High <i>Plasmodium</i> infection intensity in naturally infected malaria vectors in Africa

Abstract The population dynamics of human-to-mosquito malaria transmission in the field has important implications for the genetics, epidemiology and control of malaria. The number of oocysts in oocysts positive mosquitoes developing from a single, naturally acquired infectious blood meal (herein referred to as parasite exposure) greatly influence the effectiveness of transmission blocking interventions but still remains poorly documented. During a year-long analysis of malaria parasite transmission in Burkina Faso we caught and dissected wild malaria vectors to assess Plasmodium oocysts prevalence and load (the number of oocysts counted in mosquitoes with detectable oocysts) and the prevalence of salivary gland sporozoites. This was compared to malaria endemicity in the human population assessed in cross-sectional surveys. Data was analyzed using a novel transmission mathematical model to estimate the per-bite transmission probability and the average parasite exposure of mosquitoes for each location. Observed oocysts load and estimated parasite exposure in naturally infected mosquitoes is substantially higher than previous estimates (ranging from 3.2 to 24.5 according to seasons and locations) and indicates a strong positive association between parasite exposure of mosquitoes and parasite prevalence in human. This work suggests that highly infected mosquitoes may have a greater influence on the epidemiology and genetics of the parasite and that novel partially effective transmission blocking interventions may become more effective at halting transmission as parasite exposure is diminished.