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

Abstract

Background: Ethiopia scaled up net distribution markedly starting in 2006. Information on expected net life under field conditions (physical durability and persistence of insecticidal activity) is needed to improve planning for net replacement. Standardization of physical durability assessment methods is lacking.

Methods: Permanet®2.0 long-lasting insecticidal bed nets (LLINs), available for distribution in early 2007, were collected from households at three time intervals. The number, size and location of holes were recorded for 189 nets used for three to six months from nine sites (2007) and 220 nets used for 14 to 20 months from 11 sites (2008). In 2009, a "finger/fist" sizing method classified holes in 200 nets used for 26 to 32 months from ten sites into small (<2 cm), medium (> = 2 to < =10 cm) and large (>10 cm) sizes. A proportionate hole index based on both hole number and area was derived from these size classifications.

Results: After three to six months, 54.5% (95% CI 47.1-61.7%) of 189 LLINs had at least one hole 0.5 cm (in the longest axis) or larger; mean holes per net was 4.4 (SD 8.4), median was 1.0 (Inter Quartile Range [IQR] 0-5) and median size was 1 cm (IQR 1-2). At 14 to 20 months, 85.5% (95% CI 80.1-89.8%) of 220 nets had at least one hole with mean 29.1 (SD 50.1) and median 12 (IQR 3-36.5) holes per net, and median size of 1 cm (IQR 1-2). At 26 to 32 months, 92.5% of 200 nets had at least one hole with a mean of 62.2 (SD 205.4) and median of 23 (IQR 6-55.5) holes per net. The mean hole index was 24.3, 169.1 and 352.8 at the three time periods respectively. Repairs were rarely observed. The majority of holes were in the lower half of the net walls. The proportion of nets in 'poor' condition (hole index >300) increased from 0% at three to six months to 30% at 26 to 32 months.

Conclusions: Net damage began quickly: more than half the nets had holes by three to six months of use, with 40% of holes being larger than 2 cm. Holes continued to accumulate until 92.5% of nets had holes by 26 to 32 months of use. An almost complete lack of repairs shows the need for promoting proper use of nets and repairs, to increase LLIN longevity. Using the hole index, almost one third of the nets were classed as unusable and ineffective after two and a half years of potential use.

Figure 1

Long-lasting insecticidal bed nets’ collection areas. Map of Ethiopia showing location of collection sites. Open circles denote areas of 2007-only collections. Closed circles denote areas where nets were recovered in both 2008 and 2009.

Figure 2

Percentage of nets with any holes, in samples collected at three time periods.

Figure 3

Mean number of holes per net by time group (log scale).

Figure 4

Frequency distribution of number of holes per net, at three time periods.

Figure 5

Percentage of holes in each size category, by time group of collection.

Figure 6

Frequency distribution of holes by location on net, for nets collected after 14 to 20 months of use from sites H through U. Frequency normalized according to the area of the net represented.