High proportion of genome-wide homology and increased basal pvcrt levels in Plasmodium vivax late recurrences: a chloroquine therapeutic efficacy study

Chloroquine (CQ) is the first-line treatment for Plasmodium vivax malaria in most endemic countries. Monitoring P.vivax CQ resistance (CQR) is critical but remains challenged by the difficulty to distinguish real treatment failure from reinfection or liver relapse. Therapeutic efficacy of CQ against uncomplicated P.vivax malaria was evaluated in Gia Lai province, Vietnam. Sixty-seven patients were enrolled and followed-up for 42 days using microscopy and (RT)qPCR. Adequate clinical and parasitological response (ACPR) was 100% (66/66) on Day 28, but 75.4% (49/65) on Day 42. Eighteen recurrences (27.7%) were detected with a median time-to-recurrence of 42 days (IQR 35, 42) and blood CQ concentration <100ng/ml. Parasite genotyping by microsatellites, SNP-barcoding and whole-genome sequencing (WGS) identified a majority of homologous recurrences, with 80% (8/10) showing >98% identity-by-descent to paired Day 0 samples. Primary infections leading to recurrence occurred in younger individuals (median age for ACPR=25 years [IQR 20, 28]; recurrences=18 [16, 21]; p=0.002), had a longer parasite clearance time (PCT for ACPR=47.5h [IQR 36.2, 59.8]; recurrences=54.2h [48.4, 62.0]; p=0.035) and higher pvcrt gene expression (median relative expression ratio for ACPR=0.09 [IQR 0.05, 0.22]; recurrences=0.20 [0.15, 0.56]; p=0.002), but there was no difference in ex vivo CQ sensitivity. This study shows that CQ remained largely efficacious to treat P.vivax in Gia Lai, i.e. recurrences occurred late (>Day 28) and in the presence of low blood CQ concentrations. However, the combination of WGS and gene expression analysis (pvcrt) with clinical data (PCT) allowed to identify potential emergence of low-grade CQR that should be closely monitored.