Monitoring the durability of the long-lasting insecticidal nets Olyset® and PermaNet® 2.0 in similar use environments in Zanzibar

Abstract

Background: Malaria transmission in Zanzibar has dramatically reduced in recent years but vector control interventions such as long-lasting insecticidal nets (LLIN) must continue to reach malaria elimination. To achieve this, the Zanzibar Malaria Elimination Programme needs actionable evidence of the durability of the LLIN brands distributed. This study compared physical and insecticidal durability of two LLIN brands: Olyset ® and PermaNet© 2.0 in two similar districts on the islands of Unguja and Pemba. Methods: This was a prospective cohort study of representative samples of households from two districts, recruited at baseline 4 months after the mass campaign. All campaign nets in these households were labelled and followed up over a period of 33 months. Primary outcome was the "proportion of nets surviving in serviceable condition" based on attrition and integrity measures and the median survival in years. The outcome for insecticidal durability was determined by bio-assay from sub-samples of campaign nets. Results: A total of 834 campaign nets (121% of target) from 299 households were included in the study. Definite outcomes could be determined for 86% of the cohort nets in Unguja (PermaNet ® 2.0) and 89% in Pemba (Olyset ®). After 33 months, physical survival in serviceable condition was 55% in Unguja and 51% in Pemba. Estimated median survival was lower in Pemba at all time points with 2.3-2.7 years compared to 3.1-3.3 yeas in Unguja. Multivariable Cox proportionate hazard models confirmed the difference between brands (p < 0.0001) and identified household net-care attitude (p = 0.007) and folding of hanging nets during the day (p < 0.0001) as significant determinants, in addition to exclusive use of nets by adults (p = 0.03) and use only over a finished bedframe (p = 0.01). Optimal insecticidal effectiveness was 80% or higher for both brands at all time points when both cone bio-assays and tunnel tests were applied. Conclusions: After 3 years of follow-up, Olyset ® LLIN showed significantly lower physical survival compared to PermaNet ® 2.0 LLIN even after adjusting for other variables of net-use environment and net handling. This suggests that the differences were driven by the textile characteristics of the LLIN brands.

Evaluation of the Durability of Long-Lasting Insecticidal Nets in Guatemala

“…The median survival time reported in our study is higher than similar studies conducted in Africa that have also evaluated PermaNet ® 2.0 LLINs. In Tanzania the estimated survival was 2.5 years, in Zanzibar it was 3.1-3.3 years and in Zambia it was 2.5-3 years (Haji et al, 2020;Lorenz et al, 2019;Tan et al, 2016). In our study, the physical integrity of the LLINs-a key component for the functional survivorship-was high in each of the survey timepoints (range: 84-94%), whereas other studies have reported lower values (e.g.…”

“…In our study, the physical integrity of the LLINs-a key component for the functional survivorship-was high in each of the survey timepoints (range: 84-94%), whereas other studies have reported lower values (e.g. 68% at 36 months) (Haji et al, 2020). These differences are further highlighted by the median total hole area, which in our study was only 34 cm 2 at 32 months, whereas other studies evaluating PemaNet ® 2.0 LLINs reported values of 390 cm 2 and 106 cm 2 (Craig et al, 2015; Vanden Eng et al, 2015).…”

The fabric of life: what if mosquito nets were durable and widely available but insecticide-free?

“…Field evidence increasingly shows that current ITNs do not last as long as expected under real use settings, and that there is a wide variation in both bio-efficacy and durability in different settings [181][182][183][184]. In one study in Tanzania, scientists examined the durability of ITNs distributed by the government between 2009 and 2011 in eight districts [185].…”

The Consequences of Declining Population Access to Insecticide Treated Nets (ITNs) on Net Use Patterns and Physical Degradation of Nets after 22 Months of Ownership

“…NetProtect® was two times more likely to be serviceable when compared to Olyset® in this setting. When compared to PermaNet®, Olyset® nets have been observed to have more holes in both Mozambique [46], Zambia [47] and Zanzibar [48] and mainland Tanzania [32]. In Madagascar [49], 55.6% of NetProtect® ITNs were in good condition after a year when compared to Royal Sentry® (56.8%) and Yorkool® (69.2%), which is lower than in the current study, indicating the importance of considering location when estimating ITN durability as cultural in uences, net care and attitudes as well as the physical environment all impact on the expected life of ITNs.…”