Evolution of pathogen virulence across space during an epidemic

We explore pathogen virulence evolution during the spa- tial 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 obser- vations of the Mycoplasma gallisepticum invasion into North Amer- ican house finch (Haemorhous mexicanus) populations; however, our results likely have important applications to other emerging infec- tious 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), path- ogen transmission, and host movement, we find that pathogen vir- ulence levels near the epidemic front (that maximize wave speed) are lower than those that have a short-term growth rate advantage or that ultimately prevail (i.e., are evolutionarily stable) near the epicenter and where infection becomes endemic (i.e., that maximize 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 fre- quency after invasion where disease is endemic. These results have important implications for observing and interpreting spatiotem- poral epidemic data and may help explain transient virulence dy- namics of emerging infectious diseases.