title: Bed Net Durability Assessments: Exploring a Composite Measure of Net Damage.
abstract: BACKGROUND
  The durability of Long Lasting Insecticidal Nets (LLINs) in field conditions is of great importance for malaria prevention and control efforts; however, the physical integrity of the net fabric is not well understood making it challenging to determine overall effectiveness of nets as they age. The 2011 World Health Organization Pesticide Evaluation Scheme (WHOPES) guidelines provide a simple, standardized method using a proportional hole index (PHI) for assessing net damage with the intent to provide national malaria control programs with guidelines to assess the useful life of LLINS and estimate the rate of replacement.
  
  
  METHODS
  We evaluated the utility of the PHI measure using 409 LLINs collected over three years in Nampula Province, Mozambique following a mass distribution campaign in 2008. For each LLIN the diameter and distance from the bottom of the net were recorded for every hole. Holes were classified into four size categories and a PHI was calculated following WHOPES guidelines. We investigate how the size, shape, and location of holes influence the PHI. The areas of the WHOPES defined categories were compared to circular and elliptical areas based on approximate shape and actual measured axes of each hole and the PHI was compared to cumulative damaged surface area of the LLIN.
  
  
  RESULTS
  The damaged area of small, medium, large, and extra-large holes was overestimated using the WHOPES categories compared to elliptical areas using the actual measured axes. Similar results were found when comparing to circular areas except for extra-large holes which were underestimated. (Wilcoxon signed rank test of differences p< 0.0001 for all sizes). Approximating holes as circular overestimated hole surface area by 1.5 to 2 times or more. There was a significant difference in the mean number of holes < 0.5 cm by brand and there were more holes of all sizes on the bottom of nets than the top. For a range of hypothetical PHI thresholds used to designate a "failed LLIN", roughly 75 to 80% of failed LLINs were detected by considering large and extra-large holes alone, but sensitivity varied by brand.
  
  
  CONCLUSIONS
  Future studies may refine the PHI to better approximate overall damaged surface area. Furthermore, research is needed to identify whether or not appropriate PHI thresholds can be used to deem a net no longer protective. Once a cutoff is selected, simpler methods of determining the effective lifespan of LLINs can help guide replacement strategies for malaria control programs.
authors.affiliation: Malaria Branch, Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, GA, United States of America.
authors.forename: Jodi L
authors.initials: JL
authors.lastname: Vanden Eng
authors.affiliation: Entomology Branch, Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, GA, United States of America.
authors.forename: Adeline
authors.initials: A
authors.lastname: Chan
authors.affiliation: Instituto Nacional da Saúde, Ministério da Saúde, Maputo, Mozambique.
authors.forename: Ana Paula
authors.initials: AP
authors.lastname: Abílio
authors.affiliation: Malaria Branch, Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, GA, United States of America.
authors.forename: Adam
authors.initials: A
authors.lastname: Wolkon
authors.affiliation: Malaria Branch, Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, GA, United States of America; United States President's Malaria Initiative, Centers for Disease Control and Prevention, Atlanta, GA, United States of America.
authors.forename: Gabriel
authors.initials: G
authors.lastname: Ponce de Leon
authors.affiliation: Entomology Branch, Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, GA, United States of America.
authors.forename: John
authors.initials: J
authors.lastname: Gimnig
authors.affiliation: Malaria Branch, Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, GA, United States of America; United States President's Malaria Initiative, Centers for Disease Control and Prevention, Atlanta, GA, United States of America.
authors.forename: Juliette
authors.initials: J
authors.lastname: Morgan
mesh_terms: D019544:Equipment Failure Analysis; D006801:Humans; D057148:Insecticide-Treated Bednets; D008288:Malaria / Q000517:prevention & control; D009032:Mosquito Control / Q000295:instrumentation* / Q000379:methods; D009073:Mozambique