Bed Net Durability Assessments: Exploring a Composite Measure of Net Damage

RESEARCH ARTICLE Bed Net Durability Assessments: Exploring a Composite Measure of Net Damage Jodi L. Vanden Eng1*, Adeline Chan2, Ana Paula Abílio3, Adam Wolkon1, Gabriel Ponce de Leon1,4, John Gimnig2, Juliette Morgan1,4 1 Malaria Branch, Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, GA, United States of America, 2 Entomology Branch, Division of Parasitic Diseases and Malaria, Centers for Disease Control and Prevention, Atlanta, GA, United States of America, 3 Instituto Nacional da Saúde, Ministério da Saúde, Maputo, Mozambique, 4 United States President’s Malaria Initiative, Centers for Disease Control and Prevention, Atlanta, GA, United States of America * Abstract Background OPEN ACCESS Citation: Vanden Eng JL, Chan A, Abílio AP, Wolkon A, Ponce de Leon G, Gimnig J, et al. (2015) Bed Net Durability Assessments: Exploring a Composite Measure of Net Damage. PLoS ONE 10(6): e0128499. doi:10.1371/journal.pone.0128499 Academic Editor: Thomas A. Smith, Swiss Tropical & Public Health Institute, SWITZERLAND Received: July 15, 2014 Accepted: April 27, 2015 Published: June 5, 2015 Copyright: This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This study was funded by the U.S. President’s Malaria Initiative through the U.S. Agency for International Development. Two co-authors are affiliated with the President's Malaria Initiative. The funders had no role in the study design, data collection and analysis, decision to publish or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. 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. PLOS ONE | DOI:10.1371/journal.pone.0128499 June 5, 2015 1 / 21 Bed Net Durability Assessments: A Composite Measure of Net Damage 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. Introduction Recent escalation of national bed net distributions and universal coverage campaigns stimulated concern and investigation into the integrity and durability of long lasting insecticidal nets (LLINs) under ordinary use conditions. Mass distribution campaigns are recommended to deliver LLINs to households, rapidly increasing population coverage and improving equity of bed net ownership [1]. Attrition occurs following these campaigns, and LLINs degrade due to ordinary use and washing, compromising the physical integrity and chemical composition of the bed net. As a result, malaria control programs are challenged with devising LLIN replacement strategies that include considerations for net degradation and attrition in order to maintain high levels of protective coverage in the community. In 2005, the World Health Organization Pesticide Evaluation Scheme (WHOPES) developed “Guidelines for laboratory and field testing of LLINS” [2]. This document focused on measuring the bioefficacy of LLINs. Methodologies to test the bioefficacy of LLINs have been thoroughly developed and standardized, including laboratory investigations using cone bioassay and tunnel tests [3] [4]. Moreover, following these guidelines, a variety of experimental hut studies have been performed to investigate the penetration through, feeding success, and mortality of mosquitoes among treated and untreated nets with and without holes [5–18]. In 2011, WHOPES published a second document “Guidelines for monitoring the durability of LLINs under operational conditions” [19]. In addition to the previously described measures of insecticidal activity (bioefficacy), these guidelines suggest two additional outcome measures to assess the useful life of a LLIN: a general measure of attrition and the physical condition or fabric integrity. However, until the recent mass distribution of LLINs, concern about the durability of bed nets in actual field conditions and investigation of the integrity and physical durability of the fabric have been lagging. The integrity of different fabrics in laboratory settings have been compared using standard burst and tear strength tests identifying differences between weight, textures, and weave patterns [20]. Techniques to measure fabric integrity in the field; however, can be laborious and challenging. Precise laboratory measures of irregularly shaped holes are oft (...truncated)