MUTATION AND THE EVOLUTION OF VIRULENCE

The evolution of parasite virulence can be influenced by intra-host mutation of the parasite and competition among different parasite mutants in the same host. During the course of an infection, parasite variants may be generated that have some competitive advantage over the strains that initiated the infection. This intra-host competition may select for a short-term advantage. It can lead to variants without any transmission potential or variants with extremely high virulence. We develop a mathematical model which describes intra- and inter-host parasite evolution. The major conclusions are: (i) intra-host competition generates a virulence polymorphism in the parasite population; (ii) intra-host competition can shift the mean virulence beyond what would maximize the reproductive rate of the parasite in the host population; and (iii) the parasite population can evolve to intermediate levels of virulence even if there is no trade-off between transmission rate and virulence. Our model applies to highly mutating parasites with long infection periods, such as, for example, the human immunodeficiency virus (HIV).