Coinfection and the evolution of parasite virulence

TLDR

Coinfection, where hosts harbor multiple parasite strains that transmit independently, complicates the selection pressures on virulence and contrasts with superinfection models that impose a dominance hierarchy. The study investigates how coinfection with many parasite strains shapes the evolutionary dynamics of host–parasite associations. The authors analyze these dynamics under the assumption that each strain transmits at a rate unaffected by others within the same host. In highly diverse parasite populations, coinfection selects for strains whose virulence lies in a narrow band near the maximum compatible with a basic reproductive ratio R0 greater than one.

Abstract

Analyses of the selection pressures acting on parasite virulence are made more complicated when individual hosts can simultaneously harbour many different strains or genotypes of a parasite. Here we explore the evolutionary dynamics of host-parasite associations in which individual hosts can be coinfected with many different parasite strains. (We take coinfection to mean that each strain transmits at a rate unaffected by the presence of others in the same host.) This study thus represents the opposite extreme to our earlier work on superinfection in which there is a dominance hierarchy such that only the most virulent strain present in a host is transmitted. For highly diverse populations of parasite strains, we find that such coinfection leads to selection for strains whose virulence-levels lie in a relatively narrow band close to the maximum consistent with the parasite's basic preproductive ratio, R0, exceeding unity.