Beginning in July, 2011, approximately four million polyethylene-based LLINs (Olyset® net, Sumitomo Chemical Company) were distributed throughout Benin. Following distribution, a net tracking activity to monitor the durability of the LLINs [11, 12] was implemented in four communities. Two of the communities, Kessounou and Allada, were located in Southern Benin (Figure 1), while the other two, Kandi and Malanville, were located approximately 750 km further North. Residents in Kessounou, located on the Oueme River, have ready access to water for washing nets. In contrast, residents of Allada, who want to wash a net, must carry water some five kilometers. Similar criteria (a short distance to water for washing LLINs versus a long distance) applied to the sites in the North. Malanville, located on the Niger River (water for washing nets easily accessible) and Kandi (residents must transport water for washing nets, as at Allada).
As described previously, at the time of the nationwide LLIN distribution in 2011, a sample of 2002 households, approximately 500 per site, was randomly selected [11]. Household selection at each site took into account all villages to ensure representative sampling [11]. Assessment teams identified a 2011-distribution LLIN in each selected household and, if the net was hanging and in use, enrolled it, and the household where it was located, in a LLIN durability monitoring assessment, based on World Health Organization guidelines [13]. Each selected LLIN was double tagged (a bar code, attached to the net, plus an indelible-ink symbol applied to the LLIN fabric) to ensure correct identification during later visits. The GPS coordinates of the household and the name of the head of household, or an adult person acting on behalf of the head, were also recorded to facilitate follow up. This paper reported the 12 and 18 months assessments.
Monitoring survivorship/Attrition
Due to the way in which households were selected (one enrolled LLIN per household), LLIN survivorship at T0, was set at 100%. At 6-month intervals, each selected household received a follow-up visit. If the household was open at the time of the visit, the assessment team visually confirmed the continued presence of the coded LLIN. If the coded LLIN was not in the house, the assessment teams determined how the net went missing by interviewing the owner. Owners were asked to choose one of three reasons for why the net was no longer present in the household. These were: (i) the net was thrown away because it was physically damaged and thought to be of no value, (ii) the net was removed (e.g. given away, stolen, sold etc.), and (iii) the net was re-purposed for an alternative use.
Monitoring integrity
LLIN fabric integrity was assessed by a visual examination, without removal of coded nets from selected households. Observed holes were assigned to one of four size categories:
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1.
a hole size of 0.5-2.0 cm or ‘<a thumb-sized opening’
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2.
a hole size of 2.0-10.0 cm or ‘>a thumb but<a fist’
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3.
a ‘hole size of 10–25 cm or ’>a fist but<a head’
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4.
a hole size of >25 cm or ‘>a head’
The most-likely cause of the damage, a rip in the fabric, a rip in the seam, burned burn-related hole or the result of rodent damage was also recorded.
Interview questionnaire
A questionnaire, developed by WHO [13], to identify factors associated with survivorship, was adapted for use in the assessment. Questions were programmed (ODK Collect 1.2.2 software) into Samsung Galaxy Tablets to record responses.
Data analysis
Survivorship
The equation for quantifying overall survivorship, also referred to as attrition, was:
If a household was closed, during an assessment visit, it was treated according to the non-parametric survival method of Kaplan-Meier [14]. Survivorship, plotted against time (T6, T12, T18), was compared with NetCALC net loss model curves based on 2-year and 3-year LLIN serviceable life assumptions (http://www.networksmalaria.org). Equations for calculation of LLIN survivorship/attrition associated with three different reasons for why an assessment net had gone missing were:
Attrition rate-1 (reason: physical damage):
Attrition rate-2 (reason: removal):
Attrition rate-3 (reason: re-purposed):
Two communities were reported to show significantly different survivorship/attrition if the 95% confidence limits did not overlap.
Integrity was quantified based on two measurements:
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1)
The proportion of LLINs with any hole.
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2)
The proportionate holes index (pHI) for each net [13]
1 × number of size − 1 holes + 23 × no. of size − 2 holes + 196 × no. of size − 3 holes + 576 × no. of size − 4 holes. The figures, 23, 146, and 576 refer to the estimated mean hole area for the different sized holes. Descriptive statistics were used to compare pHI values at each assessment site (mean, median, interquartile range). Based on the pHI score, LLINs were assigned to one of three condition catagories (Roll Back Malaria: Measurement of Net Durability in the Field: Current Recommended Methodology, presented in Lyon, February 2012).
pHI ≤64 -
good
pHI ≤768 -
serviceable
pHI >768 -
replace
Factors associated with loss of integrity were identified by multivariate regression analysis of nets in the ‘replace’ category and frequency of responses (by owners of the nets). Modalities with very low numbers observed were aggregated with those that have high numbers for the multivariate analysis.
Study clearance
This prospective study was planned with and approved by the Ministry of Health. Community leaders were informed before the study and all gave verbal consent before initiation. Written consent was then obtained on the day of the study from all participating households.