Evolution of Pathogen Virulence across Space during an Epidemic

Abstract

We explore pathogen virulence evolution during the spatial expansion of an infectious disease epidemic, in the presence of a novel host movement trade-off, using a simple spatially explicit mathematical model. This work is motivated by empirical observations of the Mycoplasma gallisepticum invasion into North American House Finch (Haemorhous mexicanus) populations; however, our results likely have important applications to other emerging infectious diseases in mobile hosts. We assume that infection reduces host movement and survival, and that across pathogen strains the severity of these reductions increases with pathogen infectiousness. Assuming these trade-offs between pathogen virulence (host mortality), pathogen transmission, and host movement, we find that pathogen virulence levels near the epidemic front (that maximize wave speed) are lower than the virulence level with a short-term growth rate advantage or that ultimately prevails (i.e., are evolutionarily stable) near the epicenter and where infection becomes endemic (i.e., that maximizes the pathogen basic reproductive ratio). We predict that, under these trade-offs, less virulent pathogen strains will dominate the periphery of an epidemic, and that more virulent strains will increase in frequency after invasion where disease is endemic. These results have important implications for observing and interpreting spatio-temporal epidemic data, and may help explain transient virulence dynamics of emerging infectious diseases.