Evolutionary theory argues that ecological interactions between pathogens within an infection can be a potent source of selection shaping traits such as virulence, drug resistance, and infectiousness. In humans, malaria infections are frequently genetically diverse, with mixed genotype infections the norm. A wide variety of evidence shows that crowding occurs within infections, with the population densities of individual genotypes suppressed by the presence of others. Public health interventions are expected to impact on levels of immunity experienced by pathogens, indirectly by reducing the rate of acquisition of natural immunity by reducing the force of infection, and directly in the case of vaccination programs. Here we ask how enhanced host immunity affects competitive interactions between malaria parasites within hosts and thus the strength of in-host selection on traits such as virulence. We used a model malaria system, Plasmodium chabaudi in laboratory mice, where it has been previously shown that less virulent parasites are competitively suppressed by more virulent strains, generating within-host selection for increased virulence. We found that immunization with either a recombinant antigen or with live parasites suppressed parasite densities, but that there was no evidence that immunization relieved or exacerbated competitive suppression, or affected the relative frequency of clones within infections. There is thus no reason to think that immunization strengthens or alleviates the potentially very potent selection on parasite traits arising from interactions between pathogen genotypes within infections.
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Vol. 62 • No. 9