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 begin addressing these gaps, we characterized aggregation of the fungal pathogen Batrachochytrium dendrobatidis (Bd) in amphibian hosts. Utilizing the slope of Taylor’s Power Law, we found Bd intensity distributions were more aggregated than many 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 (i.e., 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 contains signatures of potential biological processes of amphibian-Bd systems. Our work can inform future modeling approaches and be extended to other fungal pathogens to elucidate host-fungal interactions and unite host-fungal dynamics under a common theoretical framework.

Methods
This dataset contains records of aggregated populations of amphibians and their fungal pathogen Batrachochytrium dendrobatidis (Bd) loads or intensities. Bd infection intensity obtained from amphibian skin swabs collected in the field. Bd loads were obtained through DNA extraction and qPCR, which detects the number of genomic equivalents or ITS1 copy number of Bd on amphibian skin. These methods were standardized across four dataset utilized for this study and published elsewhere. These included data taken from sites in São Paulo, Brazil [1; and manuscript in review], the East Bay region [2] and Sierra Nevada Mountains [3;4] in California, and across four states in the Eastern US [5]. Samples within each dataset were grouped based on host species, life stage (larva (i.e., tadpole in anuran species), subadult, or adult), research site, season (Brazil: Wet or Dry; East Bay and Sierra: Summer; Eastern US: winter, spring, summer, or fall), and year. 

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