Early Bio-Efficacy Loss of Nets Mass Distributed for Malaria Vector Control in Madagascar in 2018: Implications for Malaria Prevention

In 2018, insecticide-treated nets (ITNs) were mass distributed across Madagascar. The bio-efficacy of DawaPlus® 2.0 and PermaNet® 2.0 ITNs was assessed upon arrival and at 12, 24, and 36 months after distribution. Chemical analyses of insecticide residue on ITNs were also conducted. On arrival, mosquito mortality rates observed when exposed to DawaPlus 2.0 (86.4%) and PermaNet 2.0 nets (83.6%) exceeded the WHO's threshold of 80.0%. At 12, 24, and 36 months after distribution, mosquito mortality rates were <56% for all districts. Moreover, the knockdown effect was below the WHO threshold of 95.0% for all districts and at all time points, even for new ITNs. With the exception of the new DawaPlus 2.0, the deltamethrin residue on ITNs was also lower than the expected ranges of 80 mg/m2 ± 25% for DawaPlus 2.0 and 55 mg/m2 ± 25% for PermaNet 2.0; regardless of ITN age, the concentration of deltamethrin was <66 mg/m2 for DawaPlus 2.0 and <36 mg/m2 for PermaNet 2.0 ITNs. According to the manufacturers, ITNs are effective for 36 months; therefore, mass distribution campaigns are organized every 3 years. However, the DawaPlus 2.0 and PermaNet 2.0 ITNs exhibited a loss of bio-efficacy within 1 year of distribution. This bio-efficacy loss could be due to a manufacturing problem, poor storage and transportation conditions, or poor use and net care practices in Madagascar. Understanding and correcting the root causes of this issue is critical for guiding corrective actions, such as improving manufacturing processes, replacing ITNs more frequently, and increasing education on ITN care.