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ABSTRACT Wild populations face unprecedented pressure from an assortment of anthropogenic environmental changes and parasites. We sought to understand how host–parasite interactions are affected by the interactive effects of multiple environmental stressors and subsequent parasite infection.We focused on American bullfrog tadpoles (Lithobates catesbeianus) that can become infected by several parasite species (e.g., trematodes in the family Echinostomatidae genus Echinstoma, andBatrachochytrium dendrobatidis; “Bd”) and are affected by abiotic stressors including road salt and variable temperatures attributed to climate change. In a multi‐phase laboratory experiment, we exposed tadpoles to one of two salt treatments (0 and 1.5 g L−1added NaCl sublethal salt) and one of two temperature treatments (constant 23°C and fluctuating between 20°C and 25°C). Next, we exposed these tadpoles to one of four parasite treatments (parasite absent,Bdonly, trematodes only, andBd& trematodes together) at a constant temperature (23°C) without added salt. We then recorded morphological measurements (length, mass, developmental stage) and quantified infection intensity.Tadpoles exposed toBdwere larger in mass than the other parasite treatments, and the effects of trematode exposure on length varied by temperature. We did not detect any effect of our treatments on developmental stage. We found an interaction between the salinity and temperature treatments where tadpoles exposed toBdonly or to fluctuating temperatures andBdand trematodes together had higherBdinfection in the no salt added treatment compared to the sublethal salt treatment. Tadpoles in stable temperatures and no salt treatments had lower subsequentBdinfection loads than those in stable temperatures and elevated salt. With trematode infection, we found that when no salt was added, tadpoles exposed to trematodes only had higher parasite abundance compared to the other treatments (i.e., a salinity by parasite interaction).Our results suggest that the interaction of temperatures and salt pollution in freshwater can increase subsequentBdinfection and that the parasites (Bd& echinostome trematodes) can interact with each other, altering infection levels.
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Many roads to reservoirs? How susceptibility and shedding shape host competence in amphibians
Abstract Host competence—the ability to acquire, harbour and transmit infections—drives pathogen spread and persistence in multi‐host communities. Evaluating species‐specific competence is critical for predicting transmission, particularly for generalist fungal pathogens likeBatrachochytrium dendrobatidis(Bd). Despite its central role in disease dynamics, we lack an epidemiologically grounded competence metric that rigorously accounts for how infection intensity affects a host's competence. This knowledge gap limits our ability to compare mechanisms across species and assess their roles in pathogen persistence. To address these challenges, we developed a novel, load‐dependent competence metric using host–pathogen Integral Projection Models (IPMs) that integrates variation in susceptibility, within‐host pathogen growth and pathogen shedding dynamics.We applied this metric to laboratory‐based challenge experiments with three common North American amphibians (Notophthalmus viridescens,Rana clamitansandRana catesbeianus) that persist endemically with Bd. Using dose–response assays and repeated pathogen shedding measurements across species, we asked: (i) is there a consistent, non‐linear relationship between infection intensity and pathogen shedding across species? and (ii) which load‐based traits best predict host competence? We quantified four of five components of host competence—susceptibility, pathogen growth, pathogen survival and load‐dependent shedding—and used these to parameterize species‐specific IPMs, integrating competence into a single relative metric across species.We found that Bd shedding increased non‐linearly with infection intensity, contradicting the standard assumption that Bd shedding is linearly related to infection intensity.Notophthalmus viridescensandR. catesbeianuswere the most competent hosts but through distinct pathways: high susceptibility inN. viridescensand elevated shedding rates inR. catesbeianus. In contrast, density‐dependent reductions in pathogen growth and shedding limitedR. clamitanscompetence. Thus, species‐level competence is not determined by a single trait, but emerges from interactions among multiple load‐based processes.Our results demonstrate that variation in competence emerges from distinct, species‐specific processes across multiple dimensions of competence. By linking individual infection dynamics to population‐level transmission potential, our integrative framework provides a more mechanistic approach to predicting host contributions to community‐level pathogen persistence. Read the freePlain Language Summaryfor this article on the Journal blog.
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- Award ID(s):
- 2044897
- PAR ID:
- 10673164
- Publisher / Repository:
- British Ecological Society
- Date Published:
- Journal Name:
- Functional Ecology
- Volume:
- 40
- Issue:
- 3
- ISSN:
- 0269-8463
- Page Range / eLocation ID:
- 585 to 598
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1111/1365-2435.70250
