Relevance of Undetectably Rare Resistant Malaria Parasites in Treatment Failure: Experimental Evidence from Plasmodium chabaudi

Abstract

Abstract. Resistant malaria parasites are frequently found in mixed infections with drug-sensitive parasites. Particularly early in the evolutionary process, the frequency of these resistant mutants can be extremely low and below the level of molecular detection. We tested whether the rarity of resistance in infections impacted the health outcomes of treatment failure and the potential for onward transmission of resistance. Mixed infections of different ratios of resistant and susceptible Plasmodium chabaudi parasites were inoculated in laboratory mice and dynamics tracked during the course of infection using highly sensitive genotype-specific quantitative polymerase chain reaction (qPCR). Frequencies of resistant parasites ranged from 10% to 0.003% at the onset of treatment. We found that the rarer the resistant parasites were, the lower the likelihood of their onward transmission, but the worse the treatment failure was in terms of parasite numbers and disease severity. Strikingly, drug resistant parasites had the biggest impact on health outcomes when they were too rare to be detected by any molecular methods currently available for field samples. Indeed, in the field, these treatment failures would not even have been attributed to resistance.

The impact of within-host ecology on the fitness of a drug-resistant parasite

“…Highly sensitive detection techniques are needed to provide important information on competitive interactions between resistant and sensitive P. falciparum strains in field situations [ 62 , 63 ]. Standard PCR assays used on field samples would struggle to detect resistant parasites at time of treatment in all but the 10 5 and 10 6 treatment groups in the current experiment [ 64 ]. Nonetheless, a more thorough understanding of the determinants of in-host competition would help evaluate the evolutionary risks associated with treating patients who are not sick.…”

“…Note that a subset of the data from this experiment has been published elsewhere [ 64 ]. The aim of that earlier analysis was to determine the impact of the frequency of resistance in an infection on the probability of treatment failure, and so it concentrated only on the mixed infections.…”