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1. Skin Defenses and Host-Environment Microbiome Interactions in Spotted Salamanders

   https://doi.org/10.1093/icb/icaf098  

   Urrutia-Carter, Julian; Madison, Joseph_D; Frederick, John_Adam; Muletz Wolz, Carly_R (June 2025, Integrative And Comparative Biology)

   Synopsis Emerging infectious diseases have been of particular interest as a major threat to global biodiversity. In amphibians, two fungal sister taxa, Batrachochytrium dendrobatidis (Bd) and Batrachochytrium salamandrivorans (Bsal), along with the viral pathogen ranavirus, have affected global populations. Factors such as host traits, abiotic and biotic environmental conditions, and pathogen prevalence contribute to species-specific disease susceptibility. The eastern United States is home to the Appalachian Mountain system, known as a “hotspot” for salamander biodiversity. Bd and ranavirus are present throughout the Appalachians, and a Bsal emergence could be imminent. Throughout the Appalachians are the spotted salamanders, Ambystoma maculatum, a mostly terrestrial salamander that participates in mass breeding migration to ponds and vernal pools in the late spring. Previous experimental studies have shown that spotted salamanders appear to be resistant to Bd and Bsal infection, but the mechanisms behind Bd defense remain unknown. Spotted salamanders emerging from their overwintering habitats were hypothesized to have potent anti-Bd function expressed in their mucus and in their skin microbiomes, as a countermeasure to annual Bd re-emergence. We used non-invasive sampling at two pools during the spotted salamander annual breeding event to (I) determine pathogen prevalence, (II) quantify the antifungal potential of salamander skin mucus, and (III) characterize the diversity and composition of the salamander skin microbiome and contrast it to that of the corresponding environmental microbiome. We did not detect any Bd, Bsal, or ranavirus in the salamanders. The salamander mucus did not inhibit Bd growth in vitro, and anti-Bd bacteria were at low relative abundance in the microbiome. The salamander microbiome sourced a proportion of bacteria from the environment and appeared to select rare taxa from their respective pools; however, their functional relevance in pathogen defense is unclear. Our results suggest that the spotted salamander mucosal secretions and skin microbiome are not the mechanisms of defense against Bd. Rather, elements not captured by the mucosome (e.g., immune cell gene expression) may confer resistance. This study contributes to the understanding of salamander intraspecies variation in disease susceptibility. 

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2. Prior heat waves improve survival of field but not domesticated populations of tobacco hornworm exposed to repeated bacterial infections

   https://doi.org/10.1111/1365-2435.70001  

   Malinski, Katherine_H; Willett, Christopher_S; Kingsolver, Joel_G (February 2025, Functional Ecology)

   Abstract In insects and other invertebrates, prior pathogen exposures can improve immune responses and survival to subsequent infections through immune priming. Alternatively, stress and metabolic costs of multiple infections can impair host immunity and survival. The effects of high‐temperature extremes on host–pathogen interactions are not well understood despite the increasing occurrence of heat waves caused by climate change.The response of insects to heat waves and pathogens depends on recent evolutionary history with selective pressures. Domestication of insect pests has occurred in lab colonies of model species, reducing selective pressures for immune and heat stress responses. Lab strains are often used in immunological or heat stress experiments to represent wild field strains, but the efficacy of this approach is seldom evaluated.Using the tobacco hornworm (Manduca sexta), we tested the impact of a heat wave during initial pathogen exposure on survival of a secondary infection withBacillus thuringiensisbacteria. We used a domesticated lab population and a naturally occurring field population ofM. sextato evaluate the impacts of recent domestication on immune and thermal responses.A heat wave during initial infection significantly increased survival of the secondaryB. thuringiensisinfection in the field, but not the lab population ofM. sexta.In the field population, survival of the repeated infection was temperature dependent: exposure to an initial infection event reduced survival of the secondary infection at the control temperature regime, consistent with a stress effect. However, a heat wave during the initial infection event increased survival of the secondary infection, consistent with immune priming effects.The results of this study demonstrate that (a) insect response to thermal stress and pathogens can depend on recent domestication and (b) responses of hosts to repeat pathogen exposures can be temperature‐dependent, suggesting that cross‐talk between the heat stress and immune memory pathways may have important consequences for host–pathogen outcomes under heat wave events. Read the freePlain Language Summaryfor this article on the Journal blog. 

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3. Migration and tolerance shape host behaviour and response to parasite infection

   https://doi.org/10.1111/1365-2656.13539  

   Kim, Dongmin; Shaw, Allison K. (June 2021, Journal of Animal Ecology)

   Abstract Numerous theoretical models have demonstrated that migration, a seasonal animal movement behaviour, can minimize the risks and costs of parasite infection. Past work on migration–infection interactions assumes migration is the only strategy available to organisms for dealing with the parasite infection, that is they migrate to a different environment to recover or escape from infection. Thus, migration is similar to the non‐spatial strategy of resistance, where hosts prevent infection or kill parasites once infected. However, an alternative defence strategy is to tolerate the infection and experience a lower cost to the infection. To our knowledge, no studies have examined how migration can change based on combining two host strategies (migration and tolerance) for dealing with parasites.In this paper, we aim to understand how both parasite transmission and infection tolerance can influence the host's migratory behaviour.We constructed a model that incorporates two host strategies (migration and tolerance) to understand whether allowing for tolerance affects the proportion of the population that migrates at equilibrium in response to infection.We show that the benefits of tolerance can either decrease or increase the host's migration. Also, if the benefit of migration is great, then individuals are more likely to migrate regardless of the presence of tolerance. Finally, we find that the transmission rate of parasite infection can either decrease or increase the tolerant host's migration, depending on the cost of migration.These findings highlight that adopting two defence strategies is not always beneficial to the hosts. Instead, a single strategy is often better, depending on the costs and benefits of the strategies and infection pressures. Our work further suggests that multiple host‐defence strategies as a potential explanation for the evolution of migration to minimize the parasite infection. Moreover, migration can also affect the ecological and evolutionary dynamics of parasite–host interactions. 

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4. Beyond single host, single parasite interactions: Quantifying competence for complete multi‐host, multi‐parasite communities

   https://doi.org/10.1111/1365-2435.14068  

   Stewart Merrill, Tara E.; Calhoun, Dana M.; Johnson, Pieter T. J. (May 2022, Functional Ecology)

   Abstract Understanding parasite transmission in communities requires knowledge of each species' capacity to support transmission. This property, ‘competence’, is a critical currency for modelling transmission under community change and for testing diversity–disease theory. Despite the central role of competence in disease ecology, we lack a clear understanding of the factors that generate competence and drive its variation.We developed novel conceptual and quantitative approaches to systematically quantify competence for a multi‐host, multi‐parasite community. We applied our framework to an extensive dataset: five amphibian host species exposed to four parasitic trematode species across five ecologically realistic exposure doses. Together, this experimental design captured 20 host–parasite interactions while integrating important information on variation in parasite exposure. Using experimental infection assays, we measured multiple components of the infection process and combined them to produce competence estimates for each interaction.With directly estimated competence values, we asked which components of the infection process best explained variation in competence: barrier resistance (the initial fraction of administered parasites blocked from infecting a host), internal clearance (the fraction of established parasites lost over time) or pre‐transmission mortality (the probability of host death prior to transmission). We found that variation in competence among the 20 interactions was best explained by differences in barrier resistance and pre‐transmission mortality, underscoring the importance of host resistance and parasite pathogenicity in shaping competence.We also produced dose‐integrated estimates of competence that incorporated natural variation in exposure to address questions on the basis and extent of variation in competence. We found strong signals that host species identity shaped competence variation (as opposed to parasite species identity). While variation in infection outcomes across hosts, parasites, individuals and doses was considerable, individual heterogeneity was limited compared to among‐species differences. This finding highlights the robustness of our competence estimates and suggests that species‐level values may be strong predictors for community‐level transmission in natural systems.Competence emerges from distinct underlying processes and can have strong species‐level characteristics; thus, this property has great potential for linking mechanisms of infection to epidemiological patterns. Read the freePlain Language Summaryfor this article on the Journal blog. 

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5. Variability in environmental persistence but not per capita transmission rates of the amphibian chytrid fungus leads to differences in host infection prevalence

   https://doi.org/10.1111/1365-2656.13612  

   Rumschlag, Samantha L.; Roth, Sadie A.; McMahon, Taegan A.; Rohr, Jason R.; Civitello, David J. (November 2021, Journal of Animal Ecology)

   Abstract Heterogeneities in infections among host populations may arise through differences in environmental conditions through two mechanisms. First, environmental conditions may alter host exposure to pathogens via effects on survival. Second, environmental conditions may alter host susceptibility, making infection more or less likely if contact between a host and pathogen occurs. Further, host susceptibility might be altered through acquired resistance, which hosts can develop, in some systems, through exposure to dead or decaying pathogens and their metabolites. Environmental conditions may alter the rates of pathogen decomposition, influencing the likelihood of hosts developing acquired resistance.The present study primarily tests how environmental context influences the relative contributions of pathogen survival and per capita transmission on host infection prevalence using the amphibian chytrid fungus (Batrachochytrium dendrobatidis; Bd) as a model system. Secondarily, we evaluate how environmental context influences the decomposition of Bd because previous studies have shown that dead Bd and its metabolites can illicit acquired resistance in hosts. We conducted Bd survival and infection experiments and then fit models to discern how Bd mortality, decomposition and per capita transmission rates vary among water sources [e.g. artificial spring water (ASW) or water from three ponds].We found that infection prevalence differed among water sources, which was driven by differences in mortality rates of Bd, rather than differences in per capita transmission rates. Bd mortality rates varied among pond water treatments and were lower in ASW compared to pond water.These results suggest that variation in Bd infection dynamics could be a function of environmental factors in waterbodies that result in differences in exposure of hosts to live Bd. In contrast to the persistence of live Bd, we found that the rates of decomposition of dead Bd did not vary among water sources, which may suggest that exposure of hosts to dead Bd or its metabolites might not commonly vary among nearby sites. Ultimately, a mechanistic understanding of the environmental dependence of free‐living pathogens could lead to a deeper understanding of the patterns of outbreak heterogeneity, which could inform surveillance and management strategies. 

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