Investigating the Mechanisms Underlying Mosquito Infectiousness in Malaria

Project Summary Malaria continues to have a huge toll on morbidity and mortality in the world, responsible for over 600,000 deaths annually. The transmission stages are bottlenecks for the parasite and thus critical stages for intervention, particularly when aiming for elimination. Using the rodent model Plasmodium yoelli, we recently found that salivary gland sporozoite load of a biting mosquito strongly correlates with infection likelihood, with highly infected mosquitoes being 7.5 times more likely to initiate an infection. Importantly, the likelihood of achieving secondary infections rapidly rises at salivary gland sporozoite densities >10,000 that represent a small proportion of field- caught mosquitoes. These data challenge the assumption that all infected mosquitoes are equal, a belief that underlies epidemiological models of transmission and impacts malaria elimination strategies. In this proposal we will determine the mechanisms underlying the increased infectious potential of highly infected mosquitoes, comparing the quantity of sporozoites inoculated by mosquitoes harboring high and low numbers of sporozoites, as well as the quality of sporozoites in these two groups. In Aim 1 we will determine the inocula associated with low- and high-infected mosquitoes using both rodent malaria parasites and the human parasite P. falciparum. In Aim 2, we will compare sporozoites from mosquitoes with low and high infections using infection assays, intravital imaging, and single-cell RNA-Seq analysis. Overall these experiments will fill crucial knowledge gaps on parasite transmission probability that will greatly improve our understanding of parasite biology and epidemiology.