National distribution of long-lasting insecticidal (mosquito) nets (LLINs) is a proven malaria control intervention [1–3]. However, LLIN interventions have a limited serviceable life, and net replacement must be programmed in a timely way to maintain impact. Programs that wait too long risk operational failure; likewise, premature replacement is to be avoided for cost-effectiveness reasons. The current assumption regarding LLIN serviceable life is three to five years of use under field conditions [4, 5]. While this has been incorporated into LLIN intervention planning, questions have arisen about whether such an approach holds true everywhere nets are in use [6]. Rather than a single LLIN serviceable life everywhere, data from the field indicate that LLIN loss appears to vary based on both intrinsic (to the LLIN) and extrinsic factors [7–10]. We monitored LLIN durability in Benin, during the current national LLIN distribution – replacement cycle, so that the timing of the future replacement campaigns can be informed by Benin-specific net loss data.

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.

LLIN loss, measured by survivorship/attrition, occurred more rapidly than predicted by the ‘three-year serviceable life’ assumption, currently used in Benin to program distribution/replacement of LLINs. Of interest, however, was the observation that survivorship rates did not appear to be affected by LLIN washing frequency. In contrast, our results showed a marked effect of washing frequency on the integrity of the LLINs. Therefore, LLIN loss of physical integrity/deterioration, a factor that affects the ability of LLINs to prevent mosquito-human contact, may not be taken into account if only survivorship is monitored. In this evaluation, loss of integrity, measured in LLINs that remained in place, was so extensive that it may well have compromised the value of the LLINs still remaining in the households as a malaria prevention measures. Recent work [15] in which damaged LLINs are shown to increase man-vector contact from none to an average of five bites/man/night demonstrates the impact that a relatively small loss of integrity, increasing the pHI to 276, can have on LLIN efficacy. Therefore, we suggest that, assumptions about LLIN serviceable life/time to replacement should be informed, to the extent possible, by monitoring integrity of existing nets [13] as well as survival/attrition. Our results suggest that the assumption of a 3-year LLIN serviceable life, for the LLINs distributed in 2011 in Benin overestimated their duration of impact by as much as one year, a situation that could contribute to a rebound in malaria illness during the last year (year three) prior to net replacement. Additional programmatic questions of interest that can be addressed by LLIN durability monitoring include: ‘ Is the programmatic ‘effective life assumption realistic under local/regional conditions?’; ‘Do some LLIN products have slower loss/integrity attrition rates in a given location?’; and, ‘What programmatic changes can be implemented to improve LLIN duration of effective life?’

Net removal for all reasons was observed in 6% of the study houses at T6, in 18% at T12 and in 26% at T18. Some, perhaps most, of these nets were moved by design of the household (e.g. given away to other houses in the community) and therefore, it is possible that the nets continued to contribute to community protection. Nonetheless, the percentage of nets thrown away (25%) and nets re-purposed were surprisingly high in some communities, eg. Kessounou.

The proportionate hole index (pHI) [13] provides a standardized approach to describing changes in LLIN fabric integrity. Applying pHI thresholds [16]: ‘like new’, ‘needs repair’ and ‘needs replacement’, to our results, we observed that after 12 months, 16-22% of the LLIN need to be replaced at locations with less access to water (Allada and Kandi) versus 30-33% at locations with ready access to water for washing (Malanville and Kessounou). Estimates of net loss associated with integrity (computing loss as proportion of nets in pHI category ‘needs replacement’ were between 18% and 48% after 18months). If loss associated with fabric integrity declines at these rates, more than one half of the LLINs distributed in 2011, and still present in the household to which they were given, would need replacement prior to two transmission seasons post-distribution. Thus, based on fabric integrity alone, the LLINs would not be expected to provide adequate protection after two transmission seasons.

Cooking fuel, location of the kitchen as well as low LLINs maintenance and washing frequency were correlated with the loss of fabric integrity. Social practices regarding net care and repair, vary from community to community, and most likely influence condition of the LLINs in each region [17, 18]. It important, therefore, to reinforce awareness about the best practices related to net care and repair.

A recent study in Kenya [19] reported that people washed their LLINs more frequently than recommended and associated this practice with poor physical quality of nets. The study also noted that light colored nets were more likely to be washed than were dark colored nets. The polyethylene-based LLINs in this assessment were light blue. The majority of the nets (54-74%) were observed to be dirty. Thus, it may be that these factors also contributed to the high observed frequency of washing/rapid decline in fabric integrity. They could significantly affect the bio-efficacy of the nets that decreased from 9-58% after 6 months [11]. However, bio-efficacy results were not included in the present assessment-and represent a potential limitation of the study.

The findings of this study have important implications for the LLIN-malaria control strategy in Benin. They suggest that: the current polyethylene-based LLIN, distributed during 2011, has an effective life of two rather than three years; that behaviour change communication (BCC) strategies that support LLINs repair and that discourage subsequent removal of nets after distribution should be strengthened in an attempt to sustain higher coverage levels for a longer period of time; and that channels for routine replacement of LLINs, between national campaigns should be strengthened to replace nets that no longer meet minimum fabric integrity standards; and that other WHOPES-approved LLIN products should be evaluated to determine which is most cost-effective. Finally, it may be that local communities can adopt and use WHO guidance on assessment of integrity (counting holes) and survival to assess LLIN loss, thereby verifying ongoing impact under local conditions.

This study suggests that in Benin, survivorship/attrition of LLINs followed a 2-year net serviceable life assumption (model), rather than the currently used three-year assumption; that loss of fabric integrity in nets that were still in use, was extensive, with as many as one-third in need of replacement. The condition of the LLINs at the midpoint of its assumed serviceable life raises serious doubts about their usefulness in preventing mosquito-human contact during the subsequent 18 months prior to program replacement of nets under the current plan.

As countries consider how to sustain LLIN impact, it will be important to have meaningful information on LLIN loss in a variety of settings. This information should be used to ‘inform’ decisions on replacing LLINs thereby avoiding loss of efficacy earlier than assumed.