title: The epidemiology of subclinical malaria infections in South-East Asia: findings from cross-sectional surveys in Thailand-Myanmar border areas, Cambodia, and Vietnam.
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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. noi@tropmedres.ac.|Department of Molecular Tropical Medicine and Genetics, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. noi@tropmedres.ac.
authors.forename: Mallika
authors.initials: M
authors.lastname: Imwong
authors.affiliation: Oxford University Clinical Research Unit, Hospital for Tropical Diseases, 764 Vo Van Kiet, District 5, Ho Chi Minh City, Vietnam. nhienntt@oucru.org.
authors.forename: Thuy Nhien
authors.initials: TN
authors.lastname: Nguyen
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. Rupam@tropmedres.ac.
authors.forename: Rupam
authors.initials: R
authors.lastname: Tripura
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. Tom@tropmedres.ac.
authors.forename: Tom J
authors.initials: TJ
authors.lastname: Peto
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. Sue@tropmedres.ac.|Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine, Churchill Hospital, Oxford, UK. Sue@tropmedres.ac.
authors.forename: Sue J
authors.initials: SJ
authors.lastname: Lee
authors.affiliation: Shoklo Malaria Research Unit, Faculty of Tropical Medicine, Mahidol University, Tak, Thailand. drkhin_mg_lwin@shoklo-unit.com.
authors.forename: Khin Maung
authors.initials: KM
authors.lastname: Lwin
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. Preyanan@tropmedres.ac.
authors.forename: Preyanan
authors.initials: P
authors.lastname: Suangkanarat
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. Atthanee@tropmedres.ac.
authors.forename: Atthanee
authors.initials: A
authors.lastname: Jeeyapant
authors.affiliation: Shoklo Malaria Research Unit, Faculty of Tropical Medicine, Mahidol University, Tak, Thailand. Benchawan@tropmedres.ac.
authors.forename: Benchawan
authors.initials: B
authors.lastname: Vihokhern
authors.affiliation: Armed Forces Research Institute of Medical Sciences (AFRIMS), Bangkok, Thailand. klanarongw@afrims.org.
authors.forename: Klanarong
authors.initials: K
authors.lastname: Wongsaen
authors.affiliation: Center for Malariology, Parasitology and Entomology Control, Phan Rang-Thap Cham, Ninh Thuan Province, Vietnam. huedvdr@gmail.com.
authors.forename: Dao
authors.initials: D
authors.lastname: Van Hue
authors.affiliation: Institute of Malariology-Parasitology, Entomology (IMPE) of Ho Chi Minh City, 699 Tran Hung Dao Q5, Ho Chi Minh City, Vietnam. lethanhdong@gmail.com.
authors.forename: Le Thanh
authors.initials: le T
authors.lastname: Dong
authors.affiliation: Oxford University Clinical Research Unit, Hospital for Tropical Diseases, 764 Vo Van Kiet, District 5, Ho Chi Minh City, Vietnam. uyennt@oucru.org.
authors.forename: Tam-Uyen
authors.initials: TU
authors.lastname: Nguyen
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. yoel@tropmedres.ac.
authors.forename: Yoel
authors.initials: Y
authors.lastname: Lubell
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. lorenz@tropmedres.ac.|Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine, Churchill Hospital, Oxford, UK. lorenz@tropmedres.ac.
authors.forename: Lorenz
authors.initials: L
authors.lastname: von Seidlein
authors.affiliation: WWARN Asia Regional Centre, Mahidol University, Bangkok, Thailand. mehul.dhorda@wwarn.org.
authors.forename: Mehul
authors.initials: M
authors.lastname: Dhorda
authors.affiliation: WWARN Asia Regional Centre, Mahidol University, Bangkok, Thailand. cholrawee.promnarate@wwarn.org.
authors.forename: Cholrawee
authors.initials: C
authors.lastname: Promnarate
authors.affiliation: Sorbonne Universités, UPMC Univ Paris 06, UPMC UMRS CR7, 75005, Paris, France. georges.snounou@upmc.fr.|Centre d'Immunologie et de Maladies Infectieuses (CIMI), Paris, Institut National de la Santé et de la Recherche Médicale (Inserm) U1135, Centre National de la Recherche Scientifique (CNRS) ERL 8255, 75013, Paris, France. georges.snounou@upmc.fr.
authors.forename: Georges
authors.initials: G
authors.lastname: Snounou
authors.affiliation: Singapore Immunology Network (SIgN), Agency for Science, Technology and Research (A*STAR), Biopolis, Singapore, 278177, Singapore. Benoit_Malleret@immunol.a-star.edu.sg.|Department of Microbiology, Yong Loo Lin School of Medicine, National University of Singapore, National University Health System, 5 Science Drive 2, Blk MD4, Level 3, Singapore, 117597, Singapore. Benoit_Malleret@immunol.a-star.edu.sg.
authors.forename: Benoit
authors.initials: B
authors.lastname: Malleret
authors.affiliation: Singapore Immunology Network (SIgN), Agency for Science, Technology and Research (A*STAR), Biopolis, Singapore, 278177, Singapore. laurentrenia@gmail.com.
authors.forename: Laurent
authors.initials: L
authors.lastname: Rénia
authors.affiliation: Shoklo Malaria Research Unit, Faculty of Tropical Medicine, Mahidol University, Tak, Thailand. lily@shoklo-unit.com.
authors.forename: Lilly
authors.initials: L
authors.lastname: Keereecharoen
authors.affiliation: Department of Tropical Hygiene, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. pratap.sin@mahidol.ac.th.
authors.forename: Pratap
authors.initials: P
authors.lastname: Singhasivanon
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. Pasathorn@tropmedres.ac.
authors.forename: Pasathorn
authors.initials: P
authors.lastname: Sirithiranont
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. Jem@tropmedres.ac.
authors.forename: Jem
authors.initials: J
authors.lastname: Chalk
authors.affiliation: National Center for Parasitology, Entomology and Malaria Control, No. 372, Preah Monivong, Phnom Penh, 12302, Cambodia. cheanguoncnm@gmail.com.
authors.forename: Chea
authors.initials: C
authors.lastname: Nguon
authors.affiliation: Oxford University Clinical Research Unit, Hospital for Tropical Diseases, 764 Vo Van Kiet, District 5, Ho Chi Minh City, Vietnam. hientt@oucru.org.|Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine, Churchill Hospital, Oxford, UK. hientt@oucru.org.
authors.forename: Tran Tinh
authors.initials: TT
authors.lastname: Hien
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. Nickd@tropmedres.ac.|Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine, Churchill Hospital, Oxford, UK. Nickd@tropmedres.ac.
authors.forename: Nicholas
authors.initials: N
authors.lastname: Day
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. nickwdt@tropmedres.ac.|Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine, Churchill Hospital, Oxford, UK. nickwdt@tropmedres.ac.
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: White
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. arjen@tropmedres.ac.|Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine, Churchill Hospital, Oxford, UK. arjen@tropmedres.ac.
authors.forename: Arjen
authors.initials: A
authors.lastname: Dondorp
authors.affiliation: Mahidol Oxford Research Unit, Faculty of Tropical Medicine, Mahidol University, Bangkok, Thailand. francois@tropmedres.ac.|Shoklo Malaria Research Unit, Faculty of Tropical Medicine, Mahidol University, Tak, Thailand. francois@tropmedres.ac.|Centre for Tropical Medicine and Global Health, Nuffield Department of Medicine, Churchill Hospital, Oxford, UK. francois@tropmedres.ac.
authors.forename: Francois
authors.initials: F
authors.lastname: Nosten
mesh_terms: D000293:Adolescent; D000328:Adult; D001210:Asia, Southeastern / Q000453:epidemiology; D058345:Asymptomatic Infections / Q000453:epidemiology*; D002648:Child; D002675:Child, Preschool; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007223:Infant; D008288:Malaria / Q000453:epidemiology*; D008297:Male; D010963:Plasmodium falciparum; D010966:Plasmodium vivax; D055815:Young Adult