Does antimalarial mass drug administration increase or decrease the risk of resistance?

All antimalarial drugs have eventually succumbed to resistance. There is a general belief that the more people that are exposed to an antimalarial drug, the more likely it is for resistance to emerge. Mass drug administration is therefore considered a potent cause of antimalarial drug resistance. However it is the total number of parasites exposed and their individual probabilities of survival and spread that determine the risk, not the number of people that contain them. In malaria endemic areas a substantial proportion of the community carries malaria parasites in their blood without being ill. Although there are many more people with asymptomatic than symptomatic malaria at any time their parasite burdens are several orders of magnitude lower, and their host-defence mechanisms have proved substantially more effective. Symptomatic infections with high parasite numbers the most likely source of resistance emergence. Effective mass treatment which reduces the number of symptomatic malaria cases reduces the overall parasite load in a population and so can reduce the threat of antimalarial resistance emerging and spreading in treated populations