Decreasing pfmdr1 Copy Number Suggests that Plasmodium falciparum in Western Cambodia Is Regaining In Vitro Susceptibility to Mefloquine

Dihydroartemisinin-piperaquine is the current frontline artemisinin combination therapy (ACT) for Plasmodium falciparum malaria in Cambodia but is now failing in several western provinces. To investigate artesunate plus mefloquine (AS + MQ) as a replacement ACT, we measured the prevalence of multiple pfmdr1 copies-a molecular marker for MQ resistance-in 844 P. falciparum clinical isolates collected in 2008 to 2013. The pfmdr1 copy number is decreasing in Western Cambodia, suggesting that P. falciparum is regaining in vitro susceptibility to MQ.