Seasonality and the Coexistence of Pathogen Strains

AbstractHost-pathogen models usually explain the coexistence of pathogen strains by invoking population structure, meaning host or pathogen variation across space or individuals; most models, however, neglect the seasonal variation typical of host-pathogen interactions in nature. To determine the extent to which seasonality can drive pathogen coexistence, we constructed a model in which seasonal host reproduction fuels annual epidemics, which are in turn followed by interepidemic periods with no transmission, a pattern seen in many host-pathogen interactions in nature. In our model, a pathogen strain with low infectiousness and high interepidemic survival can coexist with a strain with high infectiousness and low interepidemic survival: seasonality thus permits coexistence. This seemingly simple type of coexistence can be achieved through two very different pathogen strategies, but understanding these strategies requires novel mathematical analyses. Standard analyses show that coexistence can occur if the competing strains differ in terms of R₀, the number of new infections per infectious life span in a completely susceptible population. A novel mathematical method of analyzing transient dynamics, however, allows us to show that coexistence can also occur if one strain has a lower R₀ than its competitor but a higher initial fitness λ₀, the number of new infections per unit time in a completely susceptible population. This second strategy allows coexisting pathogens to have quite similar phenotypes, whereas coexistence that depends on differences in R₀ values requires that coexisting pathogens have very different phenotypes. Our novel analytic method suggests that transient dynamics are an overlooked force in host-pathogen interactions.