones of the stress response.

Ophidiomycosis is associated with alterations in the acute glycemic and glucocorticoid stress response in a free-living snake species

Emerging fungal pathogens are a direct threat to vertebrate biodiversity. Elucidating the mechanisms by which mycoses impact host fitness is an important step towards effective prediction and management of disease outcomes in populations. The vertebrate acute stress response is an adaptive mechanism that allows individuals to meet challenges to homeostasis and survival in dynamic environments. Disease may cause stress, and coping with fungal infections may require shifts in resource allocation that alter the ability of hosts to mount an acute response to other external stressors. We examined the glucocorticoid and glycemic response to acute capture stress in a population of free-living pygmy rattlesnakes, Sistrurus miliarius, afflicted with an emerging mycosis (ophidiomycosis) across seasons. In all combinations of disease status and season, acute capture stress resulted in a significant glucocorticoid and glycemic response. While disease was not associated with elevated baseline or stress-induced corticosterone (CORT), disease was associated with an increased glucocorticoid stress response (post-stress minus baseline) across seasons. Both baseline and stress-induced glucose were lower in snakes with ophidiomycosis compared to uninfected snakes. The relationship between glucose and pre- and post-stress CORT depended on infection status, and positive correlations were only observed in uninfected snakes. The variables which explained CORT and glucose levels were different. The pattern of CORT was highly seasonal (winter high - summer low) and negatively related to body condition. Glucose, on the other hand, did not vary seasonally or with body condition and was strongly related to sex (male high - female low). Our results highlight the fact that circulating CORT and glucose are sensitive to different intrinsic and extrinsic predictor variables and support the hypothesis that disease alters the acute physiological stress response. Whether the effects of ophidiomycosis on the acute stress response result in sublethal effects on fitness should be investigated in future studies.

Amphibian Dispersal Traits Not Impacted by Triclopyr Exposure during the Juvenile Stage

Exposure to agrochemicals can have lethal and sublethal effects on amphibians. Most toxicology studies only examine exposure during the aquatic larval stage. Survival of the juvenile stage is the most important for population persistence and it is critical to understand the potential impacts of exposure during this life stage. We investigated how short-term exposure to triclopyr, an herbicide commonly used in forestry management, might impact several juvenile traits. To determine if juveniles perceived exposure as an environmental stressor, we measured their release of corticosterone. We also examined dispersal traits by measuring foraging and hopping behavior. We found no evidence that exposure negatively impacted these traits or was a stressor. Our results provide a preliminary assessment of the potential impact of triclopyr on juvenile amphibians, but we recommend additional research on the effects of agrochemicals on juvenile amphibians.

Contributions of water-borne corticosterone as one non-invasive biomarker in assessing nitrate pollution stress in tadpoles of Rana temporaria

Among a multitude of stressors to which wildlife is exposed, environmental pollution is a pervasive one that poses a serious threat. The permeable skin of amphibians is likely to increase direct contact of the body with pollutants, making them a group worth studying to access environmental quality. Consequently, finding reliable and complementary biomarkers that will present detectable and predictable changes in response to pollutants is essential to identify pollution sublethal effects on amphibians and to investigate whether these are in part responsible for population declines. The glucocorticoid hormone corticosterone (CORT), involved in many metabolic functions, is often used to measure the physiological stress response to environmental stressors in amphibians. In this study, we evaluated whether water-borne CORT can serve as a non-invasive biomarker for nitrate pollution stress in the European common frog (Rana temporaria) by comparing the effect of nitrate exposure on hormone release rates and on other physiological downstream biomarkers, i.e., ultimate physiological effects of the stressor. Specifically, we investigated the effect of different nitrate concentrations (0, 10, 50, and 100 mg/L) on water-borne CORT release rates, age, size, and body condition. Exposure to nitrate pollution significantly increased age at metamorphosis and water-borne CORT release rates, and led to reduced mass and body condition, but only at higher nitrate concentrations (i.e., 50 and 100 mg/L). Considering this similar sensitivity to other acknowledged biomarkers, water-borne CORT was a reliable biomarker of physiological stress in R. temporaria exposed to nitrate pollution stress in a controlled single-stressor laboratory approach. Thus, water-borne CORT is a promising method to be included in more holistic approaches. We recommend that such approaches keep testing multiple biomarker combinations, as species are exposed to several stressors likely to interact and produce varied outcomes in different biomarkers in their natural habitats.

Using dermal glucocorticoids to determine the effects of disease and environment on the critically endangered Wyoming toad

Amphibian populations are declining worldwide, and increased exposure to environmental stressors, including global climate change and pathogens like Batrachochytrium dendrobatidis (Bd), may be contributing to this decline. Our goal was to use a novel dermal swabbing method to measure glucocorticoid (GC) hormones and investigate the relationship among disease and environmental conditions in the critically endangered Wyoming toad (Anaxyrus baxteri). Our objectives were to (i) validate the use of dermal swabs to measure GCs using an adrenocorticotropic hormone (ACTH) challenge on eight captive toads (4 ACTH: 2 M, 2F and 4 saline as a control: 2 M, 2F), (ii) investigate stress physiology and disease status of toads across six reintroduction sites and (iii) compare dermal cortisol between reintroduced and captive toads. Dermal cortisol peaked immediately after the ACTH and saline injections. Faecal GC metabolites (FGMs) were significantly higher one week after the ACTH injection compared with the week before. Saline-injected toads had no change in FGM over time. Toads were only found in three reintroduction sites and dermal cortisol was similar across sites; however, reintroduced toads had higher dermal cortisol in August compared with June and compared with captive individuals. Bd status did not influence dermal cortisol concentrations. Dermal and faecal hormonal metabolite analyses can be used to study amphibian stress physiology and learn how environmental conditions are impacting population success.

Evaluating corticosterone as a biomarker for amphibians exposed to increased salinity and ambient corticosterone

Physiological biomarkers are commonly used to assess the health of taxa exposed to natural and anthropogenic stressors. Glucocorticoid (GC) hormones are often used as indicators of physiological stress in wildlife because they affect growth, reproduction and survival. Increased salinity from human activities negatively influences amphibians and their corticosterone (CORT; the main amphibian GC) physiology; therefore, CORT could be a useful biomarker. We evaluated whether waterborne CORT could serve as a biomarker of salt stress for three free-living amphibian species that vary in their sensitivity to salinity: boreal chorus frogs (Pseudacris maculata), northern leopard frogs (Rana pipiens) and barred tiger salamanders (Ambystoma mavortium). Across a gradient of contamination from energy-related saline wastewaters, we tested the effects of salinity on baseline and stress-induced waterborne CORT of larvae. Stress-induced, but not baseline, CORT of leopard frogs increased with increasing salinity. Salinity was not associated with baseline or stress-induced CORT of chorus frogs or tiger salamanders. Associations between CORT and salinity were also not related to species-specific sensitivities to salinity. However, we detected background environmental CORT (ambient CORT) in all wetlands and spatial variation was high within and among wetlands. Higher ambient CORT was associated with lower waterborne CORT of larvae in wetlands. Therefore, ambient CORT likely confounded associations between waterborne CORT and salinity in our analysis and possibly influenced physiology of larvae. We hypothesize that larvae may passively take up CORT from their environment and downregulate endogenous CORT. Although effects of some hormones (e.g. oestrogen) and endocrine disruptors on aquatic organisms are well described, studies investigating the occurrence and effects of ambient CORT are limited. We provide suggestions to improve collection methods, reduce variability and avoid confounding effects of ambient CORT. By making changes to methodology, waterborne CORT could still be a promising, non-invasive conservation tool to evaluate effects of salinity on amphibians.

Exposure to Batrachochytrium dendrobatidis affects chemical defences in two anuran amphibians, Rana dalmatina and Bufo bufo

Background

Batrachochytrium dendrobatidis (Bd) is the causative agent of chytridiomycosis, one of the major causes of worldwide amphibian biodiversity loss. Many amphibians exhibit skin-based chemical defences, which may play an important role against invading pathogens, but whether the synthesis of these chemical compounds is enhanced or suppressed in the presence of pathogens is largely unknown. Here we investigated direct and indirect effects of larval exposure to the globally distributed and highly virulent Bd-GPL strain on skin secreted chemical defences and life history traits during early ontogeny of agile frogs (Rana dalmatina) and common toads (Bufo bufo).

Results

Exposure to Bd during the larval stage did not result in enhanced synthesis of the antimicrobial peptide Brevinin-1 Da in R. dalmatina tadpoles or in increased production of bufadienolides in B. bufo tadpoles. However, exposure to Bd during the larval stage had a carry-over effect reaching beyond metamorphosis: both R. dalmatina and B. bufo froglets contained smaller quantities of defensive chemicals than their Bd-naïve conspecifics in the control treatment. Prevalence of Bd and infection intensities were very low in both larvae and metamorphs of R. dalmatina, while in B. bufo we observed high Bd prevalence and infection intensities, especially in metamorphs. At the same time, we did not find a significant effect of Bd-exposure on body mass or development rate in larvae or metamorphs in either species.

Conclusions

The lack of detrimental effect of Bd-exposure on life history traits, even parallel with high infection intensities in the case of B. bufo individuals, is surprising and suggests high tolerance of local populations of these two species against Bd. However, the lowered quantity of defensive chemicals may compromise antimicrobial and antipredatory defences of froglets, which may ultimately contribute to population declines also in the absence of conspicuous mass-mortality events.

Sickness behaviors across vertebrate taxa: proximate and ultimate mechanisms

There is nothing like a pandemic to get the world thinking about how infectious diseases affect individual behavior. In this respect, sick animals can behave in ways that are dramatically different from healthy animals: altered social interactions and changes to patterns of eating and drinking are all hallmarks of sickness. As a result, behavioral changes associated with inflammatory responses (i.e. sickness behaviors) have important implications for disease spread by affecting contacts with others and with common resources, including water and/or sleeping sites. In this Review, we summarize the behavioral modifications, including changes to thermoregulatory behaviors, known to occur in vertebrates during infection, with an emphasis on non-mammalian taxa, which have historically received less attention. We then outline and discuss our current understanding of the changes in physiology associated with the production of these behaviors and highlight areas where more research is needed, including an exploration of individual and sex differences in the acute phase response and a greater understanding of the ecophysiological implications of sickness behaviors for disease at the population level.

Stressed tadpoles mount more efficient glucocorticoid negative feedback in anthropogenic habitats due to phenotypic plasticity

Coping with anthropogenic environmental change is among the greatest challenges faced by wildlife, and endocrine flexibility is a potentially crucial coping mechanism. Animals may adapt to anthropogenic environments by dampening their glucocorticoid stress response, but empirical tests of this hypothesis have provided mixed evidence. An alternative hypothesis is that a non-attenuated stress response and efficient negative feedback are favored in anthropogenic habitats. To test this idea, we non-invasively sampled corticosterone release rates of common toad (Bufo bufo) tadpoles in agricultural, urban, and natural habitats, and quantified their stress response and negative feedback by a standardized stress-and-recovery protocol. We repeated the same sampling with tadpoles raised from eggs from the same ponds in a common-garden experiment to infer if the differences observed between populations in different habitats were due to individual phenotypic plasticity rather than microevolution or transgenerational effects. We found that, compared to tadpoles in natural ponds, urban tadpoles had higher baseline and stressed corticosterone release rates, and tadpoles in agricultural ponds had similar corticosterone release rates but greater stress-induced change, indicating stronger stress responses in both types of anthropogenic habitats. As predicted, tadpoles in both agricultural and urban ponds showed more efficient negative feedback than did tadpoles in natural ponds. Water pollution levels, as indicated by the concentrations of carbamazepine and corticoid-disrupting compounds in pond water, contributed to elevating the stress response regardless of land use. Infection by neither Batrachochytrium dendrobatidis nor Ranavirus was detected in free-living tadpoles. No habitat-related glucocorticoid differences persisted in the common-garden experiment. These results suggest that toad tadpoles in anthropogenic habitats increased their glucocorticoid flexibility via phenotypic plasticity. The coupling of stronger stress response and stronger negative feedback in these habitats supports the importance of rapidly "turning on and off" the stress response as a mechanism for coping with anthropogenic environmental change.

Excreted testosterone and male sexual proceptivity: A hormone validation and proof-of-concept experiment in túngara frogs

Conventional methods for sampling hormones often preclude strong inference experimental designs, including repeated measures of both hormones and behavior and balanced or simultaneous designs for hormone-behavior sampling. In amphibians there is an opportunity to non-invasively and repeatedly sample excreted steroids in the water. We examined testosterone (T) in túngara frogs (Physalaemus (=Engystomops) pustulosus) using minimally invasive water-borne methods. First, we validated procedures for the collection, extraction and measurement of T in adult males and females using pharmacological challenges coupled with estimates of parallelism and recovery determination. Next, we extended the timeline of sampling over 9 days in order to evaluate the kinetics of excretion (baseline phase, challenge phase, recovery phase), including the estimation of individual differences during baseline sampling. We also estimated concentrations of creatinine (Cr) in each water sample and evaluated whether correcting for this proxy of urine concentration significantly decreased error variance in T estimates. Lastly, we incorporated a standardized and repeated measures assay of male sexual proceptivity (phonotaxis) during the predicted peak T and recovery T timepoints. We found strong evidence supporting the utility of these methods for precise, biologically informative estimates of T in both sexes. Males had higher T than females and responded to pharmacological challenges by elevating T substantially within 48 h of challenge (hCG, GnRH). Males exhibited repeatability in baseline T and phonotaxis frequencies were positively associated with higher T. Adjusting T levels for the simultaneous measure of Cr significantly improved model fit, which in conjunction with marked variation in urine concentration, suggests that urine likely serves as the major source of excreted T. In summary, this proof-of-concept and methods study demonstrates the utility and accuracy of measuring water-borne T and behavior in amphibians.

Changes in physiology and microbial diversity in larval ornate chorus frogs are associated with habitat quality

Environmental change associated with anthropogenic disturbance can lower habitat quality, especially for sensitive species such as many amphibians. Variation in environmental quality may affect an organism's physiological health and, ultimately, survival and fitness. Using multiple health measures can aid in identifying populations at increased risk of declines. Our objective was to measure environmental variables at multiple spatial scales and their effect on three indicators of health in ornate chorus frog (Pseudacris ornata) tadpoles to identify potential correlates of population declines. To accomplish this, we measured a glucocorticoid hormone (corticosterone; CORT) profile associated with the stress response, as well as the skin mucosal immune function (combined function of skin secretions and skin bacterial community) and bacterial communities of tadpoles from multiple ponds. We found that water quality characteristics associated with environmental variation, including higher water temperature, conductivity and total dissolved solids, as well as percent developed land nearby, were associated with elevated CORT release rates. However, mucosal immune function, although highly variable, was not significantly associated with water quality or environmental factors. Finally, we examined skin bacterial diversity as it aids in immunity and is affected by environmental variation. We found that skin bacterial diversity differed between ponds and was affected by land cover type, canopy cover and pond proximity. Our results indicate that both local water quality and land cover characteristics are important determinants of population health for ornate chorus frogs. Moreover, using these proactive measures of health over time may aid in early identification of at-risk populations that could prevent further declines and aid in management decisions.

Batrachochytrium salamandrivorans ELICITS ACUTE STRESS RESPONSE IN SPOTTED SALAMANDERS BUT NOT INFECTION OR MORTALITY

The emerging fungal pathogen Batrachochytrium salamandrivorans (Bsal) is a major threat to amphibian species worldwide with potential to infect many species if it invades salamander biodiversity hotspots in the Americas. Bsal can cause the disease chytridiomycosis, and it is important to assess the risk of Bsal-induced chytridiomycosis to species in North America. We evaluated the susceptibility to Bsal of the common and widespread spotted salamander, Ambystoma maculatum, across life history stages and monitored the effect of Bsal exposure on growth rate and response of the stress hormone, corticosterone. We conclude that spotted salamanders appear resistant to Bsal because they showed no indication of disease or infection, and experienced minor effects on growth upon exposure. While we focused on a single population for this study, results were consistent across conditions of exposure including high or repeated doses of Bsal, life-stage at exposure, environmental conditions including two temperatures and two substrates, and promoting pathogen infectivity by conditioning Bsal cultures with thyroid hormone. Exposure to high levels of Bsal elicited an acute but not chronic increase in corticosterone in spotted salamanders, and reduced growth. We hypothesize that the early acute increase in corticosterone facilitated mounting an immune response to the pathogen, perhaps through immunoredistribution to the skin, but further study is needed to determine immune responses to Bsal. These results will contribute to development of appropriate Bsal management plans to conserve species at risk of emerging disease.

Relationships between glucocorticoids and infection with Batrachochytrium dendrobatidis in three amphibian species

It is often hypothesized that organisms exposed to environmental change may experience physiological stress, which could reduce individual quality and make them more susceptible to disease. Amphibians are amongst the most threatened taxa, particularly in the context of disease, but relatively few studies explore links between stress and disease in amphibian species. Here, we use the fungal pathogen Batrachochytrium dendrobatidis (Bd) and amphibians as an example to explore relationships between disease and glucocorticoids (GCs), metabolic hormones that comprise one important component of the stress response. While previous work is limited, it has largely identified positive relationships between GCs and Bd-infection. However, the causality remains unclear and few studies have integrated both baseline (GC release that is related to standard, physiological functioning) and stress-induced (GC release in response to an acute stressor) measures of GCs. Here, we examine salivary corticosterone before and after exposure to a stressor, in both field and captive settings. We present results for Bd-infected and uninfected individuals of three amphibian species with differential susceptibilities to this pathogen (Rana catesbeiana, R. clamitans, and R. sylvatica). We hypothesized that prior to stress, baseline GCs would be higher in Bd-infected animals, particularly in more Bd-susceptible species. We also expected that after exposure to a stressor, stress-induced GCs would be lower in Bd-infected animals. These species exhibited significant interspecific differences in baseline and stress induced corticosterone, though other variables like sex, body size, and day of year were usually not predictive of corticosterone. In contrast to most previous work, we found no relationships between Bd and corticosterone for two species (R. catesbeiana and R. clamitans), and in the least Bd-tolerant species (R. sylvatica) animals exhibited context-dependent differences in relationships between Bd infection and corticosterone: Bd-positive R. sylvatica had significantly lower baseline and stress-induced corticosterone, with this pattern being stronger in the field than in captivity. These results were surprising, as past work in other species has more often found elevated GCs in Bd-positive animals, a pattern that aligns with well-documented relationships between chronically high GCs, reduced individual quality, and immunosuppression. This work highlights the potential relevance of GCs to disease susceptibility in the context of amphibian declines, while underscoring the importance of characterizing these relationships in diverse contexts.

Distinct effects of acute versus chronic corticosterone exposure on Zebra finch responses to West Nile virus

Wild animals are exposed to both short- (acute) and long-term (chronic) stressors. The glucocorticoid hormones, such as corticosterone (CORT), facilitate coping with such stressors, but these hormones can have quite distinct effects contingent on the duration of their elevation. Previously, we found that experimental elevation of CORT for 2 days (via implantation) affected zebra finch (Taeniopygia guttata) responses to West Nile virus (WNV). CORT-elevated birds had higher viremia for at least 2 days longer than controls, and West Nile virus (WNV)-associated mortality occurred only in CORT-elevated birds. Here, we queried how acute elevations of CORT, via injection an hour prior to WNV exposure, would affect host responses, as short-term CORT elevations can be protective in other species. Although CORT injections and implantations elevated circulating CORT to a similar degree, the type of CORT exposure had quite distinct effects on WNV responses. CORT-implanted individuals reached higher viremia and suffered more mortality to WNV than control and CORT-injected individuals. However, CORT-implanted birds maintained body mass better during infection than the other two groups. Our results further support the possibility that chronic physiological stress affects aspects of host competence and potentially community-level WNV disease dynamics.

Water-borne and plasma corticosterone are not correlated in spotted salamanders

Water-borne hormone measurement is a noninvasive method suitable for amphibians of all sizes that are otherwise difficult to sample. For this method, containment-water is assayed for hormones released by the animal. Originally developed in fish, the method has expanded to amphibians, but requires additional species-specific validations. We wanted to determine physiological relevance of water-borne corticosterone in spotted salamanders (Ambystoma maculatum) by comparing concentrations to those taken using established corticosterone sampling methods, such as plasma. Using a mixture of field and laboratory studies, we compared water-borne corticosterone levels to other traditional methods of sampling corticosterone for spotted salamander larvae, metamorphs, and adults. Despite multiple attempts, and detecting differences between age groups, we found no correlations between water-borne and plasma corticosterone levels in any age group. Water-borne sampling measures a rate of release; whereas plasma is the concentration circulating in the blood. The unique units of measurement may inherently prevent correlations between the two. These two methods may also require different interpretations of the data and the physiological meaning. We also note caveats with the method, including how to account for differences in body size and life history stages. Collectively, our results illustrate the importance of careful validation of water-borne hormone levels in each species in order to understand its physiological significance.

Steroid hormones in hair reveal sexual maturity and competition in wild house mice (Mus musculus domesticus)

Endocrine data from wild populations provide important insight into social systems. However, obtaining samples for traditional methods involves capture and restraint of animals, and/or pain, which can influence the animal’s stress level, and thereby undesirable release of hormones. Here, we measured corticosterone, testosterone and progesterone in the hair of 482 wild-derived house mice that experienced sexual competition while living under semi-natural conditions. We tested whether sex, age, weight and indicators of sexual maturity, reproduction and social conflicts predict hormone concentrations measured in hair (sampling at endpoint). We show that body weight, sex and age significantly predict cumulative testosterone and progesterone levels, allowing the differentiation between subadults and adults in both sexes. Corticosterone was only slightly elevated in older males compared to older females and increased with the level of visible injuries or scars. Testosterone in males positively correlated with body weight, age, testes size, and sperm number. Progesterone in females significantly increased with age, body weight, and the number of embryos implanted throughout life, but not with the number of litters when controlled for age and weight. Our results highlight the biological validity of hair steroid measurements and provide important insight into reproductive competition in wild house mice.

Effect of Individual and Combined Treatments of Pesticide, Fertilizer, and Salt on Growth and Corticosterone Levels of Larval Southern Leopard Frogs (Lithobates sphenocephala)

Human activities have introduced a variety of chemicals, including pesticides, fertilizers, and salt, into the environment, which may have deleterious effects on the organisms inhabiting these areas. Amphibians are especially susceptible to absorption of chemical pollutants. To determine the possible combined effects of these chemicals on amphibian development and stress levels, Southern leopard frog (Lithobates sphenocephala) larvae were exposed to one of eight individual or combined treatments of atrazine, ammonium nitrate fertilizer, and sodium chloride salt. Stress levels, indicated by release of the stress hormone corticosterone, were measured premetamorphosis at week 8 of development. Water hormone samples were processed to analyze corticosterone levels. Changes in tadpole growth were determined by surface area measurements taken from biweekly photographs. The combined chemical treatment of atrazine, salt, and fertilizer had a significant interactive effect by increasing stress levels before metamorphosis (p = 0.003). After a month of larval development, tadpoles exposed to ammonium nitrate had larger surface area (p = 0.035). Tadpoles exposed to atrazine had a lower growth rate throughout larval development (p = 0.025) and the lowest number of individuals reaching metamorphosis at 33%. However, the frogs in the atrazine treatment that did successfully metamorphose did so in fewer days (p = 0.002). Because amphibians are exposed to multiple chemicals simultaneously in the environment, assessing the effects of a combination of contaminants is necessary to improve application strategies and ecosystem health.

Disease and the Drying Pond: Examining Possible Links among Drought, Immune Function, and Disease Development in Amphibians

Drought can heavily impact aquatic ecosystems. For amphibian species that rely on water availability for larval development, drought can have direct and indirect effects on larval survival and postmetamorphic fitness. Some amphibian species can accelerate the timing of metamorphosis to escape drying habitats through developmental plasticity. However, trade-offs associated with premature metamorphosis, such as reduced body size and altered immune function in the recently metamorphosed individual, may have downstream effects on susceptibility to disease. Here, we review the physiological mechanisms driving patterns in larval amphibian development under low water conditions. Specifically, we discuss drought-induced accelerated metamorphosis and how it may alter immune function, predisposing juvenile amphibians to infectious disease. In addition, we consider how these physiological and immunological adjustments could play out in a lethal disease system, amphibian chytridiomycosis. Last, we propose avenues for future research that adopt an ecoimmunological approach to evaluate the combined threats of drought and disease for amphibian populations.

Out in the cold and sick: Low temperatures and fungal infections impair a frog's skin defenses

Amphibians worldwide continue to battle an emerging infectious disease, chytridiomycosis, caused by Batrachochytrium dendrobatidis (Bd). Southern leopard frogs, Rana sphenocephala, are known to become infected with this pathogen, yet they are considered ‘of least concern’ for declines due to chytridiomycosis. Previous studies have shown that R. sphenocephala secretes four antimicrobial peptides (AMPs) onto their skin which may play an important role in limiting susceptibility to chytridiomycosis. Here we examined the (1) effects of temperature and AMP depletion on infections with Bd and (2) effects of temperature and Bd infection on the capacity to secrete AMPs in juvenile leopard frogs. Pathogen burden and mortality were greater in frogs exposed to Bd at low temperature but did not increase following monthly AMP depletion. Both low temperature and Bd exposure reduced the capacity of juvenile frogs to restore peptides after monthly depletions. Frogs held at 14°C were poorly able to restore peptides in comparison with those at 26 °C. Frogs held at 26 °C were better able to restore their peptides, but when exposed to Bd, this capacity was significantly reduced. These results strongly support the hypothesis that both colder temperatures and Bd infections impair the capacity of juvenile frogs to produce and secrete AMPs, an important component of their innate defense against chytrid fungi and other pathogens. Thus, in the face of unpredictable climate changes and enzootic pathogens, assessments of disease risk should consider the potential for effects of environmental variation and pathogen exposure on the quality of host defenses.

Review of the Amphibian Immune Response to Chytridiomycosis, and Future Directions

The fungal skin disease, chytridiomycosis (caused by Batrachochytrium dendrobatidis and B. salamandrivorans), has caused amphibian declines and extinctions globally since its emergence. Characterizing the host immune response to chytridiomycosis has been a focus of study with the aim of disease mitigation. However, many aspects of the innate and adaptive arms of this response are still poorly understood, likely due to the wide range of species' responses to infection. In this paper we provide an overview of expected immunological responses (with inference based on amphibian and mammalian immunology), together with a synthesis of current knowledge about these responses for the amphibian-chytridiomycosis system. We structure our review around four key immune stages: (1) the naïve immunocompetent state, (2) immune defenses that are always present (constitutive defenses), (3) mechanisms for recognition of a pathogen threat and innate immune defenses, and (4) adaptive immune responses. We also evaluate the current hot topics of immunosuppression and immunopathology in chytridiomycosis, and discuss their respective roles in pathogenesis. Our synthesis reveals that susceptibility to chytridiomycosis is likely to be multifactorial. Susceptible amphibians appear to have ineffective constitutive and innate defenses, and a late-stage response characterized by immunopathology and Bd-induced suppression of lymphocyte responses. Overall, we identify substantial gaps in current knowledge, particularly concerning the entire innate immune response (mechanisms of initial pathogen detection and possible immunoevasion by Bd, degree of activation and efficacy of the innate immune response, the unexpected absence of innate leukocyte infiltration, and the cause and role of late-stage immunopathology in pathogenesis). There are also gaps concerning most of the adaptive immune system (the relative importance of B and T cell responses for pathogen clearance, the capacity and extent of immunological memory, and specific mechanisms of pathogen-induced immunosuppression). Improving our capacity for amphibian immunological research will require selection of an appropriate Bd-susceptible model species, the development of taxon-specific affinity reagents and cell lines for functional assays, and the application of a suite of conventional and emerging immunological methods. Despite current knowledge gaps, immunological research remains a promising avenue for amphibian conservation management.

Assay validation and interspecific comparison of salivary glucocorticoids in three amphibian species

Amphibians are one of the most threatened groups of species, facing stressors ranging from habitat degradation and pollution to disease and overexploitation. Stress hormones (glucocorticoids, GCs) provide one quantitative metric of stress, and developing non-invasive methods for measuring GCs in amphibians would clarify how diverse environmental stressors impact individual health in this taxonomic group. Saliva is an advantageous matrix for quantifying GCs, as it is sampled less invasively than plasma while still detecting both baseline and acute elevation of GCs within a short timeframe. Little work has employed this method in amphibian species, and it has never been pharmacologically and biologically validated. Here, we conduct analytical, pharmacological and biological validation experiments for measuring salivary corticosterone in three amphibian species: the American bullfrog (Rana catesbeiana), the green frog (Rana clamitans) and the northern leopard frog (Rana pipiens). These species are faced with a broad range of environmental challenges, and in part of its range R. pipiens populations are currently in decline. In addition to demonstrating that this method can be reliably used in multiple amphibian species, we present an examination of intrinsic biological factors (sex, body condition) that may contribute to GC secretion, and a demonstration that saliva can be collected from free-living animals in the field to quantify corticosterone. Our findings suggest that saliva may be useful for less invasively quantifying GCs in many amphibian species.

Effects of transportation, transport medium and re-housing on Xenopus laevis (Daudin)

Understanding the immediate and longer-term effects of transportation and re-housing in a laboratory species is crucial in order to refine the transfer process, enable the optimal introduction of new animals to a novel environment and to provide a sufficient acclimatisation period before usage. Whilst consideration of animal welfare in most model vertebrate species has received attention, little quantitative evidence exists for the optimal care of the common laboratory amphibian Xenopus laevis. Techniques for the non-invasive welfare assessment of amphibians are also limited and here a non-invasive physiological assay was developed to investigate the impacts of transportation, transport medium and re-housing on X. laevis. First the impacts of transportation and transport medium (water, damp sponge or damp sphagnum moss) were investigated. Transportation caused an increase in water-borne corticosterone regardless of transport medium. Frogs transported in damp sphagnum moss also had a greater decrease in body mass in comparison to frogs not transported, suggesting that this is the least suitable transport medium for X. laevis. Next the prolonged impacts of transportation and re-housing were investigated. Frogs were transported between research facilities with different housing protocols. Samples were collected prior to and immediately following transportation, as well as 1 day, 7 days and 35 days after re-housing. Water-borne corticosterone increased following transportation and remained high for at least 7 days, decreasing to baseline levels by 35 days. Body mass decreased following transportation and remained lower than baseline levels across the entire 35 day observation period. These findings suggest the process of transportation and re-housing is stressful in this species. Together these findings have important relevance for both improving animal welfare and ensuring optimal and efficient scientific research.

Effects of Emerging Infectious Diseases on Amphibians: A Review of Experimental Studies

Numerous factors are contributing to the loss of biodiversity. These include complex effects of multiple abiotic and biotic stressors that may drive population losses. These losses are especially illustrated by amphibians, whose populations are declining worldwide. The causes of amphibian population declines are multifaceted and context-dependent. One major factor affecting amphibian populations is emerging infectious disease. Several pathogens and their associated diseases are especially significant contributors to amphibian population declines. These include the fungi Batrachochytrium dendrobatidis and B. salamandrivorans, and ranaviruses. In this review, we assess the effects of these three pathogens on amphibian hosts as found through experimental studies. Such studies offer valuable insights to the causal factors underpinning broad patterns reported through observational studies. We summarize key findings from experimental studies in the laboratory, in mesocosms, and from the field. We also summarize experiments that explore the interactive effects of these pathogens with other contributors of amphibian population declines. Though well-designed experimental studies are critical for understanding the impacts of disease, inconsistencies in experimental methodologies limit our ability to form comparisons and conclusions. Studies of the three pathogens we focus on show that host susceptibility varies with such factors as species, host age, life history stage, population and biotic (e.g., presence of competitors, predators) and abiotic conditions (e.g., temperature, presence of contaminants), as well as the strain and dose of the pathogen, to which hosts are exposed. Our findings suggest the importance of implementing standard protocols and reporting for experimental studies of amphibian disease.

Validation of water-borne steroid hormones in a tropical frog (Physalaemus pustulosus)

Minimally invasive methods for estimating hormone concentrations in wild vertebrates offer the opportunity to repeatedly measure behavior and hormone concentrations within individuals while minimizing experimenter interference during sample collection. We examined three steroid hormones (corticosterone, CORT; 17-β estradiol, E₂; progesterone, PROG) in túngara frogs (Physalaemus pustulosus) using non-invasive water-borne methods. Using solid-phase extraction of water samples and liquid extraction of plasma and homogenate samples, coupled with enzyme immunoassays, we complimented the conventional validation approaches (parallelism, recovery determination) with dose-response assays that incorporated pharmacological challenges with adrenocorticotropic hormone (ACTH) and human chorionic gonadotropin (HCG). We also compared steroid concentrations in water to those observed in plasma and whole body homogenates. Lastly, we identified the constituent steroids in each sample type with a panel targeting 30 steroid species using high performance liquid chromatography-mass spectrometry (HPLC-MS). We found that a 60-min water-bath captures physiologically relevant changes in concentrations of CORT, E₂ and PROG. Peak levels of water-borne CORT were found at approximately 2 h after ACTH injection. Water-borne CORT and E₂ concentrations were positively correlated with their plasma and homogenate equivalents, while water-borne PROG was uncorrelated with homogenate PROG concentrations but negatively correlated with homogenate E₂ concentrations. Together, our findings indicate that sampling water-borne hormones presents a non-invasive and biologically informative approach that will be useful for behavioral endocrinologists and conservation physiologists.

Amphibian immunity-stress, disease, and climate change

Like all other vertebrate groups, amphibian responses to the environment are mediated through the brain (hypothalamic)-pituitary-adrenal/interrenal (HPA/I) axis and the sympathetic nervous system. Amphibians are facing historically unprecedented environmental stress due to climate change that will involve unpredictable temperature and rainfall regimes and possible nutritional deficits due to extremes of temperature and drought. At the same time, amphibians in all parts of the world are experiencing unprecedented declines due to the emerging diseases, chytridiomycosis (caused by Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans) and ranavirus diseases due to viruses of the genus Ranavirus in the family Iridoviridae. Other pathogens and parasites also afflict amphibians, but here I will limit myself to a review of recent literature linking stress and these emerging diseases (chytridiomycosis and ranavirus disease) in order to better predict how environmental stressors and disease will affect global amphibian populations.

Differences in chytridiomycosis infection costs between two amphibian species from Central Europe

Batrachochytrium dendrobatidis (Bd) causes the disease chytridiomycosis associated with amphibian declines. Response and costs of infection varies greatly between species. Bd can induce a stress response in amphibians resulting in elevated corticosterone (CORT). We exposed Bombina variegata and Hyla arborea tadpoles to Bd+ or Bd- Salamandra salamandra larvae and measured CORT release rates, Bd infection loads, and survival through metamorphosis. Tadpoles of both species exposed to Bd+ larvae had elevated CORT release rates compared to tadpoles exposed to Bd- larvae. Bombina variegata appear less resistant to infection than H. arborea, showing higher Bd loads and more infected individuals. Within species, we did not find differences in cost of infection on survival, however more B. variegata tadpoles reached metamorphosis than H. arborea. The differences in resistance may be species specific, owing to higher immunity defenses with H. arborea having higher overall CORT release rates, and differences in antimicrobial peptides, or to differences in Bd strain or other unexplored mechanisms.

Exposure to Corticosterone Affects Host Resistance, but Not Tolerance, to an Emerging Fungal Pathogen

Host responses to pathogens include defenses that reduce infection burden (i.e., resistance) and traits that reduce the fitness consequences of an infection (i.e., tolerance). Resistance and tolerance are affected by an organism's physiological status. Corticosterone (“CORT”) is a hormone that is associated with the regulation of many physiological processes, including metabolism and reproduction. Because of its role in the stress response, CORT is also considered the primary vertebrate stress hormone. When secreted at high levels, CORT is generally thought to be immunosuppressive. Despite the known association between stress and disease resistance in domesticated organisms, it is unclear whether these associations are ecologically and evolutionary relevant in wildlife species. We conducted a 3x3 fully crossed experiment in which we exposed American toads (Anaxyrus [Bufo] americanus) to one of three levels of exogenous CORT (no CORT, low CORT, or high CORT) and then to either low or high doses of the pathogenic chytrid fungus Batrachochytrium dendrobatidis (“Bd”) or a sham exposure treatment. We assessed Bd infection levels and tested how CORT and Bd affected toad resistance, tolerance, and mortality. Exposure to the high CORT treatment significantly elevated CORT release in toads; however, there was no difference between toads given no CORT or low CORT. Exposure to CORT and Bd each increased toad mortality, but they did not interact to affect mortality. Toads that were exposed to CORT had higher Bd resistance than toads exposed to ethanol controls/low CORT, a pattern opposite that of most studies on domesticated animals. Exposure to CORT did not affect toad tolerance to Bd. Collectively, these results show that physiological stressors can alter a host’s response to a pathogen, but that the outcome might not be straightforward. Future studies that inhibit CORT secretion are needed to better our understanding of the relationship between stress physiology and disease resistance and tolerance in wild vertebrates.

Assessing the ability of swab data to determine the true burden of infection for the amphibian pathogen Batrachochytrium dendrobatidis

Batrachochytrium dendrobatidis (Bd) is a pathogenic fungus which causes the disease chytridiomycosis in amphibians by infecting the animals' epidermis. The most commonly applied method for the detection of Bd is the use of a sterile swab, rubbed over the keratinized areas of an amphibian and then processed to yield DNA for detection by qPCR. This method has been used to infer a threshold of lethal infection in some species; however, how reliable and reproducible the swabbing method is at detecting the true burden of infection suffered by individuals is not known. European midwife toads, Alytes obstetricans, are susceptible to chytridiomycosis and are highly parasitised by Bd across Europe. By quantifying Bd-load throughout the entire skin and comparing this to swab results taken from the same individual, we determined whether epidermal swabs provide a quantifiable and accurate indication of the true fungal burden suffered. Further, we examined whether we could infer a threshold for lethal infection based on comparison of swab data taken from infected A. obstetricans exhibiting different clinical states. From swab data, we detected significantly higher fungal burdens from moribund metamorphs compared to visually healthy individuals; however, the ability of these swab data to provide an accurate indication of the true fungal burden was not reliable. These data suggest that fungal load dynamics play an important role in disease-induced mortality in A. obstetricans at these sites, but that using swab data to infer an exact threshold for Bd-associated mortality might be inappropriate and misleading.