Rapid evolution, seasonality, and the termination of parasite epidemics.

Why do epidemics end? This simple question has puzzled ecologists and epidemiologists for decades. Early explanations focused on drops in host density arising from highly virulent parasites and, later, on the effects of acquired immunity. More recently, however, two additional epidemic-ending mechanisms have surfaced: environmental change (including seasonality) and rapid evolution of increased resistance of hosts to infection. Both mechanisms, via either decreasing seasonal temperatures or evolution of resistance, act by altering transmission rates. To explore these possibilities, we tracked five epidemics of a virulent yeast parasite in lake populations of Daphnia dentifera from late summer through autumn. We then fit and compared performance of time-series models that included temperature-dependent and/or evolutionary changes in transmission rates. The analyses show evolution to be the better explanation of epidemic dynamics. Thus, by integrating data and models, this study highlights the potential role of evolution in driving the termination of epidemics in natural populations.