Asymptomatic Plasmodium infections in 18 villages of southern Savannakhet Province, Lao PDR (Laos)

Abstract

BackgroundA large fraction of Plasmodium infections do not cause clinical signs and symptoms of disease and persist at densities in blood that are not detectable by microscopy or rapid diagnostic tests. These infections may be critical as a transmission reservoir in areas of low malaria endemicity. Understanding the epidemiology of these infections would be helpful for malaria elimination.MethodsA cross-sectional survey was conducted in Thapangthong and Nong Districts of Savannakhet Province, Lao PDR, to determine the prevalence of parasitaemia. A total of 888 blood samples were collected from afebrile volunteers aged ≥15 years in 18 villages during March and July 2015. Plasmodium infections were diagnosed by rapid diagnostic tests (RDT) and high volume, ultra-sensitive quantitative polymerase chain reaction (uPCR).ResultsuPCR detected Plasmodium infections in 175 of 888 samples (20 %). The species distribution was Plasmodiumfalciparum 3.6 % (32/888), Plasmodium vivax 11.1 % (99/888), mixed infections with P. falciparum and P. vivax 1.6 % (14/888) and Plasmodium of undetermined species 3.4 % (30/888). RDT identified only 2 % (18/888) positive cases. Using uPCR as reference, the sensitivity and specificity of RDTs were 28 and 100 %, respectively, in detecting P. falciparum infections, and 3 and 99 % in detecting asymptomatic P. vivax infections. The K13 kelch propeller domain C580Y mutation, associated with reduced susceptibility to artemisinin derivatives, was found in 75 % (12/18) of P. falciparum isolates from Thapangthong and in 7 % (2/28) from Nong (p < 0.001). In a multivariate analysis, males were more likely to have P. vivax infections [adjusted odds ratio (aOR) 4.76 (95 % CI 2.84–8.00)] while older villagers were at lower risk for parasitaemia [aOR for increasing age 0.98 (95 % CI 0.96–0.99)].ConclusionThere is a high prevalence of asymptomatic Plasmodium infections in southern Savannakhet. Artemisinin-resistant P. falciparum strains form an increasing proportion of the parasite population in Thapangthong District and are already present in the more remote Nong District. This worrying trend has wider implications for Laos and could reverse the gains achieved by the successful control of malaria in Laos and the Greater Mekong Sub-region (GMS). Rapid elimination of P. falciparum has to be a top priority in Laos as well as in the wider GMS.

K13 Propeller Mutations in Plasmodium falciparum Populations in Regions of Malaria Endemicity in Vietnam from 2009 to 2016

“…One particularly interesting feature was the increase in the predominance of the Cys580Tyr mutation at all sites, a pattern similar to that observed in neighboring Cambodia (8), Laos (15), and Thailand (16). One key question is whether K13 mutant parasites—and Cys580Tyr mutants in particular—were imported from other countries in the region or whether they emerged independently in Vietnam as a result of local drug pressure.…”

Assessment of subpatent Plasmodium infection in northwestern Ethiopia

“…Malaria elimination requires the detection and clearing of all Plasmodium infections. A relatively high prevalence of subpatent or low parasite density infections, that are often missed by conventional diagnostic methods (microscopy and rapid diagnostic tests) are being reported especially from low transmission settings [7][8][9][10][11][12][13][14]. Such asymptomatic, subpatent infections could be explained by acquired immunity in higher transmission settings [15].…”

Elements of effective community engagement: lessons from a targeted malaria elimination study in Lao PDR (Laos)

“…Four TME villages (two ‘intervention’ and two ‘control’) were selected out of 18 prescreened villages in two district of Savannakhet Province according to Plasmodium prevalence [20] (Figure 2). In the ‘intervention’ villages, villagers were offered antimalarials and TME-supported healthcare either at an existing health facility or through a TME physician at the village.…”