Biology subjects

Briggs, C. J.

Publications and source records attributed to Briggs, C. J..

5 recordsLinked to original sources

bioRxiv2025

A continuous-time microparasite model incorporating infection intensity and parasite aggregation

Disease outcomes depend heavily on infection intensity which is often heterogeneous across and within host populations. Most individuals carry low pathogen loads and a few carry high loads, a pattern known as aggregation. While well-characterized in macroparasite systems, aggregation and infection intensity are rarely incorporated into microparasite models. This raises key questions: Do similar mechanisms underlie aggregation in macro- and microparasite systems? And how do aggregation and load-dependent effects shape outcomes such as host suppression and virulence-transmission trade-offs? To address these questions, we developed a novel continuous time microparasite model that allows the pathogen load distribution across hosts to evolve dynamically, shaped by within- and between-host processes. We applied this framework to the amphibian chytrid fungus system involving Batrachochytrium den-drobatidis (Bd), a fungal pathogen threatening amphibian populations worldwide. Our results show that load-dependent mortality reduces aggregation, while faster within-host replication increases it. Aggregation, in turn, weakens host suppression and flat-tens virulence-transmission trade-off, shifting peak transmission to higher replication rates. Overall, our continuous-time microparasite model provides new insights into how infection intensity and aggregation influence host-pathogen dynamics and offers a valuable framework for advancing theoretical and data-driven understanding of how within-host processes scale to population-level disease dynamics for microparasites.

ecology↗

bioRxiv2024

The mean and variance of infection load reveal unseen host evolution

Heterogeneity among individuals, in number of parasites, body size, etc., can have critical interactions with population dynamics. We tease these out with a relatively simple model of such interactions in a case study of microparasite load with empirically supported assumptions to ask a key question. How does variance in microparasite load interact with population-level dynamics? We show how the mean and variance of infection load vary throughout an epidemic. Further, we show how mean and variance have mutual negative feedbacks on each other mediated by high disease-induced death rates at high loads. Helpfully, we find that mean and variance provide information into underlying processes as well. Population-level trends in the mean and variance reveal underlying trends in within-host processes, e.g., differentiating host evolution of defence that manifests as tolerance, constitutive resistance, inducible resistance, or acquired resistance. Such inference may be critical for managing endangered Mountain Yellow Legged Frog populations recovering from Batrachochytrium dendrobatidis epidemics. Lastly, we demonstrate the impact of pathogen load variance on host fitness, pathogen fitness, and host population depression by pathogens. Our results demonstrate the importance of trait heterogeneity and the insights available from relatively simple models, both for microparasite infection load and possibly other traits.

ecology↗

bioRxiv2024

Do fungi look like macroparasites? Quantifying the patterns and mechanisms of aggregation for host-fungal parasite relationships

Most hosts contain few parasites, whereas few hosts contain many. This pattern, known as aggregation, is well-documented in macroparasites where parasite intensity distribution among hosts affects host-parasite dynamics. Infection intensity also drives fungal disease dynamics, but we lack a basic understanding of host-fungal aggregation patterns, how they compare to macroparasites, and if they reflect biological processes. To address these gaps, we characterized aggregation of the fungal pathogen Batrachochytrium dendrobatidis (Bd) in amphibian hosts. Utilizing the slope of Taylors Power Law, we found Bd intensity distributions were more aggregated than macroparasites, conforming closely to lognormal distributions. We observed that Bd aggregation patterns are strongly correlated with known biological processes operating in amphibian populations, such as epizoological phase--invasion, post-invasion, and enzootic--and intensity-dependent disease mortality. Using intensity-dependent mathematical models, we found evidence of evolution of host resistance based on aggregation shifts in systems persisting with Bd following disease-induced declines. Our results show that Bd aggregation is highly conserved across disparate systems and is distinct from aggregation patterns in macroparasites, and contains signatures of potential biological processes of amphibian-Bd systems. Our work lays a foundation to unite host-fungal dynamics under a common theoretical framework and inform future modeling approaches that may elucidate host-fungus interactions.

ecology↗

bioRxiv2021

Effectiveness of antifungal treatments during chytridiomycosis epizootics in populations of an endangered frog

The recently-emerged amphibian chytrid fungus Batrachochytrium dendrobatidis (Bd) has had an unprecedented impact on global amphibian populations, and highlights the urgent need to develop effective mitigation strategies against this pathogen. We conducted field antifungal treatment experiments in populations of the endangered mountain yellow-legged frog during or immediately after Bd-caused mass die-off events. The objective of the treatments was to reduce Bd infection intensity ("load") and in doing so alter frog-Bd dynamics and increase the probability of frog population persistence despite ongoing Bd infection. Experiments included treatment of early life stages (tadpoles and subadults) with the antifungal drug itraconazole, treatment of adults with itraconazole, and augmentation of the skin microbiome of subadults with Janthinobacterium lividum, a commensal bacterium with antifungal properties. All itraconazole treatments caused immediate reductions in Bd load, and produced longer-term effects that differed between life stages. In experiments focused on early life stages, Bd load was reduced in the two months immediately following treatment and was associated with increased survival of subadults. However, Bd load and frog survival returned to pre-treatment levels in less than one year, and treatment had no effect on population persistence. In adults, treatment reduced Bd load and increased frog survival over the three-year post-treatment period, consistent with frogs having developed an effective adaptive immune response against Bd. Despite this protracted period of reduced impacts of Bd on adults, recruitment of new individuals into the adult population was limited and the population eventually declined to near-extirpation. In the microbiome augmentation experiment, bathing frogs in a J. lividum solution after Bd load reduction with itraconazole increased concentrations of this bacterium on frogs, but concentrations declined to baseline levels within one month and did not have a protective effect against Bd infection. Collectively, these results suggest that Bd mitigation efforts focused on frog populations that have recently declined due to Bd emergence are ineffective in causing long-term changes in frog-Bd dynamics and increasing population persistence, due largely to the inability of early life stages to mount an effective immune response against Bd and resulting high susceptibility. This results in repeated recruitment failure and a low probability of population persistence in the face of ongoing Bd infection.

ecology↗

bioRxiv2017

The influence of landscape and environmental factors on ranavirus epidemiology in amphibian assemblages

AimTo quantify the influence of a suite of landscape, abiotic, biotic, and host-level variables on ranavirus disease dynamics in amphibian assemblages at two biological levels (site and host-level).\n\nLocationWetlands within the East Bay region of California, USA.\n\nMethodsWe used competing models, multimodel inference, and variance partitioning to examine the influence of 16 landscape and environmental factors on patterns in site-level ranavirus presence and host-level ranavirus infection in 76 wetlands and 1,377 amphibian hosts representing five species.\n\nResultsThe landscape factor explained more variation than any other factors in site-level ranavirus presence, but biotic and host-level factors explained more variation in host-level ranavirus infection. At both the site- and host-level, the probability of ranavirus presence correlated negatively with distance to nearest ranavirus-positive wetland. At the site-level, ranavirus presence was associated positively with taxonomic richness. However, infection prevalence within the amphibian population correlated negatively with vertebrate richness. Finally, amphibian host species differed in their likelihood of ranavirus infection: American Bullfrogs had the weakest association with infection while Western Toads had the strongest. After accounting for host species effects, hosts with greater snout-vent length had a lower probability of infection.\n\nMain conclusionsStrong spatial influences at both biological levels suggest that mobile taxa (e.g., adult amphibians, birds, reptiles) may facilitate the movement of ranavirus among hosts and across the landscape. Higher taxonomic richness at sites may provide more opportunities for colonization or the presence of reservoir hosts that may influence ranavirus presence. Higher host richness correlating with higher ranavirus infection is suggestive of a dilution effect that has been observed for other amphibian disease systems and warrants further investigation. Our study demonstrates that an array of landscape, environmental, and host-level factors were associated with ranavirus epidemiology and illustrates that their importance vary with biological level.

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