Physical durability of PermaNet 2.0 long-lasting insecticidal nets over three to 32 months of use in Ethiopia

Long-lasting insecticidal bed nets (LLINs) are considered a vital component in the worldwide effort to prevent malaria transmission in malaria-endemic countries [1]. In 2007, the World Health Organization’s Global Malaria Programme recommended immediate scale up of LLIN distribution from national programmes and partners. As a result, large-scale distribution efforts have been launched to meet this goal, particularly in sub-Saharan Africa, by a host of governments, non-governmental organizations and collaborations with local governments and international charitable organizations.

Ethiopia’s impressive scale-up of net distribution started after the development of a new strategic plan in 2005, and resulted in household ownership of at least one net (any type) increasing from 4.5% in 2005 to 72.5% by 2007 [2]. In 2006, LLINs were instituted as one of the primary methods of vector control as they removed the need for regular re-treatment of impregnated nets with insecticide [1]. These “long lasting properties” were tested and standardized by the World Health Organization Pesticide Evaluation Scheme (WHOPES), requiring that LLINs retain effective insecticidal activity for at least 20 laboratory washes and under field conditions for at least three years [3]. Manufacturers’ claims regarding the life expectancy of their products are generally based on the results of these types of studies; in particular the length of time and the number of laboratory washes a net can endure before insecticidal activity is lost. However, most evaluations to date of LLIN performance and durability have been restricted to laboratory tests, experimental hut studies and limited field trials [4, 5]. To date, relatively few LLIN evaluation and monitoring studies have addressed net durability after usage in the field. Such real world information can be valuable for determining the validity of and improving the laboratory test methods, aiding future LLIN purchase decisions and determining the timing and strategy of net replacement campaigns. Moreover, such studies may identify ways these products can be further improved and also characterize the influence of specific net user behaviours on net durability.

Previous studies of net durability after real-world use have shown variable results. Smith et al.[6] reported on the condition of 255 bed nets collected in one district in Ghana, 38 months after a district-wide distribution campaign. In a subset of 50 polyester (primarily PermaNet®1.0) nets selected for detailed study, a total of 2,023 holes >0.5 cm in diameter were recorded and 31 holes >10 cm in diameter. Most holes were located on the bottom third of the net. Seam failures were seen in 50% of the nets, and a low rate of repair was also observed. A study by Killian et al. of PermaNet®1.0 and 2.0 in Uganda revealed that >70% of the nets had holes after one year and >85% had holes after two years [7]. Holes were mostly (75.6%) located on the lower half of the net. Also, the poor physical condition of the nets suggested that many campaign nets may have been worn out and discarded prior to collection, thus skewing the results by removing the unknown number of the most deteriorated nets. For Olyset™ LLINs collected in 2004 after seven years of use in Tanzania, Tami et al.[8] found that most (55%) of the nets had six to 15 holes >2 cm. The diversity of measures of net damage used shows that there is a need for standardization of methods for monitoring net durability, as studies conducted to date have not used consistent methods.

The Ethiopian Federal Ministry of Health (FMOH) set an ambitious national goal of full population coverage with a mean of two LLINs per household through distribution of about 20 million LLINs, in malarious areas, by the end of 2007. To contribute to this effort, The Carter Center assisted in procurement and distribution of three million LLINs in selected areas of three regions of Ethiopia: Amhara, Oromia and Southern Nations, Nationalities and Peoples (SNNP) Regions. The data presented here are part of the monitoring process following the large-scale distribution of PermaNet®2.0 LLINs in the three regions, starting in 2007. The aim of the work was to assess the decline in net functionality over time from samples of nets with known time of distribution, as well as establish a simple and repeatable method to assess physical durability and insecticidal activity of LLINs. These data will inform the timing and nature of the future net replacement strategies and may also improve behaviour change communication about net care and lifespan. This paper reports on the findings related to the physical durability.

This investigation was unusual in that net evaluation began very shortly after distribution. Since LLINs are expected to last for multiple years, most prior studies have been of nets a year or more after distribution. However, for programme planners preparing for replacement campaigns, it would be useful to have meaningful data about nets in the field and their rate of deterioration as early as possible. Results from this study showed that in as little as three to six months, a quantifiable picture began to emerge regarding the physical deterioration of nets in the field. Future LLIN monitoring efforts can therefore start less than a year after distribution in order to give planners a head start on developing timetables for replacement.

Although some nets were still in good physical condition even after a year-and-a-half in this study, a substantial proportion showed significant deterioration: 68% had holes and 28% were classed as ‘mediocre’ or ‘poor’ by hole index at 14 to 20 months of use. The distribution of holes per net was highly skewed, with a few nets having many holes and most nets with few holes. For this reason, statistical descriptions of overall condition of a group of nets are better described by the median and not the mean number of holes, the latter being disproportionately impacted by the existence of a few heavily damaged nets. However, the size of holes must be taken into account as well: one large hole may be as problematic (or even more so) than a large number of small holes, since large holes can let more mosquitoes inside of the net without contacting the insecticidal fibres.

The two schemes proposed [9, 10] to estimate a standardized hole index differ in both the number of size categories and the relative factors applied to each. In the first scheme, WHO [9] recommended four size categories of 0.5 to 2 cm, 2 to 10 cm, 10 to 25 cm and >25 cm, with midpoint hole diameter of 1.25, 6, 17.5 and 30 cm respectively. The relative multiplication factors based on corresponding hole areas for these groups are 1, 23, 196 and 578. Secondly, Kilian et al.[10] and Batisso et al.[11] used three size categories <2 cm, 2 to 10 cm and >10 cm (finger, fist and head). They estimated average hole areas to be 4, 36 and 225 sq cm respectively, giving multiplication factors of 1, 9 and 56 for small, medium and large holes to arrive at the proportionate hole index. The current study commenced before either of these schemes was published, but elected to use the method of Kilian et al. [10] and Batisso et al. [11] for hole index calculation as it is based on three size categories similar to those used here, and to enhance comparability between studies in Ethiopia.

A limitation of this study is that net attrition (complete loss of nets through disposal, diversion to other use, sale, or donation to others outside the household) was not measured, so net deterioration and loss may have been underestimated. Attrition has been estimated to be as high as 32% over three years (Batisso et al. [11]). Other limitations include the convenience sampling method for the sites and households, and the fact that the sites were at a range of altitudes, so net use (and as a result, net wear) may differ significantly depending on mosquito populations and perceived risk of malaria.

Repairs to nets that had developed holes were rarely observed in this study. Low repair rates were also reported by Smith et al. [6], Kilian et al. [10] and Shirayama et al. [12] so this appears to be a widespread issue. However, Bhatt et al. [13] found a total of 750 repairs, or average 1 repair per LLIN, in the forms of stitches (63.9%), knots (35.8%) and patches (0.3%) in their evaluation of Interceptor® LLINs (n = 932) in central India. It is likely that longevity of nets can be significantly improved simply by making repairs to them and perhaps by extension, encouraging behaviour that would prevent the nets from developing holes to start with.

Although the cause of the holes in the net studied here is not known, household observations suggested that fire embers together with damage from animals and sharp edges on beds or mats were major contributors. This will be explored further in risk factor analysis, using hole index and insecticide concentration as outcomes, for the 2008 and 2009 collections in a subsequent paper. Holes were observed on all sides of the net including the top panel, indicating that all parts of the nets are susceptible to damage, but net density was greater in the lower sides of the net. Of note, the relative proportion of small, medium and large holes did not change over time. Thus the hole size (and hole index) distribution at a future date could possibly be predicted using data from nets used for only a short period. Furthermore, if the proportion of small, medium and large holes is different for different net brands, it could become a measure of net quality (quality increasing as large:small hole ratio decreases). Knowing the ratio of large to small holes could have a profound impact on program planning and procurement decisions, and it would be desirable for future similar durability studies to report on this ratio.

Comparison of the physical deterioration rates between studies is difficult because of differing methodologies and time periods of assessment. Kilian et al. [10] observed that for PermaNets in Uganda, >70% had holes in them after one year (i e, <30% were undamaged). In this study, net attrition was not measured, as in the current study. Although nets were not collected after exactly one year, the findings in the current study were fairly consistent: it was observed that 54.5% of nets were damaged after three to six months and 85.5% after 14 to 20 months. In Tanzania, Maxwell et al. [14] defined “intact” nets as having <20 holes <2 cm in diameter, <5 holes 2–5 cm in diameter, and <2 holes >5 cm in diameter. Maxwell et al. found that 84.4% of Olyset nets in highland villages were ‘intact’ after four to five years and 38.4% of the nets in lowland villages were ‘intact’ after six to seven years [14]. In the current study, the time period is shorter and the hole size comparison is not strictly similar. However, it was found that 65.5% of nets had <20 holes (any size) at 14 to 20 months and 46.8% had <20 holes at 26 to 32 months, suggesting that these PermaNet®2.0 were deteriorating more quickly than the Olyset nets observed in Maxwell’s study.

In Chad, Allan et al. [15] used a proportionate hole index to assess LLIN durability in Interceptor, Olyset, and PermaNet after one year of household use, and assessed damage as holes < 2 cm, >2-5 cm and >5 cm. A total of 876 LLINS were assessed and 25% had a hole index greater than 300; these nets were classified in their study as “unserviceable and irreparable”. The largest percentage of nets (i.e. 44.5%), classified as “in partial service” had proportionate hole indices in the middle ranges of 25–174 and 175-299 [15]. The most relevant comparison for our study is to that of Batisso et al. [11] in Ethiopia, who observed that after three years, 70% of nets were still in ‘good’ or ‘fair’ condition as defined by a hole index of <175, whereas in this study the percentage overall in these two categories was lower at a slightly earlier time period: 50.5% at 26 to 32 months. As noted by Batisso et al., work still needs to be done to determine when nets should be classified as ‘unserviceable’, as well as comparisons of net life in different circumstances within the same country and between countries.

The current study provides a description of the deterioration of PermaNet®2.0 LLINs distributed in Ethiopia with support from The Carter Center in 2007. Many nets were physically damaged after only three to six months’ use (54.5%, ranging from 20-68% by site), which increased to 85% on average (range 65-100% by site) after 14 to 20 months and to 93% (range 80-100% by site) after 26 to 32 months. The condition of nets was skewed, with fewer nets being heavily damaged and many nets showing relatively light damage. The mean hole index was 24.3, 169.1 and 352.8 at the three time periods respectively. An almost complete lack of repairs suggests that a programme of teaching and encouraging net repairs may be a means of increasing LLIN longevity. Using the hole index, almost one third of the nets still present in households were classed as unusable and ineffective after two and a half years of potential use.