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

Developmental plasticity to pond drying has carryover costs on metamorph performance

Increasing variable hydroperiods may leave ectotherms with complex life cycles more vulnerable to the impacts of environmental drying. While developmental plasticity may enable some species to escape drying ponds, this plasticity might result in trade-offs with performance and subsequent fitness in adults. Here, we used rice paddy frogs (Fejervarya limnocharis) to test how pond drying influences the developmental plasticity of tadpoles, and the resulting carryover effects on body size and jumping performance. We predicted that tadpoles under simulated drought conditions (2-0.25 cm depth) compared to low stable water level conditions (0.25 cm depth) would develop faster, and the resulting metamorphs would be smaller and exhibit lower jumping performance. We show that tadpoles in drying conditions had a faster developmental rate than tadpoles in stable low water level treatments. The size of metamorphs from the drying treatment was similar to the high-water treatments (2 cm depth), but maximum jumping distance of individuals from the drying condition was lower than that of the high-water treatment. These results indicate that drying conditions for F. limnocharis increase development rate without a reduction in size at metamorphosis, but with poorer mass-independent locomotor performance, which can potentially impact their survival.

Framework for multi-stressor physiological response evaluation in amphibian risk assessment and conservation

Controlled laboratory experiments are often performed on amphibians to establish causality between stressor presence and an adverse outcome. However, in the field, identification of lab-generated biomarkers from single stressors and the interactions of multiple impacts are difficult to discern in an ecological context. The ubiquity of some pesticides and anthropogenic contaminants results in potentially cryptic sublethal effects or synergistic effects among multiple stressors. Although biochemical pathways regulating physiological responses to toxic stressors are often well-conserved among vertebrates, different exposure regimes and life stage vulnerabilities can yield variable ecological risk among species. Here we examine stress-related biomarkers, highlight endpoints commonly linked to apical effects, and discuss differences in ontogeny and ecology that could limit interpretation of biomarkers across species. Further we identify promising field-based physiological measures indicative of potential impacts to health and development of amphibians that could be useful to anuran conservation. We outline the physiological responses to common stressors in the context of altered functional pathways, presenting useful stage-specific endpoints for anuran species, and discussing multi-stressor vulnerability in the larger framework of amphibian life history and ecology. This overview identifies points of physiological, ecological, and demographic vulnerability to provide context in evaluating the multiple stressors impacting amphibian populations worldwide for strategic conservation planning.

Spatio-temporal variability in transmission risk of human schistosomes and animal trematodes in a seasonally desiccating East African landscape

Different populations of hosts and parasites experience distinct seasonality in environmental factors, depending on local-scale biotic and abiotic factors. This can lead to highly heterogeneous disease outcomes across host ranges. Variable seasonality characterizes urogenital schistosomiasis, a neglected tropical disease caused by parasitic trematodes (Schistosoma haematobium). Their intermediate hosts are aquatic Bulinus snails that are highly adapted to extreme rainfall seasonality, undergoing prolonged dormancy yearly. While Bulinus snails have a remarkable capacity for rebounding following dormancy, we investigated the extent to which parasite survival within snails is diminished. We conducted an investigation of seasonal snail schistosome dynamics in 109 ponds of variable ephemerality in Tanzania from August 2021 to July 2022. First, we found that ponds have two synchronized peaks of schistosome infection prevalence and observed cercariae, though of lower magnitude in the fully desiccating than non-desiccating ponds. Second, we evaluated total yearly schistosome prevalence across an ephemerality gradient, finding ponds with intermediate ephemerality to have the highest infection rates. We also investigated dynamics of non-schistosome trematodes, which lacked synonymity with schistosome patterns. We found peak schistosome transmission risk at intermediate pond ephemerality, thus the impacts of anticipated increases in landscape desiccation could result in increases or decreases in transmission risk with global change.

Heat stress and amphibian immunity in a time of climate change

As a class of vertebrates, amphibians, are at greater risk for declines or extinctions than any other vertebrate group, including birds and mammals. There are many threats, including habitat destruction, invasive species, overuse by humans, toxic chemicals and emerging diseases. Climate change which brings unpredictable temperature changes and rainfall constitutes an additional threat. Survival of amphibians depends on immune defences functioning well under these combined threats. Here, we review the current state of knowledge of how amphibians respond to some natural stressors, including heat and desiccation stress, and the limited studies of the immune defences under these stressful conditions. In general, the current studies suggest that desiccation and heat stress can activate the hypothalamus pituitary-interrenal axis, with possible suppression of some innate and lymphocyte-mediated responses. Elevated temperatures can alter microbial communities in amphibian skin and gut, resulting in possible dysbiosis that fosters reduced resistance to pathogens. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Towards the generation of gnotobiotic larvae as a tool to investigate the influence of the microbiome on the development of the amphibian immune system

The immune equilibrium model suggests that exposure to microbes during early life primes immune responses for pathogen exposure later in life. While recent studies using a range of gnotobiotic (germ-free) model organisms offer support for this theory, we currently lack a tractable model system for investigating the influence of the microbiome on immune system development. Here, we used an amphibian species (Xenopus laevis) to investigate the importance of the microbiome in larval development and susceptibility to infectious disease later in life. We found that experimental reductions of the microbiome during embryonic and larval stages effectively reduced microbial richness, diversity and altered community composition in tadpoles prior to metamorphosis. In addition, our antimicrobial treatments resulted in few negative effects on larval development, body condition, or survival to metamorphosis. However, contrary to our predictions, our antimicrobial treatments did not alter susceptibility to the lethal fungal pathogen Batrachochytrium dendrobatidis (Bd) in the adult life stage. While our treatments to reduce the microbiome during early development did not play a critical role in determining susceptibility to disease caused by Bd in X. laevis, they nevertheless indicate that developing a gnotobiotic amphibian model system may be highly useful for future immunological investigations. This article is part of the theme issue 'Amphibian immunity: stress, disease and ecoimmunology'.

Diet- and salinity-induced modifications of the gut microbiota are associated with differential physiological responses to ranavirus infection in Rana sylvatica

Greater knowledge of how host-microbiome interactions vary with anthropogenic environmental change and influence pathogenic infections is needed to better understand stress-mediated disease outcomes. We investigated how increasing salinization in freshwaters (e.g. due to road de-icing salt runoff) and associated increases in growth of nutritional algae influenced gut bacterial assembly, host physiology and responses to ranavirus exposure in larval wood frogs (Rana sylvatica). Elevating salinity and supplementing a basic larval diet with algae increased larval growth and also increased ranavirus loads. However, larvae given algae did not exhibit elevated kidney corticosterone levels, accelerated development or weight loss post-infection, whereas larvae fed a basic diet did. Thus, algal supplementation reversed a potentially maladaptive stress response to infection observed in prior studies in this system. Algae supplementation also reduced gut bacterial diversity. Notably, we observed higher relative abundances of Firmicutes in treatments with algae-a pattern consistent with increased growth and fat deposition in mammals-that may contribute to the diminished stress responses to infection via regulation of host metabolism and endocrine function. Our study informs mechanistic hypotheses about the role of microbiome mediation of host responses to infection that can be tested in future experiments in this host-pathogen system. 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'.

Spatiotemporal variability in transmission risk of human schistosomes and animal trematodes in a seasonally desiccating East African landscape

Different populations of hosts and parasites experience distinct seasonality in environmental factors, depending on local-scale biotic and abiotic factors. This can lead to highly heterogenous disease outcomes across host ranges. Variable seasonality characterizes urogenital schistosomiasis, a neglected tropical disease caused by parasitic trematodes (Schistosoma haematobium). Their intermediate hosts are aquatic Bulinus snails that are highly adapted to extreme rainfall seasonality, undergoing dormancy for up to seven months yearly. While Bulinus snails have a remarkable capacity for rebounding following dormancy, parasite survival within snails is greatly diminished. We conducted a year-round investigation of seasonal snail-schistosome dynamics in 109 ponds of variable ephemerality in Tanzania. First, we found that ponds have two synchronized peaks of schistosome infection prevalence and cercariae release, though of lower magnitude in the fully desiccating ponds than non-desiccating ponds. Second, we evaluated total yearly prevalence across a gradient of an ephemerality, finding ponds with intermediate ephemerality to have the highest infection rates. We also investigated dynamics of non-schistosome trematodes, which lacked synonymity with schistosome patterns. We found peak schistosome transmission risk at intermediate pond ephemerality, thus the impacts of anticipated increases in landscape desiccation could result in increases or decreases in transmission risk with global change.

Temperature and duration of exposure drive infection intensity with the amphibian pathogen Batrachochytrium dendrobatidis

The intensity of a pathogen infection plays a key role in determining how the host responds to infection. Hosts with high infections are more likely to transmit infection to others, and are may be more likely to experience progression from infection to disease symptoms, to being physiologically compromised by disease. Understanding how and why hosts exhibit variation in infection intensity therefore plays a major part in developing and implementing measures aimed at controlling infection spread, its effects, and its chance of persisting and circulating within a population of hosts. To track the relative importance of a number of variables in determining the level of infection intensity, we ran field-surveys at two breeding sites over a 12 month period using marked larvae of the common midwife toad (Alyes obstetricans) and their levels of infection with the amphibian pathogen Batrachochytrium dendrobatidis (Bd). At each sampling occasion we measured the density of larvae, the temperature of the water in the 48 h prior to sampling, the period of time the sampled individual had been in the water body, the developmental (Gosner) stage and the intensity of Bd infection of the individual. Overall our data suggest that the temperature and the duration of time spent in the water play a major role in determining the intensity of Bd infection within an individual host. However, although the duration of time spent in the water was clearly associated with infection intensity, the relationship was negative: larvae that had spent less than 3-6 months in the water had significantly higher infection intensities than those that had spent over 12 months, although this infection intensity peaked between 9 and 12 months. This could be due to animals with heavier infections developing more quickly, suffering increased mortality or, more likely, losing their mouthparts (the only part of anuran larvae that can be infected with Bd). Overall, our results identify drivers of infection intensity, and potentially transmissibility and spread, and we attribute these differences to both host and pathogen biology.

Carry-Over Effects of Desiccation Stress on the Oxidative Status of Fasting Anuran Juveniles

Amphibians are sensitive to deteriorating environmental conditions, especially during transition to a terrestrial environment which is full of uncertainties. Harsh conditions, such as desiccation during earlier stages, affect different larval traits with possible carry-over effects on juvenile and adult life histories. The first consequences of the effects can be seen in juveniles in the challenges to find food and the ability to survive without it in a terrestrial habitat. Body size and the internal energy reserves acquired during the larval phase play an important role in this period. Herein, we tested how different water regimes (low water availability, desiccation and constant high-water availability) during larval development reflect on the oxidative status and ability of yellow belly toad (Bombina variegata) juveniles to endure short-term fasting. The desiccation regime significantly reduced the body size of metamorphs. The same was observed after 2 weeks of fasting, while the feeding treatment reduced differences mostly in the body mass of individuals from different water regimes. This was the result of a greater gain in mass in juveniles pre-exposed to desiccation. Pre-exposure to desiccation also modified the parameters of the antioxidant system (AOS) under feeding conditions, leading to higher values of superoxide dismutase, glutathione reductase and glutathione S-transferase, glutathione and sulfhydryl group concentrations, and lower glutathione peroxidase in comparison to juveniles reared under constant water. The increase in the AOS of juveniles can be considered as a physiological carry-over effect of desiccation, probably as the result of compensatory growth and/or earlier exposure to chronic stress. However, water levels during larval development did not exert significant effects on the oxidative status of juveniles subjected to food unavailability. Fasting juveniles, both control and desiccated, were exposed to oxidative stress, significantly higher lipid peroxide concentrations, lower superoxide dismutase, glutathione peroxidase, glutathione S-transferase, glutathione and sulfhydryl group values in comparison to feeding individuals. The lack of food in juvenile anurans activated the AOS response in the same manner, regardless of body size and stress pre-exposure, suggesting that the generally accepted hypothesis about the influence of metamorphic body size on the fitness of the postmetamorphic stage should be tested further.

Sublethal, Behavioral, and Developmental Effects of the Neonicotinoid Pesticide Imidacloprid on Larval Wood Frogs (Rana sylvatica)

Imidacloprid, a neonicotinoid pesticide, is used to prevent the spread of the hemlock woolly adelgid, currently affecting Eastern Hemlock trees across North America. When the pesticide is sprayed directly onto soil around infested trees (soil drenching), it can run off into aquatic systems, with potential negative effects on biota. Simultaneously, climate change may lead to faster pool drying, which acts as an additional stressor for sensitive species such as amphibians. We evaluated the sublethal effects of imidacloprid (10 ppb), and interaction with shorter hydroperiods on the larval behavior, growth, and survival of a model organism, the wood frog (Rana sylvatica). We performed 3 behavioral experiments evaluating swimming speed, time spent swimming, and distance the larvae swam. We found that larvae raised in 10 ppb imidacloprid or shorter hydroperiod did not differ in their swimming time, distance, and speed from nonexposed larvae. Naïve larvae exposed for 20 min to 10- to 500-ppb concentrations also showed similar performance to nonexposed larvae. However, when we applied a stimulus halfway through each experiment, we found that larvae exposed to 10 ppb imidacloprid (short and long term) swam shorter distances and spent less time swimming, suggesting that imidacloprid exposure may slow reaction time, potentially increasing the risk of predation. To minimize impacts on pool-breeding amphibians, imidacloprid application to combat the invasive hemlock woolly adelgid should use trunk injection and avoid soil drenching. Environ Toxicol Chem 2021;40:1840-1849. © 2021 SETAC.

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.

Effects of Desiccation on Metamorphic Climax in Bombina variegata: Changes in Levels and Patterns of Oxidative Stress Parameters

Simple Summary

Global warming alters patterns of precipitation and drought, which are important factors in the survival of amphibian populations. Metamorphosis is affected by environmental changes; this is especially true of metamorphic climax, the crucial stage of amphibian development that is accompanied by significant morphological, physiological and behavioral adaptations necessary for the transition to a terrestrial habitat. This study investigated naturally occurring changes in the cellular oxidative status (antioxidant system and oxidative damage) of yellow-bellied toad larvae during this phase, and how exposure to exogenous factors such as desiccation affected them. Our results revealed clear changes in the antioxidant system’s (AOS) response and the levels of oxidative damage during metamorphic climax, with the highest response and damage observed at the end stage. Decreasing water levels during larval development altered the components of the AOS and increased oxidative damage, resulting in increased oxidative stress. The knowledge gained from this study could contribute to a better understanding of the oxidative stress that larvae experience during this critical stage of development, and the consequences of global warming—such as water loss—on amphibians.

Abstract

In this paper, we examined how the oxidative status (antioxidant system and oxidative damage) of Bombina variegata larvae changed during the metamorphic climax (Gosner stages: 42—beginning, 44—middle and 46—end) and compared the patterns and levels of oxidative stress parameters between individuals developing under constant water availability (control) and those developing under decreasing water availability (desiccation group). Our results revealed that larvae developing under decreasing water availability exhibited increased oxidative damage in the middle and end stages. This was followed by lower levels of glutathione in stages 44 and 46, as well as lower values of catalase, glutathione peroxidase, glutathione S-transferase and sulfhydryl groups in stage 46 (all in relation to control animals). Comparison between stages 42, 44 and 46 within treatments showed that individuals in the last stage demonstrated the highest intensities of lipid oxidative damage in both the control and desiccation groups. As for the parameters of the antioxidant system, control individuals displayed greater variety in response to changes induced by metamorphic climax than individuals exposed to desiccation treatment. The overall decrease in water availability during development led to increased oxidative stress and modifications in the pattern of AOS response to changes induced by metamorphic climax in larvae of B. variegata.

Applied ecoimmunology: using immunological tools to improve conservation efforts in a changing world

Ecoimmunology is a rapidly developing field that explores how the environment shapes immune function, which in turn influences host-parasite relationships and disease outcomes. Host immune defence is a key fitness determinant because it underlies the capacity of animals to resist or tolerate potential infections. Importantly, immune function can be suppressed, depressed, reconfigured or stimulated by exposure to rapidly changing environmental drivers like temperature, pollutants and food availability. Thus, hosts may experience trade-offs resulting from altered investment in immune function under environmental stressors. As such, approaches in ecoimmunology can provide powerful tools to assist in the conservation of wildlife. Here, we provide case studies that explore the diverse ways that ecoimmunology can inform and advance conservation efforts, from understanding how Galapagos finches will fare with introduced parasites, to using methods from human oncology to design vaccines against a transmissible cancer in Tasmanian devils. In addition, we discuss the future of ecoimmunology and present 10 questions that can help guide this emerging field to better inform conservation decisions and biodiversity protection. From better linking changes in immune function to disease outcomes under different environmental conditions, to understanding how individual variation contributes to disease dynamics in wild populations, there is immense potential for ecoimmunology to inform the conservation of imperilled hosts in the face of new and re-emerging pathogens, in addition to improving the detection and management of emerging potential zoonoses.

The amphibian complement system and chytridiomycosis

Understanding host immune function and ecoimmunology is increasingly important at a time when emerging infectious diseases (EIDs) threaten wildlife. One EID that has emerged and spread widely in recent years is chytridiomycosis, caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), which is implicated unprecedented amphibian declines around the world. The impacts of Bd have been severe for many amphibian species, but some populations have exhibited signs of persistence, and even recovery, in some regions. Many mechanisms may underpin this pattern and amphibian immune responses are likely one key component. Although we have made great strides in understanding amphibian immunity, the complement system remains poorly understood. The complement system is a nonspecific, innate immune defense that is known to enhance other immune responses. Complement activation can occur by three different biochemical pathways and result in protective mechanisms, such as inflammation, opsonization, and pathogen lysis, thereby providing protection to the host. We currently lack an understanding of complement pathway activation for chytridiomycosis, but several studies have suggested that it may be a key part of an early and robust immune response that confers host resistance. Here, we review the available research on the complement system in general as well as amphibian complement responses to Bd infection. Additionally, we propose future research directions that will increase our understanding of the amphibian complement system and other immune responses to Bd. Finally, we suggest how a deeper understanding of amphibian immunity could enhance the conservation and management of amphibian species that are threatened by chytridiomycosis.

High mortality in Bufo gargarizans eggs associated with an undescribed Saprolegnia ferax strain in the Republic of Korea

Pathogenic water molds have a significant impact on many species, especially amphibians. The genus Saprolegnia is a pathogenic oomycete restricted to aquatic and moist habitats, and its presence is strongly linked to the abundance of amphibians and fishes. We investigated the influence of Saprolegnia presence on egg mortality and egg occurrence under varying environmental conditions in the Asiatic toad Bufo gargarizans at 27 breeding sites in the Republic of Korea. We then assessed the impact of Saprolegnia on the presence of B. gargarizans at the 27 sites surveyed weekly during the B. gargarizans breeding season for 3 consecutive years. We used molecular tools to identify the water molds as belonging to an undescribed S. ferax strain. We demonstrated that the presence of S. ferax was positively associated with higher water conductivity and ponds. In addition, while S. ferax prevalence was associated with a reduction in B. gargarizans breeding activity and breeding success, we could not determine its impact on the subsequent breeding seasons. Our study highlights the potential negative effects of Saprolegnia on amphibian reproduction, although additional research is necessary to determine the relationship between Saprolegnia, its hosts and the impacts of habitat loss on amphibians.