Evaluation of the durability of long-lasting insecticidal nets in Guatemala

Abstract

Background: Insecticide-treated bed nets (ITNs) are widely used for the prevention and control of malaria. In Guatemala, since 2006, ITNs have been distributed free of charge in the highest risk malaria-endemic areas and constitute one of the primary vector control measures in the country. Despite relying on ITNs for almost 15 years, there is a lack of data to inform the timely replacement of ITNs whose effectiveness becomes diminished by routine use.

Methods: The survivorship, physical integrity, insecticide content and bio-efficacy of ITNs were assessed through cross-sectional surveys conducted at 18, 24 and 32 months after a 2012 distribution of PermaNet® 2.0 in a malaria focus in Guatemala. A working definition of 'LLIN providing adequate protection' was developed based on the combination of the previous parameters and usage of the net. A total of 988 ITNs were analysed (290 at 18 months, 349 at 24 months and 349 at 32 months).

Results: The functional survivorship of bed nets decreased over time, from 92% at 18 months, to 81% at 24 months and 69% at 32 months. Independent of the time of the survey, less than 80% of the bed nets that were still present in the household were reported to have been used the night before. The proportion of bed nets categorized as "in good condition" per World Health Organization (WHO) guidelines of the total hole surface area, diminished from 77% to 18 months to 58% at 32 months. The portion of ITNs with deltamethrin concentration less than 10 mg/m² increased over time. Among the bed nets for which bioassays were conducted, the percentage that met WHO criteria for efficacy dropped from 90% to 18 months to 52% at 32 months. The proportion of long-lasting insecticidal nets (LLINs) providing adequate protection was 38% at 24 months and 21% at 32 months.

Conclusions: At 32 months, only one in five of the LLINs distributed in the campaign provided adequate protection in terms of survivorship, physical integrity, bio-efficacy and usage. Efforts to encourage the community to retain, use, and properly care for the LLINs may improve their impact. Durability assessments should be included in future campaigns.

Keywords: Bio-efficacy; Deltamethrin; Durability; Insecticide content; Insecticide-treated bed nets; Long-lasting insecticidal nets; Malaria; Survivorship; attrition.

Fig. 1

Location of the 12 study communities in La Gomera, Escuintla, Guatemala

Fig. 2

Flow chart of the data collection and number of long-lasting insecticidal nets (LLINs) that were evaluated for each of the measurements. Invalid questionnaires represent questionnaires with incomplete or discrepant data or questionnaires carried out on nets that were not part of the PermaNet® 2.0 2012 distribution

Fig. 3

Reasons for long-lasting insecticidal nets (LLINs) loss

Fig. 4

Estimated total deltamethrin concentration (mg/m²) measured by X-ray fluorescence (XRF) at the survey time-points. The time-point of 0 represents values on unused long-lasting insecticidal nets (LLINs) that were from the same batch as those distributed. The dashed horizontal line represents the threshold of 25 mg/m² and the solid horizontal line represents the threshold of 10 mg/m²

Fig. 5

Locally weighted regression (LOESS) analysis between results of the cone bioassays measuring percent mortality at 24 h and concentration of deltamethrin (mg/m²) as measured by X-ray fluorescence (XRF) analysis for all survey time-points. Each dot represents one long-lasting insecticidal net (LLIN) plotted based corresponding cone bioasays and XRF measurements. The gray area represents the 95% confidence interval. The solid vertical line represents the threshold of 10 mg/m² and the dashed vertical line represents the threshold of 25 mg/m². The black horizontal line represents the 80% mortality threshold

Fig. 6

Estimate of effective protection of long-lasting insecticidal nets (LLINs) in La Gomera, Escuintla. The proportion of LLINs that met each of the criteria listed on the left is shown in sequential descending order, with each value shown as a subset of the criteria above. If the LLIN was able to fulfill all criteria, it was considered a LLIN that provided adequate protection. The stage of deltamethrin above 10 mg/m² as measured by XRF for the 18-month survey was not included as only a small number of LLINs were analysed with XRF at that time-point