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. Such interactions can occur within hosts, for example 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 different 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. Furthermore, these effects are amplified when parasites invade sequentially, generating pronounced patch-level spatial 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 prevalence and spatial spread of H. undulata through reductions in host susceptibility, and found evidence for spatial priority effects, observing reduced H. undulata prevalence when introduced after H. obtusa. Our theoretical and experimental results highlight that parasite-parasite interactions can have important implications for parasite spatial epidemiology, but the magnitude of those effects depend on the interaction type and the timing of invasion.