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. 2015 May 21;9(5):e0003758.
doi: 10.1371/journal.pntd.0003758. eCollection 2015 May.

High Rates of Asymptomatic, Sub-microscopic Plasmodium vivax Infection and Disappearing Plasmodium falciparum Malaria in an Area of Low Transmission in Solomon Islands

Andreea Waltmann  1 , Andrew W Darcy  2 , Ivor Harris  3 , Cristian Koepfli  1 , John Lodo  4 , Ventis Vahi  2 , David Piziki  2 , G Dennis Shanks  3 , Alyssa E Barry  1 , Maxine Whittaker  5 , James W Kazura  6 , Ivo Mueller  7
Affiliations

High Rates of Asymptomatic, Sub-microscopic Plasmodium vivax Infection and Disappearing Plasmodium falciparum Malaria in an Area of Low Transmission in Solomon Islands

Andreea Waltmann et al. PLoS Negl Trop Dis. .

Abstract

Introduction: Solomon Islands is intensifying national efforts to achieve malaria elimination. A long history of indoor spraying with residual insecticides, combined recently with distribution of long lasting insecticidal nets and artemether-lumefantrine therapy, has been implemented in Solomon Islands. The impact of these interventions on local endemicity of Plasmodium spp. is unknown.

Methods: In 2012, a cross-sectional survey of 3501 residents of all ages was conducted in Ngella, Central Islands Province, Solomon Islands. Prevalence of Plasmodium falciparum, P. vivax, P. ovale and P. malariae was assessed by quantitative PCR (qPCR) and light microscopy (LM). Presence of gametocytes was determined by reverse transcription quantitative PCR (RT-qPCR).

Results: By qPCR, 468 Plasmodium spp. infections were detected (prevalence = 13.4%; 463 P. vivax, five mixed P. falciparum/P. vivax, no P. ovale or P. malariae) versus 130 by LM (prevalence = 3.7%; 126 P. vivax, three P. falciparum and one P. falciparum/P. vivax). The prevalence of P. vivax infection varied significantly among villages (range 3.0-38.5%, p<0.001) and across age groups (5.3-25.9%, p<0.001). Of 468 P. vivax infections, 72.9% were sub-microscopic, 84.5% afebrile and 60.0% were both sub-microscopic and afebrile. Local residency, low education level of the household head and living in a household with at least one other P. vivax infected individual increased the risk of P. vivax infection. Overall, 23.5% of P. vivax infections had concurrent gametocytaemia. Of all P. vivax positive samples, 29.2% were polyclonal by MS16 and msp1F3 genotyping. All five P. falciparum infections were detected in residents of the same village, carried the same msp2 allele and four were positive for P. falciparum gametocytes.

Conclusion: P. vivax infection remains endemic in Ngella, with the majority of cases afebrile and below the detection limit of LM. P. falciparum has nearly disappeared, but the risk of re-introductions and outbreaks due to travel to nearby islands with higher malaria endemicity remains.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Ngella sampling sites and spatial distribution of P. vivax prevalence (qPCR).
The 9 island provinces of SI are shown in the top right inset. A (inset). Central Islands Province. B (inset). Ngella. B. Ngella study catchments and prevalence. Anchor Island (catchments shown in yellow), Bay (catchments in red), Channel (catchments in purple), North Coast (catchments in orange) and South Coast (catchments in blue). The size of the prevalence pie chart reflects village sample size.
Fig 2
Fig 2. Age trends of P. vivax infections.
A: P. vivax blood stage parasite prevalence by LM and qPCR (error bars represent binomial 95% confidence intervals, CI95). B: P. vivax parasite densities by qPCR (18S DNA copies/l) and LM counts (parasites/l) (error bars represent 95% confidence intervals, CI95). C: Gametocyte prevalence (in the total sampled population) and positivity (only among P. vivax infected) (error bars represent 95% confidence intervals, CI95). D: P. vivax multiplicity of infection (MOI) of blood stage parasites (error bars represent 95% confidence intervals, CI95).
Fig 3
Fig 3. Diversity of P. vivax genotypic markers msp1F3 and MS16.
Allelic frequencies of P. vivax msp1F3, MS16 and the combined msp1F3 / MS16 haplotypes. For msp1F3 and MS16, the frequency of the four most common alleles and respective sizes (in basepairs) are shown. The number of unique alleles identified and the estimated expected heterozygosity (HE) are given below the figure.
See this image and copyright information in PMC

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