Competitive suppression and release in artemisinin-resistant Plasmodium falciparum field isolates

Infection with multiple genetically distinct Plasmodium falciparum parasites is common in human malaria cases. Within an infected patient, a particular genotype may thrive, influencing its chance of transmission. Competitive growth assays provide a quantitative index to assess relative dynamic fitness disparities among co-infecting parasites across experimentally controlled, physiologically relevant conditions to test specific hypotheses. The ability to assess the relative competitive fitness of parasite genotypes present in patient-derived samples will be particularly valuable to track the spread of partial artemisinin (ART) resistance now emerging in Africa.

Pairwise competitive growth outcomes of genetically distinct ART-resistant (ART-R) and ART-sensitive clones isolated from the Thailand-Myanmar border were evaluated after perturbation with dihydroartemisinin. Fluorescent labeled microsatellite markers were used to measure the relative growth densities of each competing parasite. Resistant and susceptible clones were mixed and grown alone in the presence and absence of drug to determine dynamic fitness relationships among the co-infecting parasites.

ART-R strains demonstrate a competitive advantage when grown in the presence of a high drug dose. However, the loss of this advantage varies in the absence of drug, producing a range of fitness phenotypes for sensitive and resistant strains.

Not surprisingly, drug-resistant parasites outcompete susceptible parasites under drug; however, additional variables influence whether resistant parasites ultimately prevail when drug is removed, suggesting independent or interacting genetic mechanisms. Knowing relative fitness advantages and costs of specific genotypes can inform how they will spread and evolve in a competitive environment, directing novel intervention strategies.

Malaria infections often contain both drug-resistant and drug-sensitive parasites. Parasites were co-grown with and without antimalarials and competitive outcomes varied: sometimes resistant parasites won, sometimes sensitives parasites won, and in one case, the winner changed with antimalarial treatment. Drug-resistant parasite success depends on competing parasites, complicating the prediction of resistance spread.