Monitoring long-lasting insecticidal net (LLIN) durability to validate net serviceable life assumptions, in Rwanda

Abstract

Background: To validate assumptions about the length of the distribution-replacement cycle for long-lasting insecticidal nets (LLINs) in Rwanda, the Malaria and other Parasitic Diseases Division, Rwanda Ministry of Health, used World Health Organization methods to independently confirm the three-year LLIN serviceable life span recommendation of WHO.

Methods: Approximately 3,000 coded LLINs, distributed as part of a national campaign, were monitored in six sites, by means of six-monthly visits to selected houses. Two indicators, survivorship/attrition, a measure of the number of nets remaining, and fabric integrity, the proportion of remaining nets in either 'good', 'serviceable' or 'needs replacement' condition, based on holes in the net material, were tracked. To validate the assumption that the intervention would remain effective for three years, LLIN coverage, calculated using either survivorship, or integrity, by removing nets in the 'needs replacement' category from the survivorship total, was compared with the predicted proportion of nets remaining, derived from a net loss model, that assumes an LLIN serviceable life of three years.

Results: After two years, there was close agreement between estimated LLIN survivorship at all sites, 75% (range 64-84%), and the predicted proportion of nets remaining, 75%. However, when integrity was considered, observed survivorship at all sites, declined to 42% (range 10-54%).

Conclusions: More than half, 58%, of the LLINs fell into the 'needs replacement' category after two years. While these nets were counted for survivorship, they were judged to be of little-to-no benefit to a user. Therefore, when integrity was taken into account, survivorship was significantly lower than predicted, suggesting that net serviceable life was actually closer to two, rather than three years, and, by extension, that the impact of the intervention during year three of the LLIN distribution-replacement cycle could be well below that seen in years one and two.

Figure 1

Peri-urban LLIN durability monitoring sites.

Figure 2

Rural LLIN durability monitoring sites (endemic transmission).

Figure 3

Rural LLIN durability monitoring sites (hypo-endemic transmission).

Figure 4

LLIN observed survivorship by site (data from Table  3 with addition of 6-, and 18-month results) versus NetCALC-predicted ‘proportion of LLIN’s present’ assuming either a 3-year (small-dash curve) or a 5-year (larger-dash curve) LLIN serviceable life.

Figure 5

LLIN observed integrity (data from Table 4 expressed as survivorship) versus NetCALC-predicted ‘proportion of LLINs present’ assuming either a 3-year or a 5-year LLIN replacement cycle. The vertical line bisecting the two year time point (x-axis) facilitates comparison of observed and predicted survivorship.