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

Assay validation of saliva glucocorticoids in Columbia spotted frogs and effects of handling and marking

Non-invasive methods are important to the field of conservation physiology to reduce negative effects on organisms being studied. Glucocorticoid (GC) hormones are often used to assess health of individuals, but collection methods can be invasive. Many amphibians are imperiled worldwide, and saliva is a non- or semi-invasive matrix to measure GCs that has been partially validated for only four amphibian species. Validation ensures that assays are reliable and can detect changes in saliva corticosterone (sCORT) after exposure to stressors, but it is also necessary to ensure sCORT concentrations are correlated with plasma concentrations. To help validate the use of saliva in assessing CORT responses in amphibians, we captured uniquely marked Columbia spotted frogs (Rana luteiventris) on sequential days and collected baseline and stress-induced (after handling) samples. For a subset of individuals, we collected and quantified CORT in both saliva and blood samples, which have not been compared for amphibians. We tested several aspects of CORT responses and, by collecting across separate days, measured repeatability of CORT responses across days. We also evaluated whether methods common to amphibian conservation, such as handling alone or handling, clipping a toe and tagging elevated sCORT. Similar to previous studies, we show that sCORT is reliable concerning parallelism, recovery, precision and sensitivity. sCORT was weakly correlated with plasma CORT (R2 = 0.21), and we detected elevations in sCORT after handling, demonstrating biological validation. Toe clipping and tagging did not increase sCORT over handling alone, but repeated handling elevated sCORT for ~72 hours. However, sCORT responses were highly variable and repeatability was low within individuals and among capture sessions, contrary to previous studies with urinary and waterborne CORT. sCORT is a semi-invasive and rapid technique that could be useful to assess effects of anthropogenic change and conservation efforts, but will require careful study design and future validation.

Comparison of Corticosterone Concentrations in Dermal Secretions and Urine in Free-Ranging Marine Toads (Rhinella marina) in Human Care

Corticosterone concentrations have been measured in amphibians by collecting blood or urine samples. However, blood sampling is invasive, and urine can be difficult to collect. A novel method of swabbing the skin of an amphibian has been utilized in numerous species but has not been verified in marine toads (Rhinella marina). This pilot study tested dermal swabs as a noninvasive method for collecting and measuring dermal corticosterone secretions. Swabs were used to collect dermal secretion samples from sixty-six free-ranging marine toads collected on Zoo Miami grounds. The subsequent day the toads were shipped to the North Carolina Zoo where dermal samples were collected again. Additional dermal and urine samples were collected on days 9, 15, 32, and 62 under human care to measure corticosterone concentrations. There was no significant correlation (P ≥ 0.05) noted between corticosterone concentrations reported in dermal swabs and those in urine samples at all four of the euthanasia time points or between the corticosterone concentrations reported in either urine or dermal swabs and the weight of the toads. Dermal swab concentrations (ng/mL) were significantly higher (P ≤ 0.05) on the day of capture (0.64 ± 0.03) and the day of arrival (0.67 ± 0.03) than on day 15 (0.47 ± 0.03). The urine corticosterone concentrations decreased while the toads were in human care with a significant decrease (P ≤ 0.05) between days 9 (0.45 ± 0.07) and 32 (0.21 ± 0.06). This study demonstrated that dermal swabs can be used to collect marine toad corticosterone concentration samples.

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.

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.

Water Pollution Increases the Risk of Chytridiomycosis in Mexican Amphibians

Chytridiomycosis is affecting amphibians worldwide, causing the decline and extinction of several amphibian populations. The disease is caused by the fungus Batrachochytrium dendrobatidis (Bd), a multihost pathogen living in freshwater habitats. While several environmental factors have been associated with the prevalence of Bd and its virulence, the effects of water quality on the pathogen are not clear yet. Some evidence suggests that water pollution may reduce amphibians' immune response and increase prevalence of Bd. To explore this hypothesis, we analyzed the relationship between water quality and the presence of Bd by using spatial data mining of 150 geolocations of Bd in amphibians from 9 families where Bd positive specimens have been previously reported, and water quality in 4,202 lentic and lotic water bodies in Mexico from 2010 to 2021. Our model showed that in the 3 main families where Bd was recorded, its presence is high in locations with low water quality, i.e., water polluted likely contaminated with urban and industrial waste. Using this model, we inferred areas suitable for Bd in Mexico; mainly in poorly studied areas along the gulf and on the pacific slope. We further argue that actions to reduce water pollution should become an integral part of public policies to prevent the spread of Bd and protect amphibians from this deadly pathogen.

Host species is linked to pathogen genotype for the amphibian chytrid fungus (Batrachochytrium dendrobatidis)

Host-pathogen specificity can arise from certain selective environments mediated by both the host and pathogen. Therefore, understanding the degree to which host species identity is correlated with pathogen genotype can help reveal historical host-pathogen dynamics. One animal disease of particular concern is chytridiomycosis, typically caused by the global panzootic lineage of the amphibian chytrid fungus (Batrachochytrium dendrobatidis, Bd), termed Bd-GPL. This pathogen lineage has caused devastating declines in amphibian communities around the world. However, the site of origin for the common ancestor of modern Bd-GPL and the fine-scale transmission dynamics of this lineage have remained a mystery. This is especially the case in North America where Bd-GPL is widespread, but disease outbreaks occur sporadically. Herein, we use Bd genetic data collected throughout the United States from amphibian skin swabs and cultured isolate samples to investigate Bd genetic patterns. We highlight two case studies in Pennsylvania and Nevada where Bd-GPL genotypes are strongly correlated with host species identity. Specifically, in some localities bullfrogs (Rana catesbeiana) are infected with Bd-GPL lineages that are distinct from those infecting other sympatric amphibian species. Overall, we reveal a previously unknown association of Bd genotype with host species and identify the eastern United States as a Bd diversity hotspot and potential site of origin for Bd-GPL.

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.

Shifts in sensitivity of amphibian metamorphosis to endocrine disruption: the common frog (Rana temporaria) as a case study

Effective conservation actions require knowledge on the sensitivity of species to pollution and other anthropogenic stressors. Many of these stressors are endocrine disruptors (EDs) that can impair the hypothalamus-pituitary-thyroid axis and thus alter thyroid hormone (TH) levels with physiological consequences to wildlife. Due to their specific habitat requirements, amphibians are often sentinels of environmental degradation. We investigated how altered TH levels affected the bioenergetics of growth and development (i.e. age, size, metabolism, cardiac function and energy stores) before, during and after metamorphosis in the European common frog (Rana temporaria). We also determined how ontogenetic stage affected susceptibility to endocrine disruption and estimated juvenile performance. TH levels significantly affected growth and energetics at all developmental stages. Tadpoles and froglets exposed to high TH levels were significantly younger, smaller and lighter at all stages compared to those in control and low TH groups, indicating increased developmental and reduced growth rates. Across all ontogenetic stages tested, physiological consequences were rapidly observed after exposure to EDs. High TH increased heart rate by an average of 86% and reduced energy stores (fat content) by 33% compared to controls. Effects of exposure were smallest after the completion of metamorphosis. Our results demonstrate that both morphological and physiological traits of the European common frog are strongly impacted by endocrine disruption and that ontogenetic stage modulates the sensitivity of this species to endocrine disruption. Since endocrine disruption during metamorphosis can impair the physiological stress response in later life stages, long-term studies examining carry-over effects will be an important contribution to the conservation physiology of amphibians.

Beyond corticosterone: The acute stress response increases DNA damage in house sparrows

Although corticosterone (Cort) has been the predominant metric used to assess acute stress in birds, it does not always accurately reflect how an animal copes with a stressor. Downstream measurements may be more reliable. In the current study, we tested the hypothesis that acute increases in DNA damage could be used to assess stressor exposure. Studies have shown DNA damage increases in response to stress-related hormones in vitro; however, this has not yet been thoroughly applied in wild animals. We exposed house sparrows (Passer domesticus) to a 30- or 120-min restraint stressor and took blood samples at 0, 30, 60, and 120 min to measure Cort, DNA damage, and uric acid. Both treatments increased DNA damage and Cort, and decreased uric acid. It thus appears that DNA damage can reflect acute stressor exposure. To improve the usability of DNA damage as a metric for stress, we also tested the impacts of sample storage on DNA damage. Leaving red blood cells on ice for up to 24 hr, only slightly influenced DNA damage. Freezing blood samples for 1-4 weeks substantially increased DNA damage. These findings emphasize the importance of reducing variation between samples by assaying them together whenever possible. Overall, these results indicate that assessing DNA damage is a valid method of assessing acute stressor exposure that is suitable for both laboratory- and field-based studies; however, additional research is needed on the molecular dynamics of nucleated red blood cells, including whether and how their DNA is repaired.