Evaluation of the durability and use of long-lasting insecticidal nets in Nicaragua

Abstract

Background: Vector control for malaria prevention relies most often on the use of insecticide-treated bed net (ITNs) and indoor residual spraying. Little is known about the longevity of long-lasting insecticidal nets (LLINs) in the Americas. The physical integrity and insecticide retention of LLINs over time were monitored after a bed net distribution campaign to assess community practices around LLIN care and use in Waspam, northeastern Nicaragua.

Methods: At least 30 nets were collected at 6, 12, 24, and 36 months post distribution. Physical integrity was measured by counting holes and classifying nets into categories (good, damaged, and too torn) depending on a proportionate hole index (pHI). Insecticide bioefficacy was assessed using cone bioassays, and insecticide content measured using a cyanopyrethroid field test (CFT).

Results: At 6 months, 87.3 % of LLINs were in good physical condition, while by 36 months this decreased to 20.6 %, with 38.2 % considered 'too torn.' The median pHI increased from 7 at the 6-month time point to 480.5 by 36 months. After 36 months of use, median mortality in cone bioassays was 2 % (range: 0-6 %) compared to 16 % (range: 2-70 %) at 6 months. There was a decrease in the level of deltamethrin detected on the surface of the LLINs with 100 % of tested LLINs tested at 12 months and 24 months crossing the threshold for being considered a failed net by CFT.

Conclusions: This first comprehensive analysis of LLIN durability in Central America revealed rapid loss of chemical bioefficacy and progressive physical damage over a 36-month period. Use of these findings to guide future LLIN interventions in malaria elimination settings in Nicaragua, and potentially elsewhere in the Americas, could help optimize the successful implementation of vector control strategies.

Fig. 1

Diagram of the long-lasting insecticidal nets (LLINs) showing position of swatches collected for processing

Fig. 2

Proportionate hole index (pHI) by collection time point, Waspam, Nicaragua (n = 163 nets). Footnotes: Width of violin-shaped figures represents number of nets with a similar pHI. Boxplots shows the median, interquartile limits, and outliers. The y-axis uses the log₁₀ scale

Fig. 3

Mosquito mortality by cone bioassay per collection time point, Waspam, Nicaragua (n = 160 nets). Footnote: Estimated mosquito mortality per net calculated by taking the mean of all swatches

Fig. 4

Cyanopyrethroid field test results per collection time point, Waspam, Nicaragua (n = 109 nets). Footnote: Threshold for net failure set at 0.15 mg/m², equivalent to the surface concentration required to achieve 80 % mortality with susceptible Anopheles gambiae in cone bioassays

Fig. 5

Pairwise-comparisons between high-performance liquid chromatography (HPLC), cyanopyrethroid field test (CFT), and cone bioassay results from LLINs, Waspam, Nicaragua (a, n = 108 nets; b, n = 26 nets; c, n = 26 nets). Footnote: HPLC comparison considered only LLINs collected at 6 months