Parasite evolution in an age-structured population

Abstract

Although mortality increases with age in most organisms, senescence is missing from models of parasite evolution. Since virulence evolves according to the host’s mortality, and since virulence influences the intensity of transmission, which determines the average age at infection and thus the mortality rate of a senescing host, we expected that epi-evolutionary feedbacks would underlie the evolution of virulence in a population of senescing hosts. We tested this idea by extending an age-structured model of epidemiological dynamics with the parasite’s evolution. A straightforward prediction of our model is that stronger senescence forces the evolution of higher virulence. However, the model also reveals that the evolved virulence depends on the average age at infection, giving an evolutionary feedback with the epidemiological situation, a prediction not found when assuming a constant mortality rate with age. Additionally, and in contrast to most models of parasite evolution, we found that the virulence at the evolutionary equilibrium is influenced by whether the force of infection depends on the density or on the frequency of infected hosts, due to changes in the average age at infection. Our findings suggest that ignoring age-specific effects, and in particular senescence, can give misleading predictions about parasite evolution.