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.