Effects of liver-stage clearance by Primaquine on gametocyte carriage of Plasmodium vivax and P. falciparum

Abstract

Background: Primaquine (PQ) is the only currently licensed antimalarial that prevents Plasmodium vivax (Pv) relapses. It also clears mature P. falciparum (Pf) gametocytes, thereby reducing post-treatment transmission. Randomized PQ treatment in a treatment-to-reinfection cohort in Papua New Guinean children permitted the study of Pv and Pf gametocyte carriage after radical cure and to investigate the contribution of Pv relapses.

Methods: Children received radical cure with Chloroquine, Artemether-Lumefantrine plus either PQ or placebo. Blood samples were subsequently collected in 2-to 4-weekly intervals over 8 months. Gametocytes were detected by quantitative reverse transcription-PCR targeting pvs25 and pfs25.

Results: PQ treatment reduced the incidence of Pv gametocytes by 73%, which was comparable to the effect of PQ on incidence of blood-stage infections. 92% of Pv and 79% of Pf gametocyte-positive infections were asymptomatic. Pv and to a lesser extent Pf gametocyte positivity and density were associated with high blood-stage parasite densities. Multivariate analysis revealed that the odds of gametocytes were significantly reduced in mixed-species infections compared to single-species infections for both species (ORPv = 0.39 [95% CI 0.25-0.62], ORPf = 0.33 [95% CI 0.18-0.60], p<0.001). No difference between the PQ and placebo treatment arms was observed in density of Pv gametocytes or in the proportion of Pv infections that carried gametocytes. First infections after blood-stage and placebo treatment, likely caused by a relapsing hypnozoite, were equally likely to carry gametocytes than first infections after PQ treatment, likely caused by an infective mosquito bite.

Conclusion: Pv relapses and new infections are associated with similar levels of gametocytaemia. Relapses thus contribute considerably to the Pv reservoir highlighting the importance of effective anti-hypnozoite treatment for efficient control of Pv.

Trial registration: ClinicalTrials.gov NCT02143934.

Fig 1. Pv (top) and Pf (bottom) gametocyte positivity among 5019 follow-up samples.

(A, D) Detection of blood stage parasites and gametocytes by LM and molecular methods, using pv18S or pf18S rRNA qPCR for detection of blood-stage parasites and pvs25 or pfs25 qRT-PCR for detection of gametocytes. Black: gametocyte positive samples. White: parasite positive samples without gametocytes. (B, E) Proportion of gametocytes positives (by molecular methods) in submicroscopic and LM-positive samples. (C, F) Proportion of gametocyte positives (by molecular methods) in symptomatic and asymptomatic infections. Error bars indicate 95% confidence intervals by X² distribution.

Fig 2

Prevalence of blood-stage parasites and gametocytes of Pv (A) and Pf (B) during follow-up by treatment arm.

Fig 3. Gametocyte positivity and density in first Pv (top) and Pf (bottom) infections after treatment with blood-stage antimalarials alone (placebo) or blood-stage antimalarials plus PQ (PQ).

(A, D). Proportion of Pv and Pf gametocyte carriers among first infections by treatment arm. Figures within the bars indicate absolute numbers of gametocyte-positive first infections following treatment. Error bars indicate 95% confidence intervals by X² distribution. (B, E). Normalized Pv and Pf gametocyte densities in first infections by treatment arm. Densities were normalized by dividing pvs25 or pfs25 transcript numbers/μl by Pv- or Pf-18S rRNA copy numbers/μl. (C, F) Absolute Pv and Pf gametocyte densities in first infections by treatment arm. Densities are expressed as log₁₀ of pvs25 and pfs25 transcripts/μl.