Summary Background Artemisinin resistance in Plasmodium falciparum lengthens parasite clearance half-life during artemisinin monotherapy or artemisinin-based combination therapy. Absence of in-vitro and ex-vivo correlates of artemisinin resistance hinders study of this phenotype. We aimed to assess whether an in-vitro ring-stage survival assay (RSA) can identify culture-adapted P falciparum isolates from patients with slow-clearing or fast-clearing infections, to investigate the stage-dependent susceptibility of parasites to dihydroartemisinin in the in-vitro RSA, and to assess whether an ex-vivo RSA can identify artemisinin-resistant P falciparum infections. Methods We culture-adapted parasites from patients with long and short parasite clearance half-lives from a study done in Pursat, Cambodia, in 2010 (registered with ClinicalTrials.gov, number NCT00341003) and used novel in-vitro survival assays to explore the stage-dependent susceptibility of slow-clearing and fast-clearing parasites to dihydroartemisinin. In 2012, we implemented the RSA in prospective parasite clearance studies in Pursat, Preah Vihear, and Ratanakiri, Cambodia (NCT01736319), to measure the ex-vivo responses of parasites from patients with malaria. Continuous variables were compared with the Mann-Whitney U test. Correlations were analysed with the Spearman correlation test. Findings In-vitro survival rates of culture-adapted parasites from 13 slow-clearing and 13 fast-clearing infections differed significantly when assays were done on 0–3 h ring-stage parasites (10·88% vs 0·23%; p=0·007). Ex-vivo survival rates significantly correlated with in-vivo parasite clearance half-lives (n=30, r=0·74, 95% CI 0·50–0·87; p<0·0001). Interpretation The in-vitro RSA of 0–3 h ring-stage parasites provides a platform for the molecular characterisation of artemisinin resistance. The ex-vivo RSA can be easily implemented where surveillance for artemisinin resistance is needed. Funding Institut Pasteur du Cambodge and the Intramural Research Program, NIAID, NIH.

fThe declining efficacy of artemisinin derivatives against Plasmodium falciparum in western Cambodia is a major concern. The knowledge gap in the understanding of the mechanisms involved hampers designing monitoring tools. Here, we culture-adapted 20 isolates from Pailin and Ratanakiri (areas of artemisinin resistance and susceptibility in western and eastern Cambodia, respectively) and studied their in vitro response to dihydroartemisinin. No significant difference between the two sets of isolates was observed in the classical isotopic test. However, a 6-h pulse exposure to 700 nM dihydroartemisinin (ring-stage survival assay -RSA]) revealed a clear-cut geographic dichotomy. The survival rate of exposed ring-stage parasites (ring stages) was 17-fold higher in isolates from Pailin (median, 13.5%) than in those from Ratanakiri (median, 0.8%), while exposed mature stages were equally and highly susceptible (0.6% and 0.7%, respectively). Ring stages survived drug exposure by cell cycle arrest and resumed growth upon drug withdrawal. The reduced susceptibility to artemisinin in Pailin appears to be associated with an altered in vitro phenotype of ring stages from Pailin in the RSA.

Background: Resistance to anti-malarial drugs hampers control efforts and increases the risk of morbidity and mortality from malaria. The efficacy of standard therapies for uncomplicated Plasmodium falciparum and Plasmodium vivax malaria was assessed in Chumkiri, Kampot Province, Cambodia.

Background Plasmodium vivax is the most prevalent human malaria parasite, causing serious public health problems in malaria-endemic countries. Until recently the Duffy-negative blood group phenotype was considered to confer resistance to vivax malaria for most African ethnicities. We and others have reported that P. vivax strains in African countries from Madagascar to Mauritania display capacity to cause clinical vivax malaria in Duffy-negative people. New insights must now explain Duffy-independent P. vivax invasion of human erythrocytes.Methods/Principal FindingsThrough recent whole genome sequencing we obtained ≥70× coverage of the P. vivax genome from five field-isolates, resulting in ≥93% of the Sal I reference sequenced at coverage greater than 20×. Combined with sequences from one additional Malagasy field isolate and from five monkey-adapted strains, we describe here identification of DNA sequence rearrangements in the P. vivax genome, including discovery of a duplication of the P. vivax Duffy binding protein (PvDBP) gene. A survey of Malagasy patients infected with P. vivax showed that the PvDBP duplication was present in numerous locations in Madagascar and found in over 50% of infected patients evaluated. Extended geographic surveys showed that the PvDBP duplication was detected frequently in vivax patients living in East Africa and in some residents of non-African P. vivax-endemic countries. Additionally, the PvDBP duplication was observed in travelers seeking treatment of vivax malaria upon returning home. PvDBP duplication prevalence was highest in west-central Madagascar sites where the highest frequencies of P. vivax-infected, Duffy-negative people were reported.Conclusions/SignificanceThe highly conserved nature of the sequence involved in the PvDBP duplication suggests that it has occurred in a recent evolutionary time frame. These data suggest that PvDBP, a merozoite surface protein involved in red cell adhesion is rapidly evolving, possibly in response to constraints imposed by erythrocyte Duffy negativity in some human populations.

BackgroundAn estimated 2.85 billion people live at risk of Plasmodium vivax transmission. In endemic countries vivax malaria causes significant morbidity and its mortality is becoming more widely appreciated, drug-resistant strains are increasing in prevalence, and an increasing number of reports indicate that P. vivax is capable of breaking through the Duffy-negative barrier long considered to confer resistance to blood stage infection. Absence of robust in vitro propagation limits our understanding of fundamental aspects of the parasite's biology, including the determinants of its dormant hypnozoite phase, its virulence and drug susceptibility, and the molecular mechanisms underlying red blood cell invasion.Methodology/Principal FindingsHere, we report results from whole genome sequencing of five P. vivax isolates obtained from Malagasy and Cambodian patients, and of the monkey-adapted Belem strain. We obtained an average 70–400 X coverage of each genome, resulting in more than 93% of the Sal I reference sequence covered by 20 reads or more. Our study identifies more than 80,000 SNPs distributed throughout the genome which will allow designing association studies and population surveys. Analysis of the genome-wide genetic diversity in P. vivax also reveals considerable allele sharing among isolates from different continents. This observation could be consistent with a high level of gene flow among parasite strains distributed throughout the world.ConclusionsOur study shows that it is feasible to perform whole genome sequencing of P. vivax field isolates and rigorously characterize the genetic diversity of this parasite. The catalogue of polymorphisms generated here will enable large-scale genotyping studies and contribute to a better understanding of P. vivax traits such as drug resistance or erythrocyte invasion, partially circumventing the lack of laboratory culture that has hampered vivax research for years.