Who carries malaria parasites over the dry season?

Transmission of Plasmodium parasites that cause malaria is often seasonal with very low transmission during the dry season and high transmission in the rainy season. Plasmodium falciparum parasites can survive the dry season within some humans. For malaria elimination efforts, it is crucial to learn more about this parasite reservoir in humans. We use a mathematical model incorporating random mosquito bites and slow acquisition of non-sterilizing general immunity to explore which factors influence whether someone carries parasites over the dry season. Based on model simulations, we hypothesize that parasite carriage over the dry season is exposure mediated. With increasing exposure, i.e., with higher age and Force Of Infection (FOI, the mean number of infectious mosquito bites per day), immunity increases. Higher levels of immunity lead to longer infections and a higher probability of carrying parasites over the dry season. We then test this hypothesis in data from a longitudinal study in Mali and find that carriers are significantly older, have a higher FOI, and develop clinical malaria later than non-carriers.