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Prevalence of asymptomatic malaria and genotyping of Glucose 6-Phosphate (G6PD) deficiencies in a vivax-predominant setting, Lao PDR: Implications for sub-national elimination goals

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Background Even in low transmission settings many malaria infections may remain unidentified by routine passive surveillance within the heath sector. The northern half of Lao PDR has had rapid declines in reported incidence, and Plasmodium vivax now accounts for the vast majority of cases. Consequently, the national malaria program has prioritized a series of studies to confirm and understand these changes to inform policy to accelerate elimination nationally. Materials and Methods Two-stage cluster sampling was used to randomly select 1,500 households across 100 villages in four epidemiologically-diverse districts in northern Lao PDR. Household heads were interviewed using a demographic and risk factor survey, and all residents >18 months were invited to participate in parasitemia testing using rapid diagnostic tests (RDTs) and dried blood spots for PCR. Data collection occurred from September through October 2016, and a total of 5,082 persons were enrolled. PCR-based testing for parasitemia (all samples) and G6PDd genotyping (random subset of all males) were performed at Institut Pasteur in Cambodia. Results With sample demographics consistent with national census data, zero infections were detected by RDTs; 39 infections were detected by PCR (weighted prevalence 0.77%; 95% CI: 0.40-1.47%), with 3 P. falciparum, 28 P. vivax, 2 P. malariae, 5 P. falciparum/P. vivax, and 1 P. vivax/P. malariae infections. Cases included all age groups, and both sexes, with clustering at both household- and village-level. In preliminary analyses, lack of household bednet ownership, and having any household members sleeping overnight in forest or rice field areas were associated with PCR-positivity (χ2 p< 0.001); G6PD genotyping results are pending. Conclusions The low overall prevalence for parasitemia suggests these areas are very feasible targets for subnational elimination, but the demographics of PCR-positives suggests the assumption of adult males being at highest risk may need re-examining in some areas of Laos. The clustering suggests that targeting of interventions should be considered, and the high proportion of P. vivax infections highlights the critical need for G6PDd data to ensure safe and efficacious deployment of primaquine towards elimination goals.