Strategies for understanding and reducing the Plasmodium vivax and Plasmodium ovale hypnozoite reservoir in Papua New Guinean children: a randomised placebo-controlled trial and mathematical model

Abstract

Background: The undetectable hypnozoite reservoir for relapsing Plasmodium vivax and P. ovale malarias presents a major challenge for malaria control and elimination in endemic countries. This study aims to directly determine the contribution of relapses to the burden of P. vivax and P. ovale infection, illness, and transmission in Papua New Guinean children.

Methods and findings: From 17 August 2009 to 20 May 2010, 524 children aged 5-10 y from East Sepik Province in Papua New Guinea (PNG) participated in a randomised double-blind placebo-controlled trial of blood- plus liver-stage drugs (chloroquine [CQ], 3 d; artemether-lumefantrine [AL], 3 d; and primaquine [PQ], 20 d, 10 mg/kg total dose) (261 children) or blood-stage drugs only (CQ, 3 d; AL, 3 d; and placebo [PL], 20 d) (263 children). Participants, study staff, and investigators were blinded to the treatment allocation. Twenty children were excluded during the treatment phase (PQ arm: 14, PL arm: 6), and 504 were followed actively for 9 mo. During the follow-up time, 18 children (PQ arm: 7, PL arm: 11) were lost to follow-up. Main primary and secondary outcome measures were time to first P. vivax infection (by qPCR), time to first clinical episode, force of infection, gametocyte positivity, and time to first P. ovale infection (by PCR). A basic stochastic transmission model was developed to estimate the potential effect of mass drug administration (MDA) for the prevention of recurrent P. vivax infections. Targeting hypnozoites through PQ treatment reduced the risk of having at least one qPCR-detectable P. vivax or P. ovale infection during 8 mo of follow-up (P. vivax: PQ arm 0.63/y versus PL arm 2.62/y, HR = 0.18 [95% CI 0.14, 0.25], p < 0.001; P. ovale: 0.06 versus 0.14, HR = 0.31 [95% CI 0.13, 0.77], p = 0.011) and the risk of having at least one clinical P. vivax episode (HR = 0.25 [95% CI 0.11, 0.61], p = 0.002). PQ also reduced the molecular force of P. vivax blood-stage infection in the first 3 mo of follow-up (PQ arm 1.90/y versus PL arm 7.75/y, incidence rate ratio [IRR] = 0.21 [95% CI 0.15, 0.28], p < 0.001). Children who received PQ were less likely to carry P. vivax gametocytes (IRR = 0.27 [95% CI 0.19, 0.38], p < 0.001). PQ had a comparable effect irrespective of the presence of P. vivax blood-stage infection at the time of treatment (p = 0.14). Modelling revealed that mass screening and treatment with highly sensitive quantitative real-time PCR, or MDA with blood-stage treatment alone, would have only a transient effect on P. vivax transmission levels, while MDA that includes liver-stage treatment is predicted to be a highly effective strategy for P. vivax elimination. The inclusion of a directly observed 20-d treatment regime maximises the efficiency of hypnozoite clearance but limits the generalisability of results to real-world MDA programmes.

Conclusions: These results suggest that relapses cause approximately four of every five P. vivax infections and at least three of every five P. ovale infections in PNG children and are important in sustaining transmission. MDA campaigns combining blood- and liver-stage treatment are predicted to be a highly efficacious intervention for reducing P. vivax and P. ovale transmission.

Trial registration: ClinicalTrials.gov NCT02143934.

Fig 1. Drug administration schedule.

The drug regimen for the PQ and PL treatment arms was administered with direct observation on 20 d (Monday to Friday) over a 4-wk (26-d) period.

Fig 2. Study design and follow-up schedule.

After the drug treatment period, children were actively monitored for infection and illness every 2 wk for the first 12 wk. From week 14 to 32, children were actively monitored for illness every 2 wk and for infection every 4 wk.

Fig 3. Consort diagram: study design, randomisation, and retention of study participants during follow-up.

In the analysis of time to first infection and clinical episode, children were censored on the last day before the first of two consecutive missed clinical visits.

Fig 4. Time to first Plasmodium spp. infection and P. vivax clinical episode.

Kaplan-Meier plots showing the time to first (or only) (A) P. vivax infection by qPCR, (B) P. vivax infection by LM, (C) P. vivax clinical episode, (D) P. falciparum infection by qPCR, (E) P. malariae infection by qPCR, and (F) P. ovale infection by qPCR, in the two treatment arms. Dashed lines represent the respective 95% confidence intervals.

Fig 5. Kaplan-Meier plots showing the time to first (or only) P. vivax infection by qPCR in the PL and PQ arms, stratified by Plasmodium infection status at enrolment.

Dashed lines represent the respective 95% confidence intervals. PV, P. vivax.

Fig 6. Mathematical-model-based predictions of impact of MDA and MSAT with either blood-stage drugs only or blood- plus liver-stage drugs on the population prevalence of P. vivax and P. falciparum infections.

The effect of two rounds (6 mo apart) of MDA (A and B) or MSAT (C and D) with anti-malarial drugs at 80% coverage on P. vivax (A and C) and P. falciparum (B and D) blood-stage parasite prevalence, as predicted by a stochastic model in a human population of size 5,000. The lines represent the mean of 1,000 repeat simulations, and the shaded areas represent the envelopes containing 95% of stochastic simulations. The grey and green shaded bars denote the duration of prophylactic protection for DHA-PIP/CQ and tafenoquine, respectively, after each treatment round. DHA-PIP and CQ were assumed to be administered as part of a 3-d regimen, providing prophylaxis for 1 mo. PQ was assumed to be administered as part of a 14-d regimen, providing prophylaxis for 15 d. Tafenoquine was assumed to be administered via a single dose, providing prophylaxis for 2 mo.