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.

Effect of short-term heating on bioefficacy of deltamethrin-coated long-lasting insecticidal nets

“…Intriguingly, despite predelivery inspection reports indicating adequate total AI content, the bioefficacy of these LLINs in standardized WHO cone bioassays was low and highly variable with an average 24h mortality of just 39.4% for new nets that had never been used or washed 7 . A study from Nicaragua, published in early 2021, showed similar deficiencies for the same LLIN product, with strikingly low mortality rates for pyrethroid susceptible mosquitoes (less than 20%) just 6 months after distribution 8 . These findings stand in stark contrast to the bioefficacy of Permanet 2.0 ® manufactured prior to 2013, which was consistently 100% in cone bioassays, even after extended periods of usage 9 .…”

From the factory to the field: considerations of product characteristics for insecticide-treated net (ITN) bioefficacy testing

“…Thorough characterization of resistant strains before each evaluation, or close to the time of an evaluation is recommended since even standard strains vary in their phenotypic expression of resistance through time [ 43 ]. The use of resistant strains for evaluation of regeneration time of pyrethroid only nets may give supplementary information on true regeneration time, although the use of the median time to knock down [ 37 ] or median time to take off [ 30 ] may give a more sensitive measure of surface concentration using susceptible test systems. New videography methods that quantify mosquito interaction with the net surface show great promise in standardization of these measurements [ 44 ].…”

“…A method has been developed using net titration before and after washes in soap water or insolvents, that is designed to measure the time it takes for the surface concentration of one or more insecticides to become stable, i.e., the regeneration time (Skovmand, pers.commun.). Surface concentration of permethrin-treated nets has been assessed by rubbing the surface of an ITN with a cotton sample using a Martindale machine (normally used to determine the abrasion and pilling resistance of all kinds of textiles) and analysing the chemical on the cotton sample by HPLC, which correlated with bio efficacy via median time to knock down [ 37 ]. Greater uptake of existing methods is needed to generate the data required for recommendation of such tests as a regular part of product testing.…”

Comparison of cone bioassay estimates at two laboratories with different Anopheles mosquitoes for quality assurance of pyrethroid insecticide-treated nets

“…Keywords: Bioefficacy, Bioassay, Cone bioassay, Tunnel test, Insecticide treated nets, ITN, Long lasting insecticidal nets, LLIN, Pyrethroid, Mosquito, Anopheles, Malaria, Quality assurance chemical tests such as high performance liquid chromatography and gas chromatography with mass spectrometry have not yet been found to correspond well to bioefficacy results [34].…”