In November 2014, the Senegalese National Malaria Control Program (NMCP) supported by the U.S. President Malaria Initiative (PMI), launched a universal coverage campaign in four regions of the central zone (Diourbel, Fatick, Kaffrine and Kaolack) and in March 2015 in a region of the West zone (Thiès). Distribution of LLINs was done in order to obtain urban and rural stratification in different regions. Urban areas were localized in Diourbel and Kaolack respectively represented by districts of Leona and Grand Diourbel. Rural areas were localized in Fatick represented by the villages of Ndiongolor and in Kaffrine represented by the villages of Nganda and Kathiote. The western region, Thiès, was divided into two rural and urban areas represented by Malicounda and Thienaba respectively (Fig. 1).
These regions are characterized by a Sahelian, Sahelo-Sudanian or Sudano-Sahelian climate with a long dry season (eight to nine months) and a short rainy season (three to four months). In the central zone, the average annual rainfall is 750 to 800 mm with a rotation farming system of peanuts, millet, maize, cowpeas, etc. In the western zone, the average rainfall is 400 to 500 mm, and gardening is practiced because of water availability a few meters below the ground. The incidence of malaria varies: between 5 and 15 cases per 1000 inhabitants in Thiès and Fatick, between 15 and 25 per 1000 inhabitants in Kaffrine, and over 25 per 1000 inhabitants in Diourbel and Kaolack [26]. Plasmodium falciparum is responsible for 90% of malaria infections and the dominant vectors are Anopheles gambiae, An. coluzzii, An. arabiensis and An. funestus [23].
LLIN distribution
A total of 10,608 nets were distributed during the campaign, one per sleeping space in a numerical order classified by region, LLIN brand and zone. Individual households received one of eight different net product types. LLIN distributed by households was done by randomization and if some brands ran out, others were substituted, so some households received different LLIN brands. All eight LLIN brands distributed (Interceptor®, Life Net®, MAGNet™, Netprotect®, Olyset® Net, PermaNet® 2.0 R, PermaNet® 2.0 C, Yorkool® LN) were recommended by WHOPES (World Health Organization Pesticide Evaluation Scheme) [38]. The characteristics of different types of LLINs were summarized in Table 1. Five brands (Interceptor®, Netprotect®, Olyset® Net, PermaNet® 2.0 R, PermaNet® 2.0 C) were distributed in central regions (Diourbel, Fatick, Kaolack and Kaffrine) and two others (Life Net®, Yorkool® LN) in the western region Thiès. MAGNet™ was only distributed in Kaolack and Kaffrine.
Table 1
Characteristics of different LLIN brands distributed
Brand name
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|
Product type
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Insecticide concentration
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Denier
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Shape
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|
Manufacturer
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|
WHOPES approval
|
Interceptor®
|
|
Alpha-cypermethrin coated on polyester
|
|
200 mg/m2
|
|
100
|
|
Circular
|
|
BASF
|
|
Full
|
LifeNet®
|
|
Deltamethrin incorporated into polypropylene
|
|
8,5 g/kg
|
|
100
|
|
Rectangular
|
|
BAYER
|
|
Interim
|
MAGNet™
|
|
Alpha-cypermethrin incorporated into polyethylene
|
|
5,8 g/kg
|
|
150 ± 7%
|
|
Rectangular
|
|
V.K.A polymers
|
|
Full
|
Netprotect®
|
|
Deltamethrin incorporated into polyethylene
|
|
68 mg/m2
|
|
118
|
|
Rectangular
|
|
Bestnet
|
|
withdrawn
|
Olyset® Net
|
|
Permethrin incorporated into polyethylene
|
|
1000 mg/m2
|
|
150
|
|
Rectangular
|
|
Sumitomo Chemical
|
|
Full
|
PermaNet® 2.0 R
|
|
Deltamethrin coated on polyester
|
|
55 mg/m2
|
|
100
|
|
Rectangular
|
|
Vestergaard Group
|
|
Full
|
PermaNet® 2.0 C
|
|
Deltamethrin coated on polyester
|
|
55 mg/m2
|
|
100
|
|
Circular
|
|
Vestergaard Group
|
|
Full
|
Yorkool® LN
|
|
Deltamethrin coated on polyester
|
|
55 mg/m2
|
|
100
|
|
Rectangular
|
|
Tianjin Yorkool
|
|
Full
|
Sampling and survey
A distribution database of households was generated by the NMCP classified by area, type of LLIN, number of LLINs received, and consent of the household head. From this database, random sampling of individual LLIN to follow throughout the three years was done. The study was longitudinal with follow-ups at 6, 12, 24 and 36 months. Two hundred fifty to 300 samples of each LLIN brand, stratified by rural and urban areas, were selected during the random sampling at the sixth month post distribution, establishing a cohort of 2,222 nets in households. For the random sampling, the nets included in the survey were selected using Stata software (version 11.2) with stratification for the LLIN product distributed. Due to the presence of some large households with many sleeping spaces and many nets, several LLINs (of the same product type) were sampled in these households. After consent from the household head at each round, a member of the team administered the questionnaire to a household respondent aged at least 18 years old. A double marking system was used to identify cohort LLINs: barcodes sewn onto the label and written onto the label with indelible marker in case the barcode was removed. At each follow-up, the surveys were conducted in the households available in the database. Unavailable households (due to absence, exclusion, lack of access, etc.) that were not replaced contributed to the drop in the number of surveyed households over time.
The survey questionnaire was administered using Android tablets with Open Data Kit (ODK) software. Investigators collected GPS coordinates at baseline to map households surveyed. From the tablets, data were sent to a server that hosted the main database. The 6-month data collection took place simultaneously in the 4 central regions and (Diourbel, Fatick, Kaffrine and Kaolack) in May 2015 and in the western region Thiès in September 2015. The 12, 24 and 36 month rounds were conducted in November 2015, 2016 and 2017 in the four central regions and in March 2016, 2017 and 2018 in the western region Thiès.
Physical integrity
The physical condition of nets was measured by counting the number of the category of holes that were on the surface of each net. Four categories of holes were defined by the World Health Organization (WHO) [35]: size 1 (0.5–2cm diameter), size 2 (2–10cm diameter), size 3 (10–25cm diameter) and size 4 (> 25 cm diameter). With holes classified in those size categories and counted, a proportionate hole index (pHI) was calculated for each net in order to estimate the physical damage:
pHI= # size 1 holes + (# size 2 holes × 23) + (# size 3 holes × 196) + (# size 4 holes × 576)
The pHI allowed classifying nets into three categories according to arbitrary thresholds:
Good: 0 ≤ pHI ≤ 64; Damaged: 65 ≤ pHI ≤ 642; Torn: pHI > 642.
Data preparation and analysis
Data was cleaned to improve matching of LLIN identification codes (IDs) and household IDs throughout the follow-up. For each survey round, each cohort LLIN was coded as ‘LLIN not found’ (LLIN given away, household not found), ‘LLIN discarded’ (LLIN discarded due to wear and tear-given away, but may still be in use by a neighbor, relative, etc.), or ‘LLIN present’. Data was reorganized into a matrix with household IDs, LLIN IDs as line names, and survey round as column names. Any LLIN present at a time of follow-up was considered to be present in previous follow-up even it was not seen or investigated in the last survey. For example, if an LLIN was registered as present at 24 months, it must have been present in previous follow-ups (6 and 12 months). In the case that an LLIN was recorded as destroyed or used for other purposes, it was still considered to be destroyed even if it was not seen at subsequent investigations. In case of a ‘LLIN not found’ value during a survey before an ‘LLIN discarded’ value, it is unknown whether the net was already destroyed during that survey, or still present. A Bayesian model with Gibbs sampling, capable of dealing with this uncertainty in the data, was used for the analysis. With each iteration, this model assigned the status for the unknown data points with a probability based on the known data points. This model considered the probability of net presence at a survey round with a probability equal to zero if the LLIN was assigned the status of ‘discarded’ in the previous survey round. Thus, a probability of survival since the previous follow-up was calculated for each subsequent follow-up. Using random effects for households, several Bayesian models were used: one model with independent survival estimates by LLIN product type and another model for which environment (rural/urban) was taken as explanatory variable. The Bayesian model was implemented in JAGS from the software platform R (version 3.3.2).
Attrition, the complement of survival, was defined as those nets that were missing because they were reported given away, destroyed and thrown, or repurposed. To better understand what happens to lost or present nets, the survival rate of LLIN was calculated in two different ways: ‘Survival rate’ or proportion of nets surviving in serviceable condition: defined as the proportion obtained by calculating the number of sampled LLINs physically present and usable on the number of LLIN surveyed at time x. The lost LLIN which were given away, sold, or stolen were excluded from the denominator, as their ultimate outcome is unknown. This survival rate accurately reflects the level of loss due to diminishing physical integrity over time. The number of LLIN present and serviceable was calculated by including data on the reason for the loss (either damaged and discarded, or used for other purposes) and the calculation of pHI. LLINs that had a destroyed surface less than 1000 cm2 were considered serviceable (0 ≤ pHI ≤ 642). We used same calculation proposed by the Vector Control Technical Expert Group [37] with some modifications. In this study the denominator did not include all nets from the original distribution but only the selected and surveyed nets at time x. This was due to excluded households during each follow-up, i.e. after sampling for laboratory tests or other obstacles (absence, refusal, no accessibility, etc.). This calculation allowed having a very low denominator at 36 months which can induce a slight increase of survival or retention.
Retention rate: defined as the proportion obtained by dividing the number of sampled LLIN physically present over the number of LLIN surveyed at time x. Lost nets that were damaged and discarded, or used for other purposes are excluded from the denominator. This rate reflects the level of LLIN retention within households.
Median survival time was calculated for each LLIN brand based on calculated survival and retention rates. Due to the small number cohort of LLIN found at 36 months, the calculation of the median survival time (in years) was done from data obtained at 12 and 24 months.
$$\text{T}\text{m}=t1+\frac{\left(t2-t1\right)*\left(p1-50\right)}{\left(p1-p2\right)}$$
Tm is the median survival time, t1 and t2 represent respectively first and second year, p1 and p2 represent respectively the survival and retention rate in the first and second year [36]. The confidence interval of the median net survival was obtained by applying the formula to the lower and upper limits of p1 and p2, respectively. These who used survival curve proposed by the Vector Control Technical Expert Group, it was observed that projections toward the median should only be attempted if the first time point is at least 85% or lower. In this study, survival curve was not used because some LLIN brands had a first time point (12 months) higher than 85%. The Tm is calculated separately and represented in histograms with-error-bars.
Ethics
The study protocol has been submitted to the approval of the National Committee of Ethics and Health Research (CNERS/Senegal). After approbation, study was adopted and distribution started. All households that received LLINs during the distribution campaign were eligible for the study. Prior to the distribution, the project was described to all participants by the community-based agents deployed by the health posts of the included districts. Participation was voluntary and the participants could refuse to continue the survey at any time.