A longitudinal study of the durability of long-lasting insecticidal nets in Zambia

Abstract

BackgroundA 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.Methods999 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).ResultsA 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).ConclusionsThe 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.

Field Durability of &lt;i&gt;Yorkool&lt;sup&gt;&#174;&lt;/sup&gt;LN&lt;/i&gt; Nets in the Benin Republic

“…Our results corroborate most of the reports reported by some national programs that LLINs keep their insecticidal activity for at least three years [22]. But they go against those already obtained in Madagascar, which show that more than 75% of new Yorkool ® LN mosquito nets did not meet the limit value set by WHO [31] at the beginning of the study.…”

“…However, the proportion of the remaining nets that needed replacement after 18 months was not as important to suggest that the intervention would lose its impact in the second or third year of the distribution-replacement cycle [11] monitoring in our study, 56.8%, 7.8%, and 2.6% were found to be "in good condition," "damaged," and "too torn," respectively, by one year and half-use. This result contrasts with the 15%, 42.5% and 42.5% respectively "in good condition", "damaged" and "too torn", obtained after two years of use in Madagascar [31].…”

Evaluation of the Durability of Long-Lasting Insecticidal Nets in Guatemala

“…The overall survivorship at each time-point was de ned as the number of enrolled LLINs still present in the households divided by the total number of enrolled LLINs. The functional survivorship was calculated at each time-point as follows: number of enrolled LLINs still present and "serviceable" (see de nition below) in the households divided by the number of LLINs still present + LLINs not present owed to attrition reasons (damaged and thrown away or LLIN used for other purposes) (Committee & Secretariat, 2013;Tan et al, 2016; WHO Malaria Policy Advisory Committee, 2013). We then calculated the median survival time, de ned as "the time point at which the estimate of functional LLIN survival crosses the 50% mark" (WHO Malaria Policy Advisory Committee, 2013) using the following formula: , where tm is the median survival time, t 1 and t 2 were the 24 and 32 month survey points (in years) and p 1 and p 2 were the functional survivorship proportions for the 24 and 32 month points, respectively.…”

Costs of insecticide-treated bed net distribution systems in sub-Saharan Africa

“…Net durability, or the useful life of an ITN, is assumed to be 3 years in the base case scenario. In reality the lifespan of ITNs may actually be closer to two rather than 3 years [30][31][32][33]. In this case, the impact of the intervention during year three of the ITN distribution-replacement cycle could be well below that seen in years one and two and would, therefore, have a significant effect on the cost effectiveness and efficiency of these programmes.…”

“…This recommendation is based on the assumption that the useful life of an ITN is 3 years. However, in reality, the lifespan of ITNs may actually be closer to two rather than 3 years [30][31][32][33]. Because of these gaps in coverage, there are still vulnerable households who need replacement ITNs between distributions.…”