title: Household clustering of asymptomatic malaria infections in Xepon district, Savannakhet province, Lao PDR.
abstract: BACKGROUND
  In the Lao PDR, malaria morbidity and mortality have remarkably decreased over the past decade. However, asymptomatic infections in rural villages contribute to the on-going local transmission. The primary objective of this study was to explore the characteristics of infections in a malaria-endemic district of the Lao PDR. The specific objectives were to investigate the prevalence and species of malaria parasites using molecular methods and to assess individual and household parasite levels and the characteristics associated with malaria infection.
  
  
  METHODS
  The study population included 870 participants from 236 households in 10 villages of the Xepon district. Interviews, blood examinations and body temperature measurements were conducted between August and September 2013. A multilevel logistic regression model, with adjustment for clustering effects, was used to assess the association between predictor variables and an outcome variable (malaria infection status as principally determined by PCR). The predictive factors included individual-level factors (age, gender, past fever episode, and forest activity during night time) and household-level factors (household member size, household bed net usage/density and a household with one other malaria-infected member).
  
  
  RESULTS
  Fifty-two participants (including 26 children) tested positive (positive rate: 6.0 %): Plasmodium falciparum mono-infection was the most common infection (n = 41, 78.8 %), followed by P. falciparum and Plasmodium vivax mixed infections (n = 9, 17.3 %). The majority of infected participants (n = 42, 80.8 %) had no fever episodes in the two previous weeks or a measurable fever (>37 °C) at the time of survey. Living in a household with one other malaria-infected member significantly increased the odds of infection (odds ratio 24.33, 95 % confidence interval 10.15-58.32). Among the 40 households that had at least one infected member, nine households were responsible for 40.4 % of the total infections.
  
  
  CONCLUSIONS
  Plasmodium vivax was detected more frequently than it was reported from the district hospital. Most infections were asymptomatic and sub-microscopic and were highly clustered within households. To further eliminate malaria in Xepon and other similar settings in the country, the National Malaria Control Programme should consider household-based strategies, including reactive case detection targeting the household members of index cases.
authors.affiliation: Savannakhet Provincial Health Department, Phonsavangnuea Village, Kaysone-Phomvihan District, Savannakhet, Lao PDR.|Department of Molecular and Cellular Parasitology, School of Medicine, Juntendo University, 2-1-1 Hongo, Bunkyo-ku, Tokyo, 113-8421, Japan.|SATREPS Project for Parasitic Diseases, Vientiane, Lao PDR.
authors.forename: Tiengkham
authors.initials: T
authors.lastname: Pongvongsa
authors.affiliation: SATREPS Project for Parasitic Diseases, Vientiane, Lao PDR. laodaisuke@hotmail.co.jp.|Department of Global Health, School of Health Sciences, University of the Ryukyus, 207 Uehara, Nishihara-cho, Okinawa, 903-0215, Japan. laodaisuke@hotmail.co.jp.
authors.forename: Daisuke
authors.initials: D
authors.lastname: Nonaka
authors.affiliation: SATREPS Project for Parasitic Diseases, Vientiane, Lao PDR.|Department of Tropical Medicine and Malaria, Research Institute, National Center for Global Health and Medicine, 1-21-1 Toyama, Shinjuku-ku, Tokyo, 162-8655, Japan.|Institut Pasteur du Laos, Sisattanak District, Vientiane, Lao PDR.
authors.forename: Moritoshi
authors.initials: M
authors.lastname: Iwagami
authors.affiliation: SATREPS Project for Parasitic Diseases, Vientiane, Lao PDR.|Department of Tropical Medicine and Malaria, Research Institute, National Center for Global Health and Medicine, 1-21-1 Toyama, Shinjuku-ku, Tokyo, 162-8655, Japan.
authors.forename: Masami
authors.initials: M
authors.lastname: Nakatsu
authors.affiliation: Savannakhet Provincial Health Department, Phonsavangnuea Village, Kaysone-Phomvihan District, Savannakhet, Lao PDR.
authors.forename: Panom
authors.initials: P
authors.lastname: Phongmany
authors.affiliation: Graduate School of Tropical Medicine and Global Health, Nagasaki University, 1-12-4 Sakamoto, Nagasaki-shi, Nagasaki, 852-8523, Japan.
authors.forename: Futoshi
authors.initials: F
authors.lastname: Nishimoto
authors.affiliation: SATREPS Project for Parasitic Diseases, Vientiane, Lao PDR.|Department of Global Health, School of Health Sciences, University of the Ryukyus, 207 Uehara, Nishihara-cho, Okinawa, 903-0215, Japan.
authors.forename: Jun
authors.initials: J
authors.lastname: Kobayashi
authors.affiliation: SATREPS Project for Parasitic Diseases, Vientiane, Lao PDR.|Center of Malariology, Parasitology and Entomology, Ministry of Health, Vientiane, Lao PDR.
authors.forename: Bouasy
authors.initials: B
authors.lastname: Hongvanthon
authors.affiliation: SATREPS Project for Parasitic Diseases, Vientiane, Lao PDR.|Institut Pasteur du Laos, Sisattanak District, Vientiane, Lao PDR.
authors.forename: Paul T
authors.initials: PT
authors.lastname: Brey
authors.affiliation: Graduate School of Tropical Medicine and Global Health, Nagasaki University, 1-12-4 Sakamoto, Nagasaki-shi, Nagasaki, 852-8523, Japan.
authors.forename: Kazuhiko
authors.initials: K
authors.lastname: Moji
authors.affiliation: Department of Molecular and Cellular Parasitology, School of Medicine, Juntendo University, 2-1-1 Hongo, Bunkyo-ku, Tokyo, 113-8421, Japan.|SATREPS Project for Parasitic Diseases, Vientiane, Lao PDR.
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Mita
authors.affiliation: SATREPS Project for Parasitic Diseases, Vientiane, Lao PDR.|Department of Tropical Medicine and Malaria, Research Institute, National Center for Global Health and Medicine, 1-21-1 Toyama, Shinjuku-ku, Tokyo, 162-8655, Japan.|Institut Pasteur du Laos, Sisattanak District, Vientiane, Lao PDR.
authors.forename: Shigeyuki
authors.initials: S
authors.lastname: Kano
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D058070:Asymptomatic Diseases / Q000453:epidemiology*; D001769:Blood / Q000469:parasitology; D001831:Body Temperature; D002648:Child; D002675:Child, Preschool; D016000:Cluster Analysis; D060085:Coinfection / Q000453:epidemiology / Q000469:parasitology; D003430:Cross-Sectional Studies; D005191:Family Characteristics; D005192:Family Health; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D007812:Laos / Q000453:epidemiology; D008288:Malaria / Q000453:epidemiology* / Q000469:parasitology; D008297:Male; D008875:Middle Aged; D010963:Plasmodium falciparum / Q000302:isolation & purification*; D010966:Plasmodium vivax / Q000302:isolation & purification*; D015995:Prevalence; D012307:Risk Factors; D055815:Young Adult
keywords: Active case detection; Asymptomatic infections; Laos; Malaria elimination; Sub-microscopic infections