The thermal mismatch hypothesis explains host susceptibility to an emerging infectious disease

Hotspot shelters stimulate frog resistance to chytridiomycosis

Many threats to biodiversity cannot be eliminated; for example, invasive pathogens may be ubiquitous. Chytridiomycosis is a fungal disease that has spread worldwide, driving at least 90 amphibian species to extinction, and severely affecting hundreds of others1-4. Once the disease spreads to a new environment, it is likely to become a permanent part of that ecosystem. To enable coexistence with chytridiomycosis in the field, we devised an intervention that exploits host defences and pathogen vulnerabilities. Here we show that sunlight-heated artificial refugia attract endangered frogs and enable body temperatures high enough to clear infections, and that having recovered in this way, frogs are subsequently resistant to chytridiomycosis even under cool conditions that are optimal for fungal growth. Our results provide a simple, inexpensive and widely applicable strategy to buffer frogs against chytridiomycosis in nature. The refugia are immediately useful for the endangered species we tested and will have broader utility for amphibian species with similar ecologies. Furthermore, our concept could be applied to other wildlife diseases in which differences in host and pathogen physiologies can be exploited. The refugia are made from cheap and readily available materials and therefore could be rapidly adopted by wildlife managers and the public. In summary, habitat protection alone cannot protect species that are affected by invasive diseases, but simple manipulations to microhabitat structure could spell the difference between the extinction and the persistence of endangered amphibians.

High temperatures reduce growth, infection, and transmission of a naturally occurring fungal plant pathogen

Climate change is rapidly altering the distribution of suitable habitats for many species as well as their pathogenic microbes. For many pathogens, including vector-borne diseases of humans and agricultural pathogens, climate change is expected to increase transmission and lead to pathogen range expansions. However, if pathogens have a lower heat tolerance than their host, increased warming could generate so-called thermal refugia for hosts. Predicting the outcomes of warming on disease transmission requires detailed knowledge of the thermal tolerances of both the host and the pathogen. Such thermal tolerance studies are generally lacking for fungal pathogens of wild plant populations, despite the fact that plants form the base of all terrestrial communities. Here, we quantified three aspects of the thermal tolerance (growth, infection, and propagule production) of the naturally occurring fungal pathogen Microbotryum lychnidis-dioicae, which causes a sterilizing anther-smut disease on the herbaceous plant Silene latifolia. We also quantified two aspects of host thermal tolerance: seedling survival and flowering rate. We found that temperatures >30°C reduced the ability of anther-smut spores to germinate, grow, and conjugate in vitro. In addition, we found that high temperatures (30°C) during or shortly after the time of inoculation strongly reduced the likelihood of infection in seedlings. Finally, we found that high summer temperatures in the field temporarily cured infected plants, likely reducing transmission. Notably, high temperatures did not reduce survival or flowering of the host plants. Taken together, our results show that the fungus is considerably more sensitive to high temperatures than its host plant. A warming climate could therefore result in reduced disease spread or even local pathogen extirpation, leading to thermal refugia for the host.

Impact of climate change and natural disasters on fungal infections

The effects of climate change and natural disasters on fungal pathogens and the risks for fungal diseases remain incompletely understood. In this literature review, we examined how fungi are adapting to an increase in the Earth's temperature and are becoming more thermotolerant, which is enhancing fungal fitness and virulence. Climate change is creating conditions conducive to the emergence of new fungal pathogens and is priming fungi to adapt to previously inhospitable environments, such as polluted habitats and urban areas, leading to the geographical spread of some fungi to traditionally non-endemic areas. Climate change is also contributing to increases in the frequency and severity of natural disasters, which can trigger outbreaks of fungal diseases and increase the spread of fungal pathogens. The populations mostly affected are the socially vulnerable. More awareness, research, funding, and policies on the part of key stakeholders are needed to mitigate the effects of climate change and disaster-related fungal diseases.

A framework for understanding climate change impacts through non-compensatory intra- and interspecific climate change responses

Understanding and predicting population responses to climate change is a crucial challenge. A key component of population responses to climate change are cases in which focal biological rates (e.g., population growth rates) change in response to climate change due to non-compensatory effects of changes in the underlying components (e.g., birth and death rates) determining the focal rates. We refer to these responses as non-compensatory climate change effects. As differential responses of biological rates to climate change have been documented in a variety of systems and arise at multiple levels of organization within and across species, non-compensatory effects may be nearly ubiquitous. Yet, how non-compensatory climate change responses combine and scale to influence the demographics of populations is often unclear and requires mapping them to the birth and death rates underlying population change. We provide a flexible framework for incorporating non-compensatory changes in upstream rates within and among species and mapping their consequences for additional downstream rates across scales to their eventual effects on population growth rates. Throughout, we provide specific examples and potential applications of the framework. We hope this framework helps to enhance our understanding of and unify research on population responses to climate change.

Thermal ecology shapes disease outcomes of entomopathogenic fungi infecting warm-adapted insects

The thermal environment is a critical determinant of outcomes in host-pathogen interactions, yet the complexities of this relationship remain underexplored in many ecological systems. We examined the Thermal Mismatch Hypothesis (TMH) by measuring phenotypic variation in individual thermal performance profiles using a model system of two species of entomopathogenic fungi (EPF) that differ in their ecological niche, Metarhizium brunneum and M. flavoviride, and a warm-adapted model host, the mealworm Tenebrio molitor. We conducted experiments across ecologically relevant temperatures to determine the thermal performance curves for growth and virulence, measured as % survival, identify critical thresholds for these measures, and elucidate interactive host-pathogen effects. Both EPF species and the host exhibited a shared growth optima at 28 °C, while the host's growth response was moderated in sublethal pathogen infections that depended on fungus identity and temperature. However, variances in virulence patterns were different between pathogens. The fungus M. brunneum exhibited a broader optimal temperature range (23-28 °C) for virulence than M. flavoviride, which displayed a multiphasic virulence-temperature relationship with distinct peaks at 18 and 28 °C. Contrary to predictions of the TMH, both EPF displayed peak virulence at the host's optimal temperature (28 °C). The thermal profile for M. brunneum aligned more closely with that of T. molitor than that for M. flavoviride. Moreover, the individual thermal profile of M. flavoviride closely paralleled its virulence thermal profile, whereas the virulence thermal profile of M. brunneum did not track with its individual thermal performance. This suggests an indirect, midrange (23 °C) effect, where M. brunneum virulence exceeded growth. These findings suggest that the evolutionary histories and ecological adaptations of these EPF species have produced distinct thermal niches during the host interaction. This study contributes to our understanding of thermal ecology in host-pathogen interactions, underpinning the ecological and evolutionary factors that shape infection outcomes in entomopathogenic fungi. The study has ecological implications for insect population dynamics in the face of a changing climate, as well as practically for the use of these organisms in biological control.

Widespread occurrence of the amphibian chytrid panzootic lineage in Uruguay is constrained by climate

The amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) causes chytridiomycosis, a disease among the main causes of amphibian declines worldwide. However, Bd studies on Neotropical amphibians from temperate areas are scarce. We present a comprehensive survey of Bd in Uruguay, in temperate central eastern South America, carried out between 2006 and 2014. Skin swabs of 535 specimens of 21 native and exotic frogs were tested by PCR. We used individual-level data to examine the relationship between infection, climatic variables, and their effects on body condition and the number of prey items found in stomach contents. Infection was widespread in free-ranging anurans with an overall prevalence of 41.9%, detected in 15 native species, wild American bullfrogs Aquarana catesbeiana, and captive specimens of Ceratophrys ornata and Xenopus laevis. Three haplotypes of the Bd ITS region were identified in native amphibians, all belonging to the global panzootic lineage (BdGPL), of which only one was present in exotic hosts. Despite high infection frequencies in different anurans, we found no evidence of morbidity or mortality attributable to chytridiomycosis, and we observed no discernible impact on body condition or consumed prey. Climatic conditions at the time of our surveys suggested that the chance of infection is associated with monthly mean temperature, mean humidity, and total precipitation. Temperatures below 21°C combined with moderate humidity and pronounced rainfall may increase the likelihood of infection. Multiple haplotypes of BdGPL combined with high frequencies of infection suggest an enzootic pattern in native species, underscoring the need for continued monitoring.

Warming alters life-history traits and competition in a phage community

Host-parasite interactions are highly susceptible to changes in temperature due to mismatches in species thermal responses. In nature, parasites often exist in communities, and responses to temperature are expected to vary between host-parasite pairs. Temperature change thus has consequences for both host-parasite dynamics and parasite-parasite interactions. Here, we investigate the impact of warming (37°C, 40°C, and 42°C) on parasite life-history traits and competition using the opportunistic bacterial pathogen Pseudomonas aeruginosa (host) and a panel of three genetically diverse lytic bacteriophages (parasites). We show that phages vary in their responses to temperature. While 37°C and 40°C did not have a major effect on phage infectivity, infection by two phages was restricted at 42°C. This outcome was attributed to disruption of different phage life-history traits including host attachment and replication inside hosts. Furthermore, we show that temperature mediates competition between phages by altering their competitiveness. These results highlight phage trait variation across thermal regimes with the potential to drive community dynamics. Our results have important implications for eukaryotic viromes and the design of phage cocktail therapies.IMPORTANCEMammalian hosts often elevate their body temperatures through fevers to restrict the growth of bacterial infections. However, the extent to which fever temperatures affect the communities of phages with the ability to parasitize those bacteria remains unclear. In this study, we investigate the impact of warming across a fever temperature range (37°C, 40°C, and 42°C) on phage life-history traits and competition using a bacterium (host) and bacteriophage (parasite) system. We show that phages vary in their responses to temperature due to disruption of different phage life-history traits. Furthermore, we show that temperature can alter phage competitiveness and shape phage-phage competition outcomes. These results suggest that fever temperatures have the potential to restrict phage infectivity and drive phage community dynamics. We discuss implications for the role of temperature in shaping host-parasite interactions more widely.

Interconnecting global threats: climate change, biodiversity loss, and infectious diseases

The concurrent pressures of rising global temperatures, rates and incidence of species decline, and emergence of infectious diseases represent an unprecedented planetary crisis. Intergovernmental reports have drawn focus to the escalating climate and biodiversity crises and the connections between them, but interactions among all three pressures have been largely overlooked. Non-linearities and dampening and reinforcing interactions among pressures make considering interconnections essential to anticipating planetary challenges. In this Review, we define and exemplify the causal pathways that link the three global pressures of climate change, biodiversity loss, and infectious disease. A literature assessment and case studies show that the mechanisms between certain pairs of pressures are better understood than others and that the full triad of interactions is rarely considered. Although challenges to evaluating these interactions-including a mismatch in scales, data availability, and methods-are substantial, current approaches would benefit from expanding scientific cultures to embrace interdisciplinarity and from integrating animal, human, and environmental perspectives. Considering the full suite of connections would be transformative for planetary health by identifying potential for co-benefits and mutually beneficial scenarios, and highlighting where a narrow focus on solutions to one pressure might aggravate another.

Consistent Negative Correlations between Parasite Infection and Host Body Condition Across Seasons Suggest Potential Harmful Impacts of Salmincola markewitschi on Wild White-Spotted Charr, Salvelinus leucomaenis

Assessing the impacts of parasites on wild fish populations is a fundamental and challenging aspect of the study of host-parasite relationships. Salmincola, a genus of ectoparasitic copepods, mainly infects salmonid species. This genus, which is notorious in aquaculture, damages host fishes, but its impacts under natural conditions remain largely unknown or are often considered negligible. In this study, we investigated the potential impacts of mouth-attaching Salmincola markewitschi on white-spotted charr (Salvelinus leucomaenis) through intensive field surveys across four seasons using host body condition as an indicator of harmful effects. The prevalence and parasite abundance were highest in winter and gradually decreased in summer and autumn, which might be due to host breeding and/or wintering aggregations that help parasite transmissions. Despite seasonal differences in prevalence and parasite abundance, consistent negative correlations between parasite abundance and host body condition were observed across all seasons, indicating that the mouth-attaching copepods could reduce the body condition of the host fish. This provides field evidence suggesting that S. markewitschi has a potential negative impact on wild white-spotted charr.

Quantification of Thermal Acclimation in Immune Functions in Ectothermic Animals

This short review focuses on current experimental designs to quantify immune acclimation in animals. Especially in the face of rapidly changing thermal regimes, thermal acclimation of immune function has the potential to impact host-pathogen relationships and the fitness of hosts. While much of the field of ecoimmunology has focused on vertebrates and insects, broad interest in how animals can acclimate to temperatures spans taxa. The literature shows a recent increase in thermal acclimation studies in the past six years. I categorized studies as focusing on (1) natural thermal variation in the environment (e.g., seasonal), (2) in vivo manipulation of animals in captive conditions, and (3) in vitro assays using biological samples taken from wild or captive animals. I detail the strengths and weaknesses of these approaches, with an emphasis on mechanisms of acclimation at different levels of organization (organismal and cellular). These two mechanisms are not mutually exclusive, and a greater combination of the three techniques listed above will increase our knowledge of the diversity of mechanisms used by animals to acclimate to changing thermal regimes. Finally, I suggest that functional assays of immune system cells (such as quantification of phagocytosis) are an accessible and non-taxa-specific way to tease apart the effects of animals upregulating quantities of immune effectors (cells) and changes in the function of immune effectors (cellular performance) due to structural changes in cells such as those of membranes and enzymes.

Temperature and intraspecific variation affect host-parasite interactions

Parasites play key roles in regulating aquatic ecosystems, yet the impact of climate warming on their ecology and disease transmission remains poorly understood. Isolating the effect of warming is challenging as transmission involves multiple interacting species and potential intraspecific variation in temperature responses of one or more of these species. Here, we leverage a wide-ranging mosquito species and its facultative parasite as a model system to investigate the impact of temperature on host-parasite interactions and disease transmission. We conducted a common garden experiment measuring parasite growth and infection rates at seven temperatures using 12 field-collected parasite populations and a single mosquito population. We find that both free-living growth rates and infection rates varied with temperature, which were highest at 18-24.5 °C and 13 °C, respectively. Further, we find intraspecific variation in peak performance temperature reflecting patterns of local thermal adaptation-parasite populations from warmer source environments typically had higher thermal optima for free-living growth rates. For infection rates, we found a significant interaction between parasite population and nonlinear effects of temperature. These findings underscore the need to consider both host and parasite thermal responses, as well as intraspecific variation in thermal responses, when predicting the impacts of climate change on disease in aquatic ecosystems.

Extreme heat reduces host and parasite performance in a butterfly-parasite interaction

Environmental temperature fundamentally shapes insect physiology, fitness and interactions with parasites. Differential climate warming effects on host versus parasite biology could exacerbate or inhibit parasite transmission, with far-reaching implications for pollination services, biocontrol and human health. Here, we experimentally test how controlled temperatures influence multiple components of host and parasite fitness in monarch butterflies (Danaus plexippus) and their protozoan parasites Ophryocystis elektroscirrha. Using five constant-temperature treatments spanning 18-34°C, we measured monarch development, survival, size, immune function and parasite infection status and intensity. Monarch size and survival declined sharply at the hottest temperature (34°C), as did infection probability, suggesting that extreme heat decreases both host and parasite performance. The lack of infection at 34°C was not due to greater host immunity or faster host development but could instead reflect the thermal limits of parasite invasion and within-host replication. In the context of ongoing climate change, temperature increases above current thermal maxima could reduce the fitness of both monarchs and their parasites, with lower infection rates potentially balancing negative impacts of extreme heat on future monarch abundance and distribution.

High temperatures augment inhibition of parasites by a honey bee gut symbiont

Temperature affects growth, metabolism, and interspecific interactions in microbial communities. Within animal hosts, gut bacterial symbionts can provide resistance to parasitic infections. Both infection and populations of symbionts can be shaped by the host body temperature. However, the effects of temperature on the antiparasitic activities of gut symbionts have seldom been explored. The Lactobacillus-rich gut microbiota of facultatively endothermic honey bees is subject to seasonal and ontogenetic changes in host temperature that could alter the effects of symbionts against parasites. We used cell cultures of a Lactobacillus symbiont and an important trypanosomatid gut parasite of honey bees to test the potential for temperature to shape parasite-symbiont interactions. We found that symbionts showed greater heat tolerance than parasites and chemically inhibited parasite growth via production of acids. Acceleration of symbiont growth and acid production at high temperatures resulted in progressively stronger antiparasitic effects across a temperature range typical of bee colonies. Consequently, the presence of symbionts reduced both the peak growth rate and heat tolerance of parasites. Substantial changes in parasite-symbiont interactions were evident over a temperature breadth that parallels changes in diverse animals exhibiting infection-related fevers and the amplitude of circadian temperature variation typical of endothermic birds and mammals, implying the frequent potential for temperature to alter symbiont-mediated resistance to parasites in endo- and ectothermic hosts. Results suggest that the endothermic behavior of honey bees could enhance the impacts of gut symbionts on parasites, implicating thermoregulation as a reinforcer of core symbioses and possibly microbiome-mediated antiparasitic defense. IMPORTANCE Two factors that shape the resistance of animals to infection are body temperature and gut microbiota. However, temperature can also alter interactions among microbes, raising the question of whether and how temperature changes the antiparasitic effects of gut microbiota. Honey bees are agriculturally important hosts of diverse parasites and infection-mitigating gut microbes. They can also socially regulate their body temperatures to an extent unusual for an insect. We show that high temperatures found in honey bee colonies augment the ability of a gut bacterial symbiont to inhibit the growth of a common bee parasite, reducing the parasite's ability to grow at high temperatures. This suggests that fluctuations in colony and body temperatures across life stages and seasons could alter the protective value of bees' gut microbiota against parasites, and that temperature-driven changes in gut microbiota could be an underappreciated mechanism by which temperature-including endothermy and fever-alters animal infection.

One Health Approach to Globalizing, Accelerating, and Focusing Amphibian and Reptile Disease Research-Reflections and Opinions from the First Global Amphibian and Reptile Disease Conference

The world's reptiles and amphibians are experiencing dramatic and ongoing losses in biodiversity, changes that can have substantial effects on ecosystems and human health. In 2022, the first Global Amphibian and Reptile Disease Conference was held, using One Health as a guiding principle. The conference showcased knowledge on numerous reptile and amphibian pathogens from several standpoints, including epidemiology, host immune defenses, wild population effects, and mitigation. The conference also provided field experts the opportunity to discuss and identify the most urgent herpetofaunal disease research directions necessary to address current and future threats to reptile and amphibian biodiversity.

A pathogen's spatial range is not constrained by geographical features in the flax rust pathosystem

Climate change and shifting environmental conditions can allow pathogens to spread into previously unburdened areas. For plant pathogens, this dynamic has the potential to disrupt natural ecosystem equilibria and human agriculture, making predicting plant pathogen range shifts increasingly important. Although such predictions will hinge on an accurate understanding of the determinants of pathogen range-namely the environmental, geographical, and host range characteristics that modulate local pathogen habitation-few studies to date have probed these in natural plant populations. Here, we characterize range determinants for the model system of Lewis flax (Linum lewisii) and its pathogen, flax rust (Melampsora lini), in the Rocky Mountains. Transect surveys were performed to assess three relationships: (i) the effect of geographical features-elevation, slope aspect, slope grade, and land cover-on flax presence and density, (ii) the effect of geographical features on flax rust presence and prevalence, and (iii) the effects of flax's local population density and metapopulation structure on flax rust presence and prevalence. We found that flax population density, but not host metapopulation structure, influences the distribution of flax rust. Additionally, we showed that, while the distribution of flax was broadly constrained to a relatively narrow range of geographical and resulting environmental features, flax rust was evenly distributed across the full range of settings measured. These results indicate that a warming environment, which is expected to modulate such features, may restrict the optimal range of the plant more than that of its pathogen. Importantly, our results also suggest that even if flax shifts its spatial range to escape increasing climatic pressures, flax rust will not face any significant barriers to track this movement.

Fluctuating temperatures have a surprising effect on disease transmission

Theory predicts that temperature fluctuations should reduce performance near an organism's thermal optimum. A new study in PLOS Biology found fluctuations increased parasite transmission instead, highlighting questions about how climate change will impact infectious diseases.

Short-term temperature fluctuations increase disease in a Daphnia-parasite infectious disease system

Climate change has profound effects on infectious disease dynamics, yet the impacts of increased short-term temperature fluctuations on disease spread remain poorly understood. We empirically tested the theoretical prediction that short-term thermal fluctuations suppress endemic infection prevalence at the pathogen's thermal optimum. This prediction follows from a mechanistic disease transmission model analyzed using stochastic simulations of the model parameterized with thermal performance curves (TPCs) from metabolic scaling theory and using nonlinear averaging, which predicts ecological outcomes consistent with Jensen's inequality (i.e., reduced performance around concave-down portions of a thermal response curve). Experimental observations of replicated epidemics of the microparasite Ordospora colligata in Daphnia magna populations indicate that temperature variability had the opposite effect of our theoretical predictions and instead increase endemic infection prevalence. This positive effect of temperature variability is qualitatively consistent with a published hypothesis that parasites may acclimate more rapidly to fluctuating temperatures than their hosts; however, incorporating hypothetical effects of delayed host acclimation into the mechanistic transmission model did not fully account for the observed pattern. The experimental data indicate that shifts in the distribution of infection burden underlie the positive effect of temperature fluctuations on endemic prevalence. The increase in disease risk associated with climate fluctuations may therefore result from disease processes interacting across scales, particularly within-host dynamics, that are not captured by combining standard transmission models with metabolic scaling theory.

Temperature and intraspecific variation affect host-parasite interactions

Parasites play key roles in regulating aquatic ecosystems, yet the impact of climate warming on their ecology and disease transmission remains poorly understood. Isolating the effect of warming is challenging as transmission involves multiple interacting species and potential intraspecific variation in temperature responses of one or more of these species. Here, we leverage a wide-ranging mosquito species and its facultative parasite as a model system to investigate the impact of temperature on host-parasite interactions and disease transmission. We conducted a common garden experiment measuring parasite growth and infection rates at seven temperatures using 12 field-collected parasite populations and a single mosquito population. We find that both free-living growth rates and infection rates varied with temperature, which were highest at 18-24.5°C and 13°C, respectively. Further, we find intraspecific variation in peak performance temperature reflecting patterns of local thermal adaptation-parasite populations from warmer source environments typically had higher thermal optima for free-living growth rates. For infection rates, we found a significant interaction between parasite population and nonlinear effects of temperature. These findings underscore the need to consider both host and parasite thermal responses, as well as intraspecific variation in thermal responses, when predicting the impacts of climate change on disease in aquatic ecosystems.

Antibiofilm Effect of Lavandula multifida Essential Oil: A New Approach for Chronic Infections

Fungal infections are associated with high morbidity and mortality rates, being highly prevalent in patients with underlying health complications such as chronic lung disease, HIV, cancer, and diabetes mellitus. To mitigate these infections, the development of effective antifungals is imperative, with plants standing out as promising sources of bioactive compounds. In the present study, we focus on the antibiofilm potential of Lavandula multifida essential oil (EO) against dermatophyte strains and Candida albicans. The EO was characterized using GC and GC-MS, and its antifungal effect was assessed on both biofilm formation and disruption. Biofilm mass, extracellular matrix, and viability were quantified using crystal violet, safranin, and XTT assays, respectively, and morphological alterations were confirmed using optical and scanning electron microscopy. L. multifida EO showed very high amounts of carvacrol and was very effective in inhibiting and disrupting fungal biofilms. The EO significantly decreased biofilm mass and viability in all tested fungi. In addition, a reduction in dermatophytes' extracellular matrix was observed, particularly during biofilm formation. Morphological alterations were evident in mature biofilms, with a clear decrease in hypha diameter. These promising results support the use of L. multifida EO in the development of effective plant-based antifungal products.

Mutualisms in a warming world

Predicting the impacts of global warming on mutualisms poses a significant challenge given the functional and life history differences that usually exist among interacting species. However, this is a critical endeavour since virtually all species on Earth depend on other species for survival and/or reproduction. The field of thermal ecology can provide physiological and mechanistic insights, as well as quantitative tools, for addressing this challenge. Here, we develop a conceptual and quantitative framework that connects thermal physiology to species' traits, species' traits to interacting mutualists' traits and interacting traits to the mutualism. We first identify the functioning of reciprocal mutualism-relevant traits in diverse systems as the key temperature-dependent mechanisms driving the interaction. We then develop metrics that measure the thermal performance of interacting mutualists' traits and that approximate the thermal performance of the mutualism itself. This integrated approach allows us to additionally examine how warming might interact with resource/nutrient availability and affect mutualistic species' associations across space and time. We offer this framework as a synthesis of convergent and critical issues in mutualism science in a changing world, and as a baseline to which other ecological complexities and scales might be added.

A big data-model integration approach for predicting epizootics and population recovery in a keystone species

Infectious diseases pose a significant threat to global health and biodiversity. Yet, predicting the spatiotemporal dynamics of wildlife epizootics remains challenging. Disease outbreaks result from complex nonlinear interactions among a large collection of variables that rarely adhere to the assumptions of parametric regression modeling. We adopted a nonparametric machine learning approach to model wildlife epizootics and population recovery, using the disease system of colonial black-tailed prairie dogs (BTPD, Cynomys ludovicianus) and sylvatic plague as an example. We synthesized colony data between 2001 and 2020 from eight USDA Forest Service National Grasslands across the range of BTPDs in central North America. We then modeled extinctions due to plague and colony recovery of BTPDs in relation to complex interactions among climate, topoedaphic variables, colony characteristics, and disease history. Extinctions due to plague occurred more frequently when BTPD colonies were spatially clustered, in closer proximity to colonies decimated by plague during the previous year, following cooler than average temperatures the previous summer, and when wetter winter/springs were preceded by drier summers/falls. Rigorous cross-validations and spatial predictions indicated that our final models predicted plague outbreaks and colony recovery in BTPD with high accuracy (e.g., AUC generally >0.80). Thus, these spatially explicit models can reliably predict the spatial and temporal dynamics of wildlife epizootics and subsequent population recovery in a highly complex host-pathogen system. Our models can be used to support strategic management planning (e.g., plague mitigation) to optimize benefits of this keystone species to associated wildlife communities and ecosystem functioning. This optimization can reduce conflicts among different landowners and resource managers, as well as economic losses to the ranching industry. More broadly, our big data-model integration approach provides a general framework for spatially explicit forecasting of disease-induced population fluctuations for use in natural resource management decision-making.

Elevated nest temperature has opposing effects on host species infested with parasitic nest flies

Environmental factors, such as elevated temperature, can have varying effects on hosts and their parasites, which can have consequences for the net outcome of this relationship. The individual direct effects of temperature must be disentangled to determine the net-effect in host-parasite relationships, yet few studies have determined the net-effects in a multi-host system. To address this gap, we experimentally manipulated temperature and parasite presence in the nests of two host species infested by parasitic blowflies (Protocalliphora sialia). We conducted a factorial experiment by increasing temperature (or not) and removing all parasites (or not) in the nests of eastern bluebirds (Sialia sialis) and tree swallows (Tachycineta bicolor). We then measured nestling morphometrics, blood loss, and survival and quantified parasite abundance. We predicted that if temperature had a direct effect on parasite abundance, then elevated temperature would cause similar directional effects on parasite abundance across host species. If temperature had a direct effect on hosts, and therefore an indirect effect on the parasite, parasite abundance would differ across host species. Swallow nests with elevated temperature had fewer parasites compared to nests without temperature manipulation. In contrast, bluebird nests with elevated temperatures had more parasites compared to nests without temperature manipulation. The results of our study demonstrate that elevated temperature can have differential effects on host species, which can impact infestation susceptibility. Furthermore, changing climates could have complex net-effects on parasite fitness and host health across multi-host-parasite interactions.

Interdisciplinary approach to solve unusual mortalities in the European common frog (Rana temporaria) in two high-mountain ponds affected by climate change

The global decline in amphibian populations is a major environmental issue. Chytridiomycosis, Ranaviruses and the red-leg syndrome have been identified in unusual mortality events. However, these infections do not account for all causes of declining amphibian populations. Moreover, several cases of amphibian mortality are difficult to solve without resorting to an interdisciplinary approach. Two cases of unusual mortality in Rana temporaria occurred at two high-mountain ponds (northwest Italy) in April and May 2021. Water and frog samples were analysed to understand the possible causes responsible for the unusual mortalities. Results of the main physicochemical (pH, conductivity, dissolved oxygen, chemical and biochemical oxygen demand) and nutrient (ammonia/ammonium, nitrite, nitrate, total phosphorus) parameters revealed a good condition of the water quality, with the absence of the main cyanotoxins (microcystins/nodularins). However, unseasonably high spring water temperatures were recorded in both ponds (12.73 °C and 14.21 °C for Frog Pond and Selleries Pond, respectively). Frogs (n = 50; snout-vent length: 7.0-9.8 cm; body mass: 85-123 g) collected from Frog Pond mainly presented bumps on the ventral cavity and dermal ulceration associated with the isolation of Carnobacterium maltaromaticum. On the other hand, frogs (n = 5; snout-vent length: 8.0-9.1 cm; body mass: 87-92 g) from Selleries Pond presented petechiae and dermal ulcerations on the rear limbs associated with the isolation of Aeromonas salmonicida and A. sobria. In both mortality events, the interdisciplinary approach revealed an association between frog mortalities and the isolation of bacteria. Isolated bacteria are considered opportunistic pathogens, and the high values of the water temperature has certainly led a stress on the frogs, favouring the spread of bacteria and the death of the frogs. Further studies are needed to assess the pathophysiological effects of the opportunistic bacteria here isolated, clarifying the interactions between emerging pathogens and climate change.

Host-driven temperature dependence of Deformed wing virus infection in honey bee pupae

The temperature dependence of infection reflects changes in performance of parasites and hosts. High temperatures often mitigate infection by favoring heat-tolerant hosts over heat-sensitive parasites. Honey bees exhibit endothermic thermoregulation-rare among insects-that can favor resistance to parasites. However, viruses are heavily host-dependent, suggesting that viral infection could be supported-not threatened-by optimum host function. To understand how temperature-driven changes in performance of viruses and hosts shape infection, we compared the temperature dependence of isolated viral enzyme activity, three honey bee traits, and infection of honey bee pupae. Viral enzyme activity varied <2-fold over a > 30 °C interval spanning temperatures typical of ectothermic insects and honey bees. In contrast, honey bee performance peaked at high (≥ 35 °C) temperatures and was highly temperature-sensitive. Although these results suggested that increasing temperature would favor hosts over viruses, the temperature dependence of pupal infection matched that of pupal development, falling only near pupae's upper thermal limits. Our results reflect the host-dependent nature of viruses, suggesting that infection is accelerated-not curtailed-by optimum host function, contradicting predictions based on relative performance of parasites and hosts, and suggesting tradeoffs between infection resistance and host survival that limit the viability of bee 'fever'.

Temperature modifies trait-mediated infection outcomes in a Daphnia-fungal parasite system

One major concern related to climate change is that elevated temperatures will drive increases in parasite outbreaks. Increasing temperature is known to alter host traits and host-parasite interactions, but we know relatively little about how these are connected mechanistically-that is, about how warmer temperatures impact the relationship between epidemiologically relevant host traits and infection outcomes. Here, we used a zooplankton-fungus (Daphnia dentifera-Metschnikowia bicuspidata) disease system to experimentally investigate how temperature impacted physical barriers to infection and cellular immune responses. We found that Daphnia reared at warmer temperatures had more robust physical barriers to infection but decreased cellular immune responses during the initial infection process. Infected hosts at warmer temperatures also suffered greater reductions in fecundity and lifespan. Furthermore, the relationship between a key trait-gut epithelium thickness, a physical barrier-and the likelihood of terminal infection reversed at warmer temperatures. Together, our results highlight the complex ways that temperatures can modulate host-parasite interactions and show that different defense components can have qualitatively different responses to warmer temperatures, highlighting the importance of considering key host traits when predicting disease dynamics in a warmer world. This article is part of the theme issue 'Infectious disease ecology and evolution in a changing world'.

Major Emerging Fungal Diseases of Reptiles and Amphibians

Emerging infectious diseases (EIDs) are caused by pathogens that have undergone recent changes in terms of geographic spread, increasing incidence, or expanding host range. In this narrative review, we describe three important fungal EIDs with keratin trophism that are relevant to reptile and amphibian conservation and veterinary practice. Nannizziopsis spp. have been mainly described in saurians; infection results in thickened, discolored skin crusting, with eventual progression to deep tissues. Previously only reported in captive populations, it was first described in wild animals in Australia in 2020. Ophidiomyces ophidiicola (formely O. ophiodiicola) is only known to infect snakes; clinical signs include ulcerating lesions in the cranial, ventral, and pericloacal regions. It has been associated with mortality events in wild populations in North America. Batrachochytrium spp. cause ulceration, hyperkeratosis, and erythema in amphibians. They are a major cause of catastrophic amphibian declines worldwide. In general, infection and clinical course are determined by host-related characteristics (e.g., nutritional, metabolic, and immune status), pathogens (e.g., virulence and environmental survival), and environment (e.g., temperature, hygrometry, and water quality). The animal trade is thought to be an important cause of worldwide spread, with global modifications in temperature, hygrometry, and water quality further affecting fungal pathogenicity and host immune response.

The effect of warming on seagrass wasting disease depends on host genotypic identity and diversity

Temperature increases due to climate change have affected the distribution and severity of diseases in natural systems, causing outbreaks that can destroy host populations. Host identity, diversity, and the associated microbiome can affect host responses to both infection and temperature, but little is known about how they could function as important mediators of disease in altered thermal environments. We conducted an 8-week warming experiment to test the independent and interactive effects of warming, host genotypic identity, and host genotypic diversity on the prevalence and intensity of infections of seagrass (Zostera marina) by the wasting disease parasite (Labyrinthula zosterae). At elevated temperatures, we found that genotypically diverse host assemblages had reduced infection intensity, but not reduced prevalence, relative to less diverse assemblages. This dilution effect on parasite intensity was the result of both host composition effects as well as emergent properties of biodiversity. In contrast with the benefits of genotypic diversity under warming, diversity actually increased parasite intensity slightly in ambient temperatures. We found mixed support for the hypothesis that a growth-defense trade-off contributed to elevated disease intensity under warming. Changes in the abundance (but not composition) of a few taxa in the host microbiome were correlated with genotype-specific responses to wasting disease infections under warming, consistent with the emerging evidence linking changes in the host microbiome to the outcome of host-parasite interactions. This work emphasizes the context dependence of biodiversity-disease relationships and highlights the potential importance of interactions among biodiversity loss, climate change, and disease outbreaks in a key foundation species.

Environmental refuges from disease in host-parasite interactions under global change

The physiological performance of organisms depends on their environmental context, resulting in performance-response curves along environmental gradients. Parasite performance-response curves are generally expected to be broader than those of their hosts due to shorter generation times and hence faster adaptation. However, certain environmental conditions may limit parasite performance more than that of the host, thereby providing an environmental refuge from disease. Thermal disease refuges have been extensively studied in response to climate warming, but other environmental factors may also provide environmental disease refuges which, in turn, respond to global change. Here, we (1) showcase laboratory and natural examples of refuges from parasites along various environmental gradients, and (2) provide hypotheses on how global environmental change may affect these refuges. We strive to synthesize knowledge on potential environmental disease refuges along different environmental gradients including salinity and nutrients, in both natural and food-production systems. Although scaling up from single host-parasite relationships along one environmental gradient to their interaction outcome in the full complexity of natural environments remains difficult, integrating host and parasite performance-response can serve to formulate testable hypotheses about the variability in parasitism outcomes and the occurrence of environmental disease refuges under current and future environmental conditions.

Pyrethroid insecticides pose greater risk than organophosphate insecticides to biocontrol agents for human schistosomiasis

Use of agrochemicals, including insecticides, is vital to food production and predicted to increase 2-5 fold by 2050. Previous studies have shown a positive association between agriculture and the human infectious disease schistosomiasis, which is problematic as this parasitic disease infects approximately 250 million people worldwide. Certain insecticides might runoff fields and be highly toxic to invertebrates, such as prawns in the genus Macrobrachium, that are biocontrol agents for snails that transmit the parasites causing schistosomiasis. We used a laboratory dose-response experiment and an observational field study to determine the relative toxicities of three pyrethroid (esfenvalerate, λ-cyhalothrin, and permethrin) and three organophosphate (chlorpyrifos, malathion, and terbufos) insecticides to Macrobrachium prawns. In the lab, pyrethroids were consistently several orders of magnitude more toxic than organophosphate insecticides, and more likely to runoff fields at lethal levels according to modeling data. At 31 water contact sites in the lower basin of the Senegal River where schistosomiasis is endemic, we found that Macrobrachium prawn survival was associated with pyrethroid but not organophosphate application rates to nearby crop fields after controlling for abiotic and prawn-level factors. Our laboratory and field results suggest that widely used pyrethroid insecticides can have strong non-target effects on Macrobrachium prawns that are biocontrol agents where 400 million people are at risk of human schistosomiasis. Understanding the ecotoxicology of high-risk insecticides may help improve human health in schistosomiasis-endemic regions undergoing agricultural expansion.

Transgenerational plasticity in a zooplankton in response to elevated temperature and parasitism

Organisms are increasingly facing multiple stressors, which can simultaneously interact to cause unpredictable impacts compared with a single stressor alone. Recent evidence suggests that phenotypic plasticity can allow for rapid responses to altered environments, including biotic and abiotic stressors, both within a generation and across generations (transgenerational plasticity). Parents can potentially "prime" their offspring to better cope with similar stressors or, alternatively, might produce offspring that are less fit because of energetic constraints. At present, it remains unclear exactly how biotic and abiotic stressors jointly mediate the responses of transgenerational plasticity and whether this plasticity is adaptive. Here, we test the effects of biotic and abiotic environmental changes on within- and transgenerational plasticity using a Daphnia-Metschnikowia zooplankton-fungal parasite system. By exposing parents and their offspring consecutively to the single and combined effects of elevated temperature and parasite infection, we showed that transgenerational plasticity induced by temperature and parasite stress influenced host fecundity and lifespan; offsprings of mothers who were exposed to one of the stressors were better able to tolerate elevated temperature, compared with the offspring of mothers who were exposed to neither or both stressors. Yet, the negative effects caused by parasite infection were much stronger, and this greater reduction in host fitness was not mitigated by transgenerational plasticity. We also showed that elevated temperature led to a lower average immune response, and that the relationship between immune response and lifetime fecundity reversed under elevated temperature: the daughters of exposed mothers showed decreased fecundity with increased hemocyte production at ambient temperature but the opposite relationship at elevated temperature. Together, our results highlight the need to address questions at the interface of multiple stressors and transgenerational plasticity and the importance of considering multiple fitness-associated traits when evaluating the adaptive value of transgenerational plasticity under changing environments.

Immunoendocrinology and Ecoimmunology in Brazilian Anurans

This paper reviews several aspects of immunoendocrinology and ecoimmunology in Brazilian species of anurans under investigation for more than a decade, including (1) patterns of annual covariation of circulating steroids, calling behavior and innate immunity, (2) endocrine and immune correlates of calling performance, (3) behavioral and physiological correlates of parasite load, (4) thermal sensitivity of immune function, and (5) endocrine and immunomodulation by experimental exposure to acute and chronic stressors, as well as to endocrine manipulations and simulated infections. Integrated results have shown an immunoprotective role of increased steroid plasma levels during reproductive activity in calling males. Moreover, a higher helminth parasite load is associated with changes in several behavioral and physiological traits under field conditions. We also found anuran innate immunity is generally characterized by eurythermy, with maximal performance observed in temperatures close to normal and fever thermal preferendum. Moreover, the aerobic scope of innate immune response is decreased at fever thermal preferendum. Experimental exposure to stressors results in increased corticosterone plasma levels and immune redistribution, with an impact on immune function depending on the duration of the stress exposure. Interestingly, the fate of immunomodulation by chronic stressors also depends in part on individual body condition. Acute treatment with corticosterone generally enhances immune function, while prolonged exposure results in immunosuppression. Still, the results of hormonal treatment are complex and depend on the dose, duration of treatment, and the immune variable considered. Finally, simulated infection results in complex modulation of the expression of cytokines, increased immune function, activation of the Hypothalamus-Pituitary-Interrenal axis, and decreased activity of the Hypothalamus-Pituitary-Gonadal axis, as well as reduced melatonin plasma levels, suggesting that anurans have a functional Immune-Pineal axis, homologous to that previously described for mammals. These integrated and complementary approaches have contributed to a better understanding of physiological mechanisms and processes, as well as ecological and evolutionary implications of anuran immunoendocrinology.

Temperature effects on teleost immunity in the light of climate change

Temperature is an important environmental modulator of teleost immune activity. Susceptibility of teleosts to temperature variation depends on the species-specific adaptive temperature range, and the activity of the teleost immune system is generally temperature-dependent. Similar to many physiological and metabolic traits of ectotherms, temperature modulates the activity of immune traits. At low temperatures, acquired immunity of many teleost species is down-modulated, and their immuno-competence mainly depends on innate immunity. At intermediate temperatures, both innate and acquired immunity are fully active and provide optimal protection, including long-lasting immunological memory. When temperatures increase and reach the upper permissive range, teleost immunity is compromised. Moreover, temperature shifts may have negative effects on teleost immune functions, in particular if shifts occur rapidly with high amplitudes. On the contrary, short-term temperature increase may help teleost immunity to fight against pathogens transiently. A major challenge to teleosts therefore is to maintain immuno-competence throughout the temperature range they are exposed to. Climate change coincides with rising temperatures, and more frequent and more extreme temperature shifts. Both are likely to influence the immuno-competence of teleosts. Nonetheless, teleosts exist in habitats that differ substantially in temperature, ranging from below zero in the Arctic's to above 40°C in warm springs, illustrating their enormous potential to adapt to different temperature regimes. The present review seeks to discuss how changes in temperature variation, induced by climate change, might influence teleost immunity.

Resource limitation determines realized thermal performance of consumers in trophodynamic models

Recent work has demonstrated that changes in resource availability can alter a consumer's thermal performance curve (TPC). When resources decline, the optimal temperature and breadth of thermal performance also decline, leading to a greater risk of warming than predicted by static TPCs. We investigate the effect of temperature on coupled consumer-resource dynamics, focusing on the potential for changes in the consumer TPC to alter extinction risk. Coupling consumer and resource dynamics generally reduces the potential for resource decline to exacerbate the effects of warming via changes to the TPC due to a reduction in top-down control when consumers near the limits of their thermal performance curve. However, if resources are more sensitive to warming, consumer TPCs can be reshaped by declining resources, leading to increased extinction risk. Our work elucidates the role of top-down and bottom-up regulation in determining the extent to which changes in resource density alter consumer TPCs.

Scaling effects of temperature on parasitism from individuals to populations

Parasitism is expected to change in a warmer future, but whether warming leads to substantial increases in parasitism remains unclear. Understanding how warming effects on parasitism in individual hosts (e.g. parasite load) translate to effects on population-level parasitism (e.g. prevalence, R0 ) remains a major knowledge gap. We conducted a literature review and identified 24 host-parasite systems that had information on the temperature dependence of parasitism at both individual host and host population levels: 13 vector-borne systems and 11 environmentally transmitted systems. We found a strong positive correlation between the thermal optima of individual- and population-level parasitism, although several of the environmentally transmitted systems exhibited thermal optima >5°C apart between individual and population levels. Parasitism thermal optima were close to vector performance thermal optima in vector-borne systems but not hosts in environmentally transmitted systems, suggesting these thermal mismatches may be more common in certain types of host-parasite systems. We also adapted and simulated simple models for both types of transmission modes and found the same pattern across the two modes: thermal optima were more strongly correlated across scales when there were more traits linking individual- to population-level processes. Generally, our results suggest that information on the temperature dependence, and specifically the thermal optimum, at either the individual or population level should provide a useful-although not quantitatively exact-baseline for predicting temperature dependence at the other level, especially in vector-borne parasite systems. Environmentally transmitted parasitism may operate by a different set of rules, in which temperature dependence is decoupled in some systems, requiring the need for trait-based studies of temperature dependence at individual and population levels.

"Heat waves" experienced during larval life have species-specific consequences on life-history traits and sexual development in anuran amphibians

Extreme temperatures during heat waves can induce mass-mortality events, but can also exert sublethal negative effects by compromising life-history traits and derailing sexual development. Ectothermic animals may, however, also benefit from increased temperatures via enhanced physiological performance and the suppression of cold-adapted pathogens. Therefore, it is crucial to address how the intensity and timing of naturally occurring or human-induced heat waves affect life-history traits and sexual development in amphibians, to predict future effects of climate change and to minimize risks arising from the application of elevated temperature in disease mitigation. We raised agile frog (Rana dalmatina) and common toad (Bufo bufo) tadpoles at 19 °C and exposed them to a simulated heat wave of 28 or 30 °C for six days during one of three ontogenetic periods (early, mid or late larval development). In agile frogs, exposure to 30 °C during early larval development increased mortality. Regardless of timing, all heat-treatments delayed metamorphosis, and exposure to 30 °C decreased body mass at metamorphosis. Furthermore, exposure to 30 °C during any period and to 28 °C late in development caused female-to-male sex reversal, skewing sex ratios strongly towards males. In common toads, high temperature only slightly decreased survival and did not influence phenotypic sex ratio, while it reduced metamorph mass and length of larval development. Juvenile body mass measured 2 months after metamorphosis was not adversely affected by temperature treatments in either species. Our results indicate that heat waves may have devastating effects on amphibian populations, and the severity of these negative consequences, and sensitivity can vary greatly between species and with the timing and intensity of heat. Finally, thermal treatments against cold-adapted pathogens have to be executed with caution, taking into account the thermo-sensitivity of the species and the life stage of animals to be treated.

Elevating Air Temperature may Enhance Future Epidemic Risk of the Plant Pathogen Phytophthora infestans

Knowledge of pathogen adaptation to global warming is important for predicting future disease epidemics and food production in agricultural ecosystems; however, the patterns and mechanisms of such adaptation in many plant pathogens are poorly understood. Here, population genetics combined with physiological assays and common garden experiments were used to analyze the genetics, physiology, and thermal preference of pathogen aggressiveness in an evolutionary context using 140 Phytophthora infestans genotypes under five temperature regimes. Pathogens originating from warmer regions were more thermophilic and had a broader thermal niche than those from cooler regions. Phenotypic plasticity contributed ~10-fold more than heritability measured by genetic variance. Further, experimental temperatures altered the expression of genetic variation and the association of pathogen aggressiveness with the local temperature. Increasing experimental temperature enhanced the variation in aggressiveness. At low experimental temperatures, pathogens from warmer places produced less disease than those from cooler places; however, this pattern was reversed at higher experimental temperatures. These results suggest that geographic variation in the thermal preferences of pathogens should be included in modeling future disease epidemics in agricultural ecosystems in response to global warming, and greater attention should be paid to preventing the movement of pathogens from warmer to cooler places.

In vitro thermal tolerance of a hypervirulent lineage of Batrachochytrium dendrobatidis: Growth arrestment by elevated temperature and recovery following thermal treatment

Chytridiomycosis, an emerging infectious disease caused by Batrachochytrium dendrobatidis (Bd), poses a serious threat to amphibians. The thermal optimum of Bd is lower than that of most amphibians, providing an opportunity to cure infected individuals with elevated temperature. However, this approach presupposes detailed knowledge about the thermal tolerance of the fungus. To determine the temperature that may effectively reduce infection burdens in vivo, detailed in vitro studies are needed to characterize thermal tolerance of the fungus without complexities introduced by the species-specific characteristics of hosts' immune systems. The aim of our study was to evaluate the thermal tolerance of a hypervirulent isolate of Bd, considering the limits of its thermal tolerance and its capacity to rebound following heat treatment. We incubated Bd cell cultures at five different temperatures (21, 25.5, 27, 29, or 30.5 C) for one of six exposure durations (3, 4, 5, 6, 7, or 8 days) and subsequently counted the number of zoospores to assess the temperature dependence of Bd growth. We observed intensive Bd growth at 21 C. At 25.5 C, the number of zoospores also increased over time, but the curve plateaued at about half of the maximum values observed in the lower temperature treatment. At temperatures of 27 C and above, the fungus showed no measurable growth. However, when we moved the cultures back to 21 C after the elevated temperature treatments, we observed recovery of Bd growth in all cultures previously treated at 27 C. At 29 C, a treatment duration of 8 days was necessary to prevent recovery of Bd growth, and at 30.5 C a treatment duration of 5 days was needed to achieve the same result, revealing that these moderately elevated temperatures applied for only a few days have merely a fungistatic rather than a fungicidal effect under in vitro conditions.

Symbiosis and host responses to heating

Virtually all organisms are colonized by microbes. Average temperatures are rising because of global climate change - accompanied by increases in extreme climatic events and heat shock - and symbioses with microbes may determine species persistence in the 21st century. Although parasite infection typically reduces host upper thermal limits, interactions with beneficial microbes can facilitate host adaptation to warming. The effects of warming on the ecology and evolution of the microbial symbionts remain understudied but are important for understanding how climate change might affect host health and disease. We present a framework for untangling the contributions of symbiosis to predictions of host persistence in the face of global change.

Contrasting temperature responses in seasonal timing of cercariae shedding by Rhipidocotyle trematodes

Global warming is likely to lengthen the seasonal duration of larval release by parasites. We exposed freshwater mussel hosts, Anodonta anatina, from 2 high-latitude populations to high, intermediate and low temperatures throughout the annual cercarial shedding period of the sympatric trematodes Rhipidocotyle fennica and R. campanula, sharing the same transmission pathway. At the individual host level, under warmer conditions, the timing of the cercarial release in both parasite species shifted towards seasonally earlier period while its duration did not change. At the host population level, evidence for the lengthening of larvae shedding period with warming was found for R. fennica. R. campanula started the cercarial release seasonally clearly earlier, and at a lower temperature, than R. fennica. Furthermore, the proportion of mussels shedding cercariae increased, while day-degrees required to start the cercariae shedding decreased in high-temperature treatment in R. fennica. In R. campanula these effects were not found, suggesting that warming can benefit more R. fennica. These results do not completely support the view that climate warming would invariably increase the seasonal duration of larval shedding by parasites, but emphasizes species-specific differences in temperature-dependence and in seasonality of cercarial release.

Growth rate determines prokaryote-provirus network modulated by temperature and host genetic traits

BACKGROUND

Prokaryote-virus interactions play key roles in driving biogeochemical cycles. However, little is known about the drivers shaping their interaction network structures, especially from the host features. Here, we compiled 7656 species-level genomes in 39 prokaryotic phyla across environments globally and explored how their interaction specialization is constrained by host life history traits, such as growth rate.

RESULTS

We first reported that host growth rate indicated by the reverse of minimal doubling time was negatively related to interaction specialization for host in host-provirus network across various ecosystems and taxonomy groups. Such a negative linear growth rate-specialization relationship (GrSR) was dependent on host optimal growth temperature (OGT), and stronger toward the two gradient ends of OGT. For instance, prokaryotic species with an OGT ≥ 40 °C showed a stronger GrSR (Pearson's r = -0.525, P < 0.001). Significant GrSRs were observed with the presences of host genes in promoting the infection cycle at stages of adsorption, establishment, and viral release, but nonsignificant with the presence of immune systems, such as restriction-modification systems and CRISPR-Cas systems. Moreover, GrSR strength was increased with the presence of temperature-dependent lytic switches, which was also confirmed by mathematical modeling.

CONCLUSIONS

Together, our results advance our understanding of the interactions between prokaryotes and proviruses and highlight the importance of host growth rate in interaction specialization during lysogenization. Video Abstract.

Signatures of functional bacteriome structure in a tropical direct-developing amphibian species

Background

Host microbiomes may differ under the same environmental conditions and these differences may influence susceptibility to infection. Amphibians are ideal for comparing microbiomes in the context of disease defense because hundreds of species face infection with the skin-invading microbe Batrachochytrium dendrobatidis (Bd), and species richness of host communities, including their skin bacteria (bacteriome), may be exceptionally high. We conducted a landscape-scale Bd survey of six co-occurring amphibian species in Brazil’s Atlantic Forest. To test the bacteriome as a driver of differential Bd prevalence, we compared bacteriome composition and co-occurrence network structure among the six focal host species.

Results

Intensive sampling yielded divergent Bd prevalence in two ecologically similar terrestrial-breeding species, a group with historically low Bd resistance. Specifically, we detected the highest Bd prevalence in Ischnocnema henselii but no Bd detections in Haddadus binotatus. Haddadus binotatus carried the highest bacteriome alpha and common core diversity, and a modular network partitioned by negative co-occurrences, characteristics associated with community stability and competitive interactions that could inhibit Bd colonization.

Conclusions

Our findings suggest that community structure of the bacteriome might drive Bd resistance in H. binotatus, which could guide microbiome manipulation as a conservation strategy to protect diverse radiations of direct-developing species from Bd-induced population collapses.

Does the thermal mismatch hypothesis predict disease outcomes in different morphs of a terrestrial salamander?

Many aspects of ectotherm physiology are temperature-dependent. The immune system of temperate-dwelling ectothermic host species is no exception and their immune function is often downregulated in cold temperatures. Likewise, species of ectothermic pathogens experience temperature-mediated effects on rates of transmission and/or virulence. Although seemingly straightforward, predicting the outcomes of ectothermic host-pathogen interactions is quite challenging. A recent hypothesis termed the thermal mismatch hypothesis posits that cool-adapted host species should be most susceptible to pathogen infection during warm temperature periods whereas warm-adapted host species should be most susceptible to pathogens during periods of cool temperatures. We explore this hypothesis using two ecologically and physiologically differentiated color morphs of the Eastern Red-backed Salamander (Plethodon cinereus) and a pathogenic chytrid fungus (Batrachochytrium dendrobatidis; hereafter "Bd") using a fully factorial laboratory experiment. At cool temperatures, unstriped salamanders (i.e., those that are tolerant of warm temperatures) had a significantly higher probability of Bd infection compared with cool-tolerant striped salamanders, consistent with the thermal mismatch hypothesis. However, we found no support for this hypothesis when salamanders were exposed to Bd at warm temperatures: the probability of Bd infection in the cool-tolerant striped salamanders was nearly identical in both cool and warm temperatures, opposite the predictions of the thermal mismatch hypothesis. Our results are most consistent with the fact that Bd grows poorly at warm temperatures. Alternatively, our data could indicate that the two color morphs do not differ in their tolerance to warm temperatures but that striped salamanders are more tolerant to cool temperatures than unstriped salamanders.

The role of abiotic variables in an emerging global amphibian fungal disease in mountains

The emergence of the chytridiomycete fungal pathogen Batrachochytrium dendrobatidis (Bd), causing the disease chytridiomycosis, has caused collapse of amphibian communities in numerous mountain systems. The health of amphibians and of mountain freshwater habitats they inhabit is also threatened by ongoing changes in environmental and anthropogenic factors such as climate, hydrology, and pollution. Climate change is causing more extreme climatic events, shifts in ice occurrence, and changes in the timing of snowmelt and pollutant deposition cycles. All of these factors impact both pathogen and host, and disease dynamics. Here we review abiotic variables, known to control Bd occurrence and chytridiomycosis severity, and discuss how climate change may modify them. We propose two main categories of abiotic variables that may alter Bd distribution, persistence, and physiology: 1) climate and hydrology (temperature, precipitation, hydrology, ultraviolet radiation (UVR); and, 2) water chemistry (pH, salinity, pollution). For both categories, we identify topics for further research. More studies on the relationship between global change, pollution and pathogens in complex landscapes, such as mountains, are needed to allow for accurate risk assessments for freshwater ecosystems and resulting impacts on wildlife and human health. Our review emphasizes the importance of using data of higher spatiotemporal resolution and uniform abiotic metrics in order to better compare study outcomes. Fine-scale temperature variability, especially of water temperature, variability of moisture conditions and water levels, snow, ice and runoff dynamics should be assessed as abiotic variables shaping the mountain habitat of pathogen and host. A better understanding of hydroclimate and water chemistry variables, as co-factors in disease, will increase our understanding of chytridiomycosis dynamics.

Intraspecific diversity alters the relationship between climate change and parasitism in a polymorphic ectotherm

Climate-modulated parasitism is driven by a range of factors, yet the spatial and temporal variability of this relationship has received scant attention in wild vertebrate hosts. Moreover, most prior studies overlooked the intraspecific differences across host morphotypes, which impedes a full understanding of the climate-parasitism relationship. In the common lizard (Zootoca vivipara), females exhibit three colour morphs: yellow (Y-females), orange (O-females) and mixed (mixture of yellow and orange, M-females). Zootoca vivipara is also infested with an ectoparasite (Ophionyssus mites). We therefore used this model system to examine the intraspecific response of hosts to parasitism under climate change. We found infestation probability to differ across colour morphs at both spatial (10 sites) and temporal (20 years) scales: M-females had lower parasite infestations than Y- and O-females at lower temperatures, but became more susceptible to parasites as temperature increased. The advantage of M-females at low temperatures was counterbalanced by their higher mortality rates thereafter, which suggests a morph-dependent trade-off between resistance to parasites and host survival. Furthermore, significant interactions between colour morphs and temperature indicate that the relationship between parasite infestations and climate warming was contingent on host morphotypes. Parasite infestations increased with temperature for most morphs, but displayed morph-specific rates. Finally, infested M-females had higher reductions in survival rates than infested Y- or O-females, which implies a potential loss of intraspecific diversity within populations as parasitism and temperatures rise. Overall, we found parasitism increases with warming temperatures, but this relationship is modulated by host morphotypes and an interaction with temperature. We suggest that epidemiological models incorporate intraspecific diversity within species for better understanding the dynamics of wildlife diseases under climate warming.