title: A longitudinal study of the durability of long-lasting insecticidal nets in Zambia.
abstract: BACKGROUND
  A key goal of malaria control is to achieve universal access to, and use of, long-lasting insecticidal nets (LLINs) among people at risk for malaria. Quantifying the number of LLINs needed to achieve and maintain universal coverage requires knowing when nets need replacement. Longitudinal studies have observed physical deterioration in LLINs well before the assumed net lifespan of 3 years. The objective of this study was to describe attrition, physical integrity and insecticide persistence of LLINs over time to assist with better quantification of nets needing replacement.
  
  
  METHODS
  999 LLINs distributed in 2011 in two highly endemic provinces in Zambia were randomly selected, and were enrolled at 12 months old. LLINs were followed every 6 months up to 30 months of age. Holes were counted and measured (finger, fist, and head method) and a proportional hole index (pHI) was calculated. Households were surveyed about net care and repair and if applicable, reasons for attrition. Functional survival was defined as nets with a pHI <643 and present for follow-up. At 12 and 24 months of age, 74 LLINs were randomly selected for examination of insecticidal activity and content using bioassay and chemical analysis methods previously described by the World Health Organization (WHO).
  
  
  RESULTS
  A total of 999 LLINs were enrolled; 505 deltamethrin-treated polyester nets and 494 permethrin-treated polyethylene nets. With 74 used to examine insecticide activity, 925 were available for full follow-up. At 30 months, 325 (33 %) LLINs remained. Net attrition was primarily due to disposal (29 %). Presence of repairs and use over a reed mat were significantly associated with larger pHIs. By 30 months, only 56 % of remaining nets met criteria for functional survival. A shorter functional survival was associated with having been washed. At 24 months, nets had reduced insecticidal activity (57 % met WHO minimal criteria) and content (5 % met WHO target insecticide content).
  
  
  CONCLUSIONS
  The median functional survival time for LLINs observed the study was 2.5-3 years and insecticide activity and content were markedly decreased by 2 years. A better measure of net survival incorporating insecticidal field effectiveness, net physical integrity, and attrition is needed.
authors.affiliation: Center for Global Health, Malaria Branch, Centers for Disease Control and Prevention, 1600 Clifton Rd. MS A6, Atlanta, GA, 30333, USA. ktan@cdc.gov.
authors.forename: Kathrine R
authors.initials: KR
authors.lastname: Tan
authors.affiliation: Jhpiego, an affiliate of Johns Hopkins University, Washington, DC, USA. jane.coleman@jhpiego.org.
authors.forename: Jane
authors.initials: J
authors.lastname: Coleman
authors.affiliation: Peace Corps, West Coast Region, San Francisco, CA, USA. bsmith@peacecorps.gov.
authors.forename: Barbara
authors.initials: B
authors.lastname: Smith
authors.affiliation: Zambia National Malaria Control Centre, Lusaka, Zambia. bossbusk@gmail.com.
authors.forename: Busiku
authors.initials: B
authors.lastname: Hamainza
authors.affiliation: Bayer (Pty) Ltd, Isando, South Africa. Cecilia.katebesakala@bayer.com.
authors.forename: Cecilia
authors.initials: C
authors.lastname: Katebe-Sakala
authors.affiliation: Success Academy Charter Schools, Brooklyn, NY, USA. caseykean@gmail.com.
authors.forename: Casey
authors.initials: C
authors.lastname: Kean
authors.affiliation: Society for Family Health, Lusaka, Zambia. Ashleyk9155@gmail.com.
authors.forename: Ashley
authors.initials: A
authors.lastname: Kowal
authors.affiliation: Center for Global Health, Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, GA, USA. jvandeneng@cdc.gov.
authors.forename: Jodi
authors.initials: J
authors.lastname: Vanden Eng
authors.affiliation: National Center for Enteric, Zoonotic, and Infectious Diseases, Centers for Disease Control and Prevention, Atlanta, GA, USA. tparris@cdc.gov.
authors.forename: Tiffany K
authors.initials: TK
authors.lastname: Parris
authors.affiliation: Center for Global Health, Entomology Branch, Centers for Disease Control and Prevention, Atlanta, GA, USA. cmapp@cdc.gov.
authors.forename: Carla T
authors.initials: CT
authors.lastname: Mapp
authors.affiliation: Center for Global Health, Entomology Branch, Centers for Disease Control and Prevention, Atlanta, GA, USA. scsmith2@cdc.gov.
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Smith
authors.affiliation: Center for Global Health, Entomology Branch, Centers for Disease Control and Prevention, Atlanta, GA, USA. rwirtz@cdc.gov.
authors.forename: Robert
authors.initials: R
authors.lastname: Wirtz
authors.affiliation: Zambia National Malaria Control Centre, Lusaka, Zambia. mkamuliwo@yahoo.co.uk.
authors.forename: Mulakwa
authors.initials: M
authors.lastname: Kamuliwo
authors.affiliation: Center for Global Health, Polio Eradication Branch, Centers for Disease Control and Prevention, Atlanta, GA, USA. acraig@cdc.gov.
authors.forename: Allen S
authors.initials: AS
authors.lastname: Craig
mesh_terms: D006801:Humans; D057148:Insecticide-Treated Bednets / Q000592:standards* / Q000706:statistics & numerical data*; D007306:Insecticides / Q000032:analysis*; D008137:Longitudinal Studies; D008288:Malaria / Q000517:prevention & control; D013997:Time Factors; D015024:Zambia
chemical_list: D007306:Insecticides