Asymptomatic malaria: a hidden reservoir for the transmission and circulation of artemisinin partial-resistant <i>Plasmodium falciparum</i> in Tanzania

Asymptomatic malaria infections contribute substantially to silent transmission, but the prevalence of artemisinin resistance (ART-R) markers in these carriers remains poorly understood. A community-based cross-sectional study was conducted in Tanzania from December 2022 to July 2023, enrolling 3,489 participants from high-transmission regions of Geita and Kigoma and a low-transmission region of Arusha. Four villages per region were randomly selected, and venous blood samples were tested using rapid diagnostic tests, microscopy, and qPCR, revealing overall positivity rate of 24.4%, 15.8%, and 26.2%, respectively, which indicate a significant proportion of submicroscopic infections. Among the 802 isolates successfully sequenced for <i>pfk13</i> and <i>pfmdr1</i>, 24 (3.0%) isolates from high-transmission areas carried validated <i>pfk13</i> partial-resistance markers Y493H (0.2%), R561H (2.0%), and A675V (0.7%), while all low-transmission isolates were wild-type. All isolates retained the <i>pfmdr1</i> N86 codon, and the NFD haplotype associated with reduced susceptibility to lumefantrine was detected in 48.1% and 48.4% of isolates in high- and low-transmission areas, respectively. Mutations were more frequent in children under five and in females. Artemether-lumefantrine (AL, 64.7%) was the most commonly used antimalarial in high-transmission areas, whereas sulfadoxine-pyrimethamine (SP, 75.9%) predominated in low-transmission areas. Higher AL use correlated with increased <i>pfmdr1</i> mutation prevalence in high-transmission regions, while NFD detection in low-transmission areas may reflect gene flow from high-transmission settings. These findings demonstrate that asymptomatic carriers are a substantial hidden reservoir of ART-resistant parasites, emphasizing the importance of integrating molecular surveillance and demographic information on asymptomatic infections into malaria control programs to detect emerging resistance and guide targeted interventions in Tanzania.