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Background  Malaria treatment efforts are hindered by the rapid emergence and spread of drug resistant parasites. Simple assays to monitor parasite drug response in direct patient samples ( ex vivo ) can detect drug resistance before it becomes clinically apparent, and can inform changes in treatment policy to prevent the spread of resistance.    Methods  Parasite drug responses to amodiaquine, artemisinin, chloroquine and mefloquine were tested in approximately 400  Plasmodium falciparum  malaria infections in Thiès, Senegal between 2008 and 2011 using a DAPI-based  ex vivo  drug resistance assay. Drug resistance-associated mutations were also genotyped in  pfcrt  and  pfmdr1 .    Results  Parasite drug responses changed between 2008 and 2011, as parasites became less sensitive to amodiaquine, artemisinin and chloroquine over time. The prevalence of known resistance-associated mutations also changed over time. Decreased amodiaquine sensitivity was associated with sustained, highly prevalent mutations in  pfcrt , and one mutation in  pfmdr1  – Y184F – was associated with decreased parasite sensitivity to artemisinin.    Conclusions  Directly measuring  ex vivo  parasite drug response and resistance mutation genotyping over time are useful tools for monitoring parasite drug responses in field samples. Furthermore, these data suggest that the use of amodiaquine and artemisinin derivatives in combination therapies is selecting for increased drug tolerance within this population.

Changes in drug sensitivity and anti-malarial drug resistance mutations over time among Plasmodium falciparum parasites in Senegal