Rapid evolution and ecological host-parasite dynamics

Why do epidemics end? Traditionally, the termination of epidemics has been attributed to ecological causes: namely, the depletion of susceptible hosts as a result of mortality or acquired immunity. Here, we suggest that epidemics can also end because of rapid host evolution. We have used a particular host-parasite system, Daphnia dentifera and the virulent yeast parasite Metschnikowia bicuspidata, as a model system for studying the effects of evolution on ecological host-parasite dynamics. We found that Daphnia from lakes with recent epidemics were more resistant to infection and had less variance in susceptibility than Daphnia from lakes without recent epidemics, indicating directional selection by the parasite. However, we found little evidence for genetic variation in infectivity or virulence in Metschnikowia. We used a quantitative genetics approach to derive an epidemiological model that incorporates the substantial genetic variation in Daphnia susceptibility, allowing for evolution of this susceptibility. This evolutionary epidemiological model reveals that rapid evolution can explain the termination of epidemics on timescales matching what occurs in lake populations. Thus, not only does this work provide rare evidence for parasite-mediated selection in natural populations, it also suggests that rapid evolution has important effects on short-term host-parasite dynamics, including the potential to drive the termination of epidemics. We are currently extending this work to study the effects of evolution on host-parasite dynamics on longer temporal and larger spatial scales.