Relationships between host infectivity to mosquitoes and asexual parasite density in Plasmodium falciparum.

We describe a statistical model for the relationship between asexual parasite densities of Plasmodium falciparum and the infectivity of the host to mosquitoes. The model takes into account the delay between asexual parasitemia and infectivity resulting from the time course of gametocytemia. It also allows for the need for the blood meal to contain gametocytes of both sexes if infection is to take place. We show that by fitting this model to data from malariatherapy patients it can explain observed patterns of infectiousness of the human host and is consistent with distributions of gametocyte densities in malariatherapy patients. By integrating this model into an individual-based simulation of human populations exposed to endemic P. falciparum transmission, we are able to predict the contributions of different host age groups to the infectious reservoir. Comparison of model predictions with published estimates of this quantity confirms that infected adults hosts are likely to make a significant contribution to the reservoir of transmission, and points to the need for improved population-based estimates of this age-dependence in infectivity of humans in endemic areas.