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. 2015 Sep 30:14:381.
doi: 10.1186/s12936-015-0906-x.

The epidemiology of subclinical malaria infections in South-East Asia: findings from cross-sectional surveys in Thailand-Myanmar border areas, Cambodia, and Vietnam

Mallika Imwong  1   2 , Thuy Nhien Nguyen  3 , Rupam Tripura  4 , Tom J Peto  5 , Sue J Lee  6   7 , Khin Maung Lwin  8 , Preyanan Suangkanarat  9 , Atthanee Jeeyapant  10 , Benchawan Vihokhern  11 , Klanarong Wongsaen  12 , Dao Van Hue  13 , Le Thanh Dong  14 , Tam-Uyen Nguyen  15 , Yoel Lubell  16 , Lorenz von Seidlein  17   18 , Mehul Dhorda  19 , Cholrawee Promnarate  20 , Georges Snounou  21   22 , Benoit Malleret  23   24 , Laurent Rénia  25 , Lilly Keereecharoen  26 , Pratap Singhasivanon  27 , Pasathorn Sirithiranont  28 , Jem Chalk  29 , Chea Nguon  30 , Tran Tinh Hien  31   32 , Nicholas Day  33   34 , Nicholas J White  35   36 , Arjen Dondorp  37   38 , Francois Nosten  39   40   41
Affiliations

The epidemiology of subclinical malaria infections in South-East Asia: findings from cross-sectional surveys in Thailand-Myanmar border areas, Cambodia, and Vietnam

Mallika Imwong et al. Malar J. .

Abstract

Background: The importance of the submicroscopic reservoir of Plasmodium infections for malaria elimination depends on its size, which is generally considered small in low transmission settings. The precise estimation of this reservoir requires more sensitive parasite detection methods. The prevalence of asymptomatic, sub-microscopic malaria was assessed by a sensitive, high blood volume quantitative real-time polymerase chain reaction method in three countries of the Greater Mekong Sub-region.

Methods: Cross-sectional surveys were conducted in three villages in western Cambodia, four villages along the Thailand-Myanmar border and four villages in southwest Vietnam. Malaria parasitaemia was assessed by Plasmodium falciparum/pan malaria rapid diagnostic tests (RDTs), microscopy and a high volume ultra-sensitive real-time polymerase chain reaction (HVUSqPCR: limit of detection 22 parasites/mL). All villagers older than 6 months were invited to participate.

Results: A census before the surveys identified 7355 residents in the study villages. Parasite prevalence was 224/5008 (4 %) by RDT, 229/5111 (5 %) by microscopy, and 988/4975 (20 %) when assessed by HVUSqPCR. Of these 164 (3 %) were infected with P. falciparum, 357 (7 %) with Plasmodium vivax, 56 (1 %) with a mixed infection, and 411 (8 %) had parasite densities that were too low for species identification. A history of fever, male sex, and age of 15 years or older were independently associated with parasitaemia in a multivariate regression model stratified by site.

Conclusion: Light microscopy and RDTs identified only a quarter of all parasitaemic participants. The asymptomatic Plasmodium reservoir is considerable, even in low transmission settings. Novel strategies are needed to eliminate this previously under recognized reservoir of malaria transmission.

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Figures

Fig. 1
Fig. 1
South East Asia, with markers for the position of the study sites in Thailand–Myanmar border areas, Cambodia and two sites in Vietnam
Fig. 2
Fig. 2
Assembly of study participants
Fig. 3
Fig. 3
The correlations between anaemia, fever and age. Younger participants were more likely to be febrile and anaemic (Hb < 11 g/dL)
Fig. 4
Fig. 4
The detection rate of microscopy and RDT in relation to parasite density
Fig. 5
Fig. 5
Prevalence summary by detection method for each study village (sorted by high volume ultra-sensitive real time polymerase chain reaction; HVUSqPCR)
Fig. 6
Fig. 6
Fever in relation to P. falciparum and P. vivax densities
See this image and copyright information in PMC

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