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

<div><p>Background</p><p>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.</p><p>Methods</p><p>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.</p><p>Results</p><p>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.</p><p>Conclusions</p><p>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.</p></div

Scatterplots of number of extra-small size holes (< 0.5 cm) vs total number of small, medium, large and extra-large size holes per net by type for 130 nets during the 3^rd year of study (2011).

<p>Each point represents one net. Polyester nets: 100 denier, traverse [3.5.50], Polyethylene nets: 150 denier, tulle [3.5.52].</p

Characteristics of nets with extra-small holes (< 0.5 cm diameter) for 130 nets in the third year 2011.^*

<p>*95% CI = 95% confidence interval for the geometric mean</p><p>** p-value = Kruskal-Wallis test. Polyester nets: 100 denier, traverse [3.5.50], Polyethylene nets: 150 denier, tulle [3.5.52].</p><p>Characteristics of nets with extra-small holes (< 0.5 cm diameter) for 130 nets in the third year 2011.^{<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0128499#t004fn001" target="_blank">*</a>}</p

Percent of holes of each WHOPES size category located at varying distances from the bottom of the net by type.

<p>Error bars represent 95% confidence limits. Polyester nets: 100 denier, traverse [3.5.50], Polyethylene nets: 150 denier, tulle [3.5.52].</p

Histogram of measured hole diameters for each assigned WHOPES hole category.

<p>The midpoint hole diameter used for the WHOPES category weight is indicated by an orange dashed vertical line. The median hole diameter in each category is indicated by a green dotted vertical line.</p

Scatterplots of number of extra-small size holes (< 0.5 cm) vs total number of small, medium, large and extra-large size holes per net by type for 130 nets during the 3^rd year of study (2011).

<p>Each point represents one net. Polyester nets: 100 denier, traverse [3.5.50], Polyethylene nets: 150 denier, tulle [3.5.52].</p

Scatterplot of surface area calculated using WHOPES Proportional Hole Index categories vs measured total holed surface area of net using A) formula of a circle, and B) formula of an ellipse by net type.

<p>Each point represents one net. The black lines represent the diagonals. Polyester nets: 100 denier, traverse [3.5.50], Polyethylene nets: 150 denier, tulle [3.5.52].</p

Summary of WHOPES hole size categories, areas and corresponding weights.

<p>* True midpoint is not known.</p><p>Summary of WHOPES hole size categories, areas and corresponding weights.</p

Crimean-Congo hemorrhagic fever (CCHF) seropositive subjects’ demographics among survey participants during an investigation of the 12 rural villages reporting at least one 2014 CCHF case from January to September 2014 in Georgia (N = 12).

<p>Crimean-Congo hemorrhagic fever (CCHF) seropositive subjects’ demographics among survey participants during an investigation of the 12 rural villages reporting at least one 2014 CCHF case from January to September 2014 in Georgia (N = 12).</p

Demographics of survey participants during an investigation of the 12 rural villages with reporting at least one 2014 Crimean-Congo hemorrhagic fever case from January to September 2014 in Georgia.

<p>Demographics of survey participants during an investigation of the 12 rural villages with reporting at least one 2014 Crimean-Congo hemorrhagic fever case from January to September 2014 in Georgia.</p