Proliferation in malaria parasites: how resource limitation can prevent evolution of greater virulence.

Abstract

For parasites, robust proliferation within hosts is crucial for establishing the infection and creating opportu- nities for onward transmission. While faster proliferation enhances transmission rates, it is often assumed to curtail transmission duration by killing the host (virulence), a tradeoff constraining parasite evolution. Yet in many diseases, including malaria, the preponderance of infections with mild or absent symptoms suggests that host mortality is not a sufficient constraint, raising the question of what restrains evolution towards faster proliferation. In malaria infections, the maximum rate of proliferation is determined by the burst size, the number of daughter parasites produced per infected red blood cell. Larger burst sizes should expand the pool of infected red blood cells that can be used to produce the specialized transmission forms needed to infect mosquitoes. We use a within-host model parameterized for rodent malaria parasites (Plasmodium chabaudi ) to project the transmission consequences of burst size, focusing on initial acute infection where re- source limitation and risk of host mortality are greatest. We find that resource limitation restricts evolution towards higher burst sizes below the level predicted by host mortality alone. Our results suggest resource lim- itation could represent a more general constraint than virulence-transmission tradeoffs, preventing evolution towards faster proliferation.