Prospective Clinical and Molecular Evaluation of Potential Plasmodium ovale curtisi and wallikeri Relapses in a High-transmission Setting

Abstract

Background: Plasmodium ovale curtisi and wallikeri are perceived as relapsing malarial parasites. Contrary to Plasmodium vivax, direct evidence for this hypothesis is scarce. The aim of this prospective study was to characterize the reappearance patterns of ovale parasites.

Methods: P. ovale spp. infected patients were treated with artemether-lumefantrine and followed biweekly for up to 1 year for the detection of reappearing parasitemia. Molecular analysis of reappearing isolates was performed to identify homologous isolates by genotyping and to define cases of relapse following predefined criteria.

Results: At inclusion, 26 participants were positive for P. ovale curtisi and/or P. ovale wallikeri. The median duration of follow-up was 35 weeks. Reappearance of the same P. ovale species was observed in 46% of participants; 61% of P. ovale curtisi and 19% of P. ovale wallikeri infection-free intervals were estimated to end with reappearance by week 32. Based on the predefined criteria, 23% of participants were identified with 1 or 2 relapses, all induced by P. ovale curtisi.

Conclusion: These findings are in line with the currently accepted relapse theory inasmuch as the reappearance of P. ovale curtisi strains following initial blood clearance was conclusively demonstrated. Interestingly, no relapse of P. ovale wallikeri was observed.

Keywords: Plasmodium ovale; Plasmodium ovale curtisi; Plasmodium ovale wallikeri; CERMEL; relapse.

Figure 1.

Patient profile/timeline plot depicting reappearance patterns throughout the observational period. The x-axis shows the time in weeks, the y-axis displays Plasmodium ovale spp. infection as categorical variable (yes/no). For better visibility, infection variables are jittered (no: −0.2 to 0.2; yes: 0.8–1.2). Missing values are represented by gaps.

Figure 2.

Infection-free intervals until first (quantitative polymerase chain reaction positive) reappearance in weeks of Plasmodium ovale curtisi parasites (A) and Plasmodium ovale wallikeri parasites (B). The unit of interest is participants. + Loss to follow-up; gray area: 95% confidence interval.

Figure 3.

Infection-free intervals until (quantitative polymerase chain reaction positive) reappearance in weeks for all observed reappearances of Plasmodium ovale curtisi parasites (A) and Plasmodium ovale wallikeri parasites (B). Following drop out due to reappearance, participants are reincluded in the Kaplan-Meier blots. The unit of interest is infection-free time to reappearance. + Loss to follow-up; gray area: 95% confidence interval.

Figure 4.

Kaplan-Meier blots show time in weeks to first relapse of Plasmodium ovale curtisi measuring participants (A) and all observed relapses of P. ovale curtisi parasites measuring time to relapse (B).

Figure 5.

Median Joining network of a 624bp section of the nuclear 18S rDNA gene of Plasmodium ovale curtisi, showing the relationships between haplotypes and information on the geographic origin by country. Numbers in the circles indicate the number of individuals sharing the same haplotype, and bars indicate the number of substitutions between haplotypes. Samples are described in detail in Supplementary Table 1 (Rep324_1: Rep324-SCR and D28; Rep 324_2: Rep324-W12).