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

ABSTRACT 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 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.