Increased Sensitivity of Plasmodium falciparum to Artesunate/Amodiaquine Despite 14 Years as First-Line Malaria Treatment, Zanzibar

Abstract

Artemisinin-based combination therapies (ACTs) are first-line treatments for uncomplicated Plasmodium falciparum malaria. ACT resistance is spreading in Asia but not yet in Africa. Reduced effects of ACT partner drugs have been reported but with little information regarding widely used artesunate/amodiaquine (ASAQ). We studied its efficacy in Zanzibar after 14 years as first-line treatment directly by an in vivo, single-armed trial and indirectly by prevalences of different genotypes in the P. falciparum chloroquine-resistance transporter, multidrug-resistance 1, and Kelch 13 propeller domain genes. In vivo efficacy was higher during 2017 (100%; 95% CI 97.4%-100%) than during 2002-2005 (94.7%; 95% CI 91.9%-96.7%) (p = 0.003). Molecular findings showed no artemisinin resistance-associated genotypes and major increases in genotypes associated with high sensitivity/efficacy for amodiaquine than before ASAQ was introduced. Thus, the efficacy of ASAQ is maintained and appears to be increased after long-term use in contrast to what is observed for other ACTs used in Africa.

Keywords: ACT; ASAQ; P. falciparum Kelch 13 propeller domain gene; P. falciparum chloroquine-resistance transporter gene; P. falciparum multidrug-resistance 1 gene; Plasmodium falciparum; Tanzania; Zanzibar; artemisinin-based combination therapy; artesunate/amodiaquine; drug resistance; genotypes; malaria; parasites; pfcrt gene; pfk13 gene; pfmdr1 gene; resistance selection; trends; vector-borne infections; zoonoses.

Figure 1

Locations of 14 study health centers, including 11 peripheral satellite health units and 3 referral health facilities for which increased sensitivity of Plasmodium falciparum to artesunate/amodiaquine despite 14 years as first-line malaria treatment was tested, Zanzibar. A) Unguja Island; B) Pemba Island.

Figure 2

Comparison of parasite clearance rates until day 3 posttreatment for 3 study periods, Zanzibar. Microscopy determined geometrical mean parasite densities. Only parasite densities >2,000 parasites/μL on day 0 were included in 2017. Microscopy negative samples were given an arbitrary value of 1 parasite/μL.

Figure 3

Frequency of polymorphisms associated with amodiaquine resistance in Plasmodium falciparum infections in Zanzibar, 2002–2017. Black bars indicate resistance alleles, gray bars indicate mixed infections, and white bars indicate wild-type alleles. Error bars indicate 95% CIs of proportions of infections harboring resistance alleles (either alone or mixed infections). Values in parentheses are the total number of genotyped samples shown next to the study year. Trend analysis: p<0.001 for pfcrt 76T + mixed, pfmdr1 86Y + mixed, pfmdr1 Y184 + mixed, pfmdr1 1246Y + mixed; p<0.001 for pfcrt 76T, pfmdr1 86Y, pfmdr1 Y184Y; and p = 0.016 for pfmdr1 1246Y. Pfcrt, P. falciparum chloroquine-resistance transporter gene; Pfmdr1, P. falciparum multidrug-resistance gene.