Differing effects of parasite-parasite interaction types on the spatial epidemiology of co-circulating parasites

Interactions among co-circulating parasite species influence infection risk and disease progression. Interactions can occur within hosts, e.g. altering susceptibility, or indirectly through host demography or movement, potentially affecting landscape-scale transmission. Despite their ubiquity, the spatial implications of these interactions have received limited attention. We combine spatially-explicit modelling with laboratory experiments to investigate how parasite-parasite interactions influence disease spread. We model within-host, demographic, and dispersal-related interactions across a linear landscape, showing that within-host interactions modifying host susceptibility have the strongest effects on parasite prevalence, spatial heterogeneity, and rate of spread. These effects are amplified when parasites invade sequentially, generating pronounced patch-level priority effects. We tested these predictions experimentally using a protist host (Paramecium caudatum) and two bacterial parasites (Holospora undulata and H. obtusa). Consistent with model predictions, we found that H. obtusa reduces the overall prevalence and spread of H. undulata across a landscape of interconnected patches, likely through reductions in host susceptibility, and evidence of priority effects, observing reduced H. undulata prevalence when introduced after H. obtusa. Our results highlight that parasite-parasite interactions can have important implications for the landscape-scale epidemiology of co-circulating parasites, but the effect magnitude depends on the interaction type and the timing of invasion.