A role for super-spreaders in carrying malaria parasites across the months-long dry season

In malaria endemic regions, transmission of Plasmodium falciparum parasites is often seasonal with very low transmission during the dry season and high transmission in the wet season. Parasites survive the dry season within some individuals who experience prolonged carriage of parasites and are thought to ‘seed’ infection in the next transmission season. We use a combination of mathematical simulations and data analysis to characterise dry season carriers and their role in the subsequent transmission season. Simulating the life-history of individuals experiencing repeated exposure to infection predicts that dry season carriage is more likely in the oldest, most exposed and most immune individuals. This hypothesis is supported by data from a longitudinal study in Mali that shows that carriers are significantly older, experience a higher biting rate at the beginning of the transmission season and develop clinical malaria later than non-carriers. Further, since the most exposed individuals in a community are most likely to be dry season carriers, we show that this is predicted to enable a more than 2-fold faster spread of parasites into the mosquito population at the start of the subsequent wet season.