title: Asymptomatic Plasmodium infections in 18 villages of southern Savannakhet Province, Lao PDR (Laos).
abstract: A 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.
  
  
  
  A 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).
  
  
  
  uPCR detected Plasmodium infections in 175 of 888 samples (20 %). The species distribution was Plasmodium falciparum 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)].
  
  
  
  There 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.
authors.affiliation: Microbiology Laboratory, Lao-Oxford-Mahosot Hospital-Wellcome Trust Research Unit (LOMWRU), Vientiane, Lao PDR.
authors.forename: Koukeo
authors.initials: K
authors.lastname: Phommasone
authors.affiliation: Mahidol Oxford Tropical Medicine Research Unit (MORU), Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
authors.forename: Bipin
authors.initials: B
authors.lastname: Adhikari
authors.affiliation: Mahidol Oxford Tropical Medicine Research Unit (MORU), Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
authors.forename: Gisela
authors.initials: G
authors.lastname: Henriques
authors.affiliation: Savannakhet Provincial Health Department, Nakhon Sawan, Savannakhet Province, Lao PDR.
authors.forename: Tiengkham
authors.initials: T
authors.lastname: Pongvongsa
authors.affiliation: Savannakhet Provincial Health Department, Nakhon Sawan, Savannakhet Province, Lao PDR.
authors.forename: Panom
authors.initials: P
authors.lastname: Phongmany
authors.affiliation: Mahidol Oxford Tropical Medicine Research Unit (MORU), Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. Lorenz@tropmedres.ac.|Centre for Tropical Medicine and Global Health, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, United Kingdom. Lorenz@tropmedres.ac.
authors.forename: Lorenz
authors.initials: L
authors.lastname: von Seidlein
authors.affiliation: Mahidol Oxford Tropical Medicine Research Unit (MORU), Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.|Centre for Tropical Medicine and Global Health, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, United Kingdom.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: White
authors.affiliation: Mahidol Oxford Tropical Medicine Research Unit (MORU), Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.|Centre for Tropical Medicine and Global Health, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, United Kingdom.
authors.forename: Nicholas P J
authors.initials: NP
authors.lastname: Day
authors.affiliation: Mahidol Oxford Tropical Medicine Research Unit (MORU), Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.|Centre for Tropical Medicine and Global Health, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, United Kingdom.
authors.forename: Arjen
authors.initials: A
authors.lastname: M Dondorp
authors.affiliation: Microbiology Laboratory, Lao-Oxford-Mahosot Hospital-Wellcome Trust Research Unit (LOMWRU), Vientiane, Lao PDR.|Centre for Tropical Medicine and Global Health, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, United Kingdom.
authors.forename: Paul N
authors.initials: PN
authors.lastname: Newton
authors.affiliation: Mahidol Oxford Tropical Medicine Research Unit (MORU), Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand.
authors.forename: Mallika
authors.initials: M
authors.lastname: Imwong
authors.affiliation: Microbiology Laboratory, Lao-Oxford-Mahosot Hospital-Wellcome Trust Research Unit (LOMWRU), Vientiane, Lao PDR.|Centre for Tropical Medicine and Global Health, Nuffield Department of Clinical Medicine, University of Oxford, Oxford, United Kingdom.|Faculty of Postgraduate Studies, University of Health Sciences, Vientiane, Lao PDR.
authors.forename: Mayfong
authors.initials: M
authors.lastname: Mayxay
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000962:Antimalarials / Q000494:pharmacology; D037621:Artemisinins / Q000494:pharmacology; D058345:Asymptomatic Infections / Q000453:epidemiology*; D002846:Chromatography, Affinity; D003430:Cross-Sectional Studies; D004351:Drug Resistance; D005260:Female; D006801:Humans; D007783:Lactones / Q000494:pharmacology; D007812:Laos / Q000453:epidemiology; D008288:Malaria / Q000453:epidemiology*; D008297:Male; D008875:Middle Aged; D010963:Plasmodium falciparum / Q000302:isolation & purification; D010966:Plasmodium vivax / Q000302:isolation & purification; D015995:Prevalence; D060888:Real-Time Polymerase Chain Reaction; D012424:Rural Population; D055815:Young Adult
chemical_list: D000962:Antimalarials; D037621:Artemisinins; D007783:Lactones; C075875:artemisin
keywords: Asymptomatic; Lao PDR; Prevalence; RDT; uPCR