Response of Plasmodium Vivax Variants to Chloroquine as Determined by Microscopy and Quantitative Polymerase Chain Reaction

Abstract Genotypic heterogeneity in the repetitive portion of the circumsporozoite (CS) protein of Plasmodium vivax has been reported from many P. vivax-endemic areas. The objective of this study was to determine if the VK210 and VK247 CS variants of P. vivax differed in their clearance rates following chloroquine (CQ) therapy. One hundred seventy-one cases of P. vivax infection occurring in patients presenting to a research treatment center in Thailand were analyzed. Finger-prick blood samples were collected for microscopy and spotted onto filter paper at presentation and on each of five days of observation through supervised CQ therapy. A portion of the CS gene was amplified from filter paper samples by the polymerase chain reaction (PCR) and genotyped by oligoprobes specific for the VK210 and VK247 CS repeat regions. The mean time to clear parasitemia as determined by thick blood smear was significantly longer for pure VK210 infections (51 hr; 95% confidence interval [CI] 47.4–54, P = 0.006) and mixed infections (53 hr; 95% CI 49.2–56.7, P = 0.0009) as compared with VK247 infections (44 hr; 95% CI 39.8–47.9). Five patients matched for parasitemia, age, sex, and previous malaria experience were selected from each of the three genotype groups in the larger study for further analysis by quantitative PCR of P. vivax genotype-specific DNA during a treatment course. The mean time to clear parasite DNA, as determined by PCR, was significantly slower for VK210 parasites (65 hr; 95% CI 51–79) than for VK247 parasites (47 hr; 95% CI 30–63, P = 0.045). In patients with mixed infections, VK210 DNA also cleared more slowly (mean 66 hr; 95% CI 40–93) than VK247 DNA (mean 35 hr; 95% CI 23–47, P = 0.056). These results suggest that strain-variable responses to CQ may exist. Detection and quantification of P. vivax genotype-specific DNA from filter paper samples may be useful as a method to monitor response to chemotherapy in a field setting.