Parasite responses to resource provisioning can be altered by within-host co-infection interactions

Anthropogenic changes to the environment significantly impact wildlife infectious diseases by modifying food resources and impacting host-parasite interactions through changes in host demography, behaviour and immune defences. Supplemental resource provisioning has been found to both enhance and mitigate parasite transmission; however, the role of co-infecting parasites in mediating these effects remains understudied. We developed a mathematical model to explore these dynamics, motivated by the empirical system of wood mice (Apodemus sylvaticus) infected with the nematode Heligmosomoides polygyrus, which suppresses co-infections by the apicomplexan microparasite Eimeria hungaryensis. Our model shows that the effects of resource provisioning on parasite epidemiology can be mediated, and potentially reversed, by within-host co-infection interactions, through effects on host-parasite contact rates and host susceptibility. Provisioning may elevate microparasite prevalence by reducing nematode burdens, thereby releasing the microparasite from the negative effects of co-infection. However, if provisioning increases host contact rates with parasite infective stages in the environment, the associated increase in nematode burdens can result in concomitant reductions in the microparasite, owing to the negative within-host co-infection interaction. Our study highlights the need for experimental designs that decouple the complex factors of provisioning on co-infecting parasite dynamics and provides a framework for interpreting outcomes in multi-parasite systems.