The Optimal Production of Gametocytes byPlasmodium falciparum

We use a simple model of the blood-stage infection dynamics of the malaria parasitePlasmodium falciparumto consider the adaptive significance of different rates of conversion from its pathogenic, asexual stages to its transmissible, sexual forms. We find that the rates that maximize transmissivity in single-strain infections are generally greater than the highest rates reported forin vitrocultures and are several times those for which the behavior of the model is consistent with clinical profiles of infection dynamics. When two strains that share a common immune agent coinfect a host through simultaneous inoculation or sequential superinfection, however, a strain with a lower, clinically-consistent value of the conversion rate inhibits the transmissivity of the one with the higher value optimal for single-strain infection. Hence we suggest that “apparent” competition by way of a common immune response might be responsible for selection of the former.