Background: Malaria prevention with long-lasting insecticidal-treated nets (LLINs) has seen a tremendous scale-up in sub-Sahara Africa in recent years; however, studies have suggested that the physical durability between LLINs may vary significantly. These differences are largely driven by environmental and behavioral factors, but may also be driven by differences in the textile qualities of the LLIN brand. Country programs should implement regular monitoring of LLIN durability. Following guidance from the U.S. President’s Malaria Initiative (PMI), durability monitoring of two brands of LLINs—each with different specifications, distributed in the 2016 mass distribution campaign in the Democratic Republic of Congo (DRC) during three years—was set up in two ecologically similar health zones in neighboring provinces: Sud Ubangi and Mongala. The National Malaria Control Program and the Kinshasa School of Public Health carried out this activity, with support from the VectorWorks project and PMI.
Methods: During this prospective cohort study, representative samples of households from each district were recruited at baseline, one to six months after the mass campaign. All campaign nets in these households were labeled and followed up during 33–36 months. A total of 240 households (80% of target) and 754 campaign nets (109%) were included in the study. Definite outcomes could be determined for 67% of the cohort nets in Sud Ubangi and 82% in Mongala. Outcomes measured for physical durability were attrition (all-cause attrition and attrition due to wear and tear) and physical integrity, based the proportionate Hole Index (pHI) and subsequent categorization of cohort nets as serviceable (pHI<643). They were then combined to provide the “proportion of nets surviving in serviceable condition” at each time point of follow up and the median survival in years (the time until 50% of cohort nets with known outcomes were no longer serviceable). In addition, a survival analysis was undertaken using a Cox proportionate hazard model to analyze determinants of LLIN survival. The outcome for insecticidal durability was determined by bio-assay (World Health Organization [WHO] cone test) from subsamples of the campaign nets; it was defined as the proportion of nets that showed optimal insecticidal effectiveness (24-hour mortality of ≥80% or 60-minute knockdown of ≥95%). In addition, demographic, socio-economic, and behavioral aspects were recorded through a structured questionnaire at each time point. The demographic characteristics of the populations were comparable between sites, which are typical for rural African populations; it did not change significantly over time. House construction at both sites was similar and very simple, with around 80% of roofs made from grass or thatch, 80–90% of walls made from mud, and 90% of floors made from earth or clay. Almost all households used firewood for cooking, had access to a pit latrine, but also used surface water from rivers and creeks for drinking. The economic situation was also very similar, with a slight advantage for Mongala, mainly due to a higher mobile phone coverage that, generally, varied because of low-to-poor provider coverage. Most durability risk factors were very similar between the two sites, with some minor differences, such as higher instances of cooking in the sleeping rooms in Sud Ubangi, higher use of finished bed frames in Sud Ubangi, but more foam mattresses in Mongala. The main difference was the much more positive attitude toward net care in Sud Ubangi, in spite of similarly low behavior change communication message exposure at both sites. This did not necessarily translate into actual repair of nets, but did impact the LLINs’ physical durability.While, overall, all-cause attrition of campaign nets (loss for any reason) was high—with 57% in Sud Ubangi and 76% in Mongala—the main observation was an exceptionally high rate of discarding nets because they were too old or were torn. This was statistically significantly higher in Mongala (48%) than Sud Ubangi (26%). In contrast, the physical condition of nets still found in the households was similar at both sites, with 23% of surviving nets in Sud Ubangi and 30% in Mongala being too torn to use. Overall survival in serviceable condition during the last survey was 37% in Sud Ubangi and just 17% in Mongala; this difference was statistically and programmatically significant (p=0.003). Estimated median survival was 1.6 years for the Dawa Plus 2.0 in Mongala (95% CI 1.3–1.9) and 2.2 years for the DuraNet in Sud Ubangi (95% CI 2.0–2.4), both of which were well below the assumed three-year median survival. Results from survival analysis of the data confirmed the magnitude of median survival (1.7 years in Mongala and 2.6 years in Sud Ubangi). It was also established in a Cox proportionate hazard model that the difference in median survival was mainly due to the LLIN brand and not to other factors, such as positive net care attitude, type of sleeping place, or dominant use by children only, all of which showed some positive or negative impact on the outcome in the Cox models. A bio-assay using the WHO cone tests showed optimal insecticidal performance up to the final survey for the DuraNet LLIN brand, but the Dawa Plus 2.0 LLIN brand only had optimal performance at >80% only, up to the 24-month data point; 53% of samples at the final survey failed, even the minimal effectiveness criteria. This suggests that insecticidal content was lower or was lost faster than expected. Whether this is relevant for vector control considerations in this case is questionable because the physical durability in this setting was only 1.6 years on average.
Conclusion: After three years of follow-up among neighboring, rural populations in the provinces of Sud Ubangi and Mongala, the 150 denier polyethylene LLIN DuraNet showed significant differences in median physical survival compared to the 100 denier polyester LLIN Dawa Plus 2.0; however, both remained well under the three-year expected median survival. The difference could be attributed mostly to the differences in the brand, because a Cox proportionate hazard model adjusting for other risk factors confirmed the brand as the strongest driver of the difference. This means that in environments like DRC, it will be preferable to distribute a more durable LLIN, such as the DuraNet or similar brands, but also to consider a distribution strategy with campaigns every two years or, alternatively, a continuous distribution strategy. Insecticidal performance was optimal for the DuraNet in Sud Ubangi, but for the Dawa Plus in Mongala optimal performance lasted only up to 24 months and it failed at 36 months. However, by this time, most of the cohort nets were already lost.
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