A rtemisinin-based combination therapy (ACT) has been first-line treatment for uncomplicated Plasmodium falciparum malaria globally for the past 10-15 years and has contributed greatly to a reduction of malaria illnesses and deaths during 2005-2015 (1,2). However, artemisinin resistance emerged in Cambodia during 2008, where it then spread and even developed de novo throughout the Great Mekong Region (3,4). Possible resistance has been reported from eastern India (5) and, Guyana in South America (6) but not yet from Africa (4). However, ACT resistance represents a continuous threat in contexts such as Zanzibar, where numerous longdistance visitors represent a special risk for imported artemisinin-resistant malaria parasites. Chloroquine resistance entered eastern Africa most probably from India in late 1970s (7). In addition, selection of resistance/tolerance to the slowly eliminated long-acting partner drugs in ACT (e.g., amodiaquine) is expected, especially in highly malaria-endemic areas of Africa (8-10), which could result in relatively reduced ACT cure rates and reduced protection against artemisinin resistance (11). Currently, however, complete ACT resistance has developed and spread only in Asia (e.g., Cambodia) (12). In Zanzibar, malaria transmission has been reduced substantially after new and reinforced malaria tools and interventions, including ACT for uncomplicated malaria (2), were implemented. The reduced parasite biomass on the islands of Zanzibar has resulted in an expected selection (bottleneck) of the parasite populations (2,13), which under strong drug exposure might select for drug resistance. The firstline ACT in Zanzibar has been artesunate/amodiaquine (ASAQ) since 2003, plus recently added single, low-dose primaquine. Artemether/lumefantrine was used as second-line treatment when ACT was first used, followed by quinine when treatment guidelines were revised in 2009 (2). Free access throughout the health systems has resulted in sustained high population coverage and compliance to ASAQ (2,14,15). The partner drug amodiaquine is relatively short-lived (half-life 2-8 hours) and is primarily metabolized to
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