Exploring How a Generalist Pathogen and Within-Host Priority Effects Alter the Risk of Being Infected by a Specialist Pathogen

AbstractIn multihost-multipathogen communities, a focal host's risk of being infected by a particular pathogen can be influenced by the presence of other host and pathogen species. We explore how indirect interactions between pathogens at the within-host level (through coinfecting the same individual) and the between-host level (through altered susceptible host densities) affect the focal host's risk of infection. We use an SI-type epidemiological model of two host species and two environmentally transmitted pathogens where one pathogen is a specialist on the focal host and the other pathogen is a generalist. We show that monotonic, unimodal, and U-shaped relationships between the specialist and generalist infectious propagule densities (proxies of the focal host's risk of infection) are driven by the way within-host priority effects alter the production of specialist infectious propagules by infected focal host individuals. Interestingly, within-host priority effects can also lead to overcompensation in density wherein increased infected host mortality results in greater specialist infectious propagule density. We interpret these results in terms of how the focal host's risk of being infected by a specialist pathogen is affected by the presence of a generalist pathogen, its alternative host, and within-host priority effects.