Dose relationships can exacerbate, mute, or reverse the impact of heterospecific host density on infection prevalence

The likelihood an individual becomes infected depends on the community in
which it is embedded. For environmentally transmitted parasites, host
community composition can alter host density, the density of parasites
that hosts encounter in the environment, and the dose to which hosts are
subsequently exposed. While some multi-host theory incorporates some of
these factors (e.g., competition among hosts), it does not currently
consider the nonlinear relationships between parasite exposure dose and
per-propagule infectivity (dose-infectivity relationships), between
exposure dose and infected host mortality (dose-mortality relationships),
and between exposure dose and parasite propagule excretion (dose-excretion
relationships). This makes it difficult to predict the impact of host
species on one another’s likelihood of infection. To understand the
implications of these non-linear dose relationships for multi-host
communities, we first performed a meta-analysis on published
dose-infectivity experiments to quantify the proportion of accelerating,
linear, or decelerating dose-infectivity relationships; we found that most
experiments demonstrated decelerating dose-infectivity relationships. We
then explored how dose-infectivity, dose-mortality, and dose-excretion
relationships might alter the impact of heterospecific host density on
infectious propagule density, infection prevalence, and density of a focal
host using two host, one parasite models. We found that dose relationships
either decreased the magnitude of the impact of heterospecific host
density on propagule density and infection prevalence via negative
feedback loops (decelerating dose-infectivity relationships, positive
dose-mortality relationships, and negative dose-excretion relationships),
or increased the magnitude of the impact of heterospecific host density on
infection prevalence via positive feedback loops (accelerating
dose-infectivity relationships and positive dose-excretion relationships).
Further, positive dose-mortality relationships resulted in hosts that
traditionally decrease disease (e.g. low-competence, strong competitors)
increasing infection prevalence, and vice versa. Finally, we found that
dose-relationships can create positive feedback loops that facilitate
friendly competition (i.e., increased heterospecific density has a
positive effect on focal host density because the reduction in disease
outweighs the negative effects of interspecific competition). This
suggests that without taking dose relationships into account, we may
incorrectly predict the effect of heterospecific host interactions, and
thus host community composition, on environmentally transmitted parasites.