Within-host priority effects and epidemic timing determine outbreak severity in coinfected populations

Coinfections of hosts by multiple pathogen species are ubiquitous, but
predicting their impact on disease remains challenging. Interactions
between coinfecting pathogens within hosts can alter pathogen
transmission, with the impact on transmission typically dependent on the
relative arrival order of pathogens within hosts (within-host priority
effects). However, it is unclear how these within-host priority effects
influence multi-pathogen epidemics, particularly when the arrival order of
pathogens at the host population scale varies. Here we combined models and
experiments with zooplankton and their naturally co-occurring fungal and
bacterial pathogens to examine how within-host priority effects influence
multi-pathogen epidemics. Epidemiological models parameterized with
within-host priority effects measured at the single host scale predicted
that advancing the start date of bacterial epidemics relative to fungal
epidemics would decrease mean bacterial prevalence in a multi-pathogen
setting, while models without within-host priority effects predicted the
opposite effect. We tested these predictions with experimental
multi-pathogen epidemics. Empirical dynamics matched predictions from the
model including within-host priority effects, providing evidence that
within-host priority effects influenced epidemic dynamics. Overall,
within-host priority effects may be a key element of predicting
multi-pathogen epidemic dynamics in the future, particularly as shifting
disease phenology alters the order of infection within hosts.