In MalariaWorld this week a study from Madagascar examines early bio-efficacy loss of ITNs. In ‘Early Bio-Efficacy Loss of Nets Mass Distributed for Malaria Vector Control in Madagascar in 2018: Implications for Malaria Prevention’, Nepomichene et al assess the bio-efficacy of DawaPlus® 2.0 and PermaNet® 2.0 ITNs upon arrival and at 12, 24, and 36 months after distribution.
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%. However, at 12, 24, and 36 months after distribution, mosquito mortality rates were <56% for all districts. With the exception 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 (Tana Netting, Bangkok, Thailand and Vestergaard–Frandsen, Lausanne, Switzerland), 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.
The logic behind the use of insecticide treated nets is that a mosquito that lands on the net will die and not be able to bite another person. This is assumed to improve the effectiveness of nets at preventing malaria. However, this has never been effectively shown. It is not even clear that net programs are effective at reducing the occurrence of malaria. In their discussion the authors state that Madagascar, reported malaria cases increased from 965,000 in 2018 and 992,000 in 2019, shortly after mass ITN distribution, and to 1,950,000 in 2020 and then to 2,344,000 in 2021. This does not suggest that they are effective at preventing malaria.
I sleep inside a mosquito net because I don’t like the discomfort of mosquito bites. It is not insecticide treated. From a health perspective I am concerned about the exposure to insecticides from such nets. It is clear from this study that the insecticide does not stay on the nets so could easily be absorbed by people using them.
And, of course, nets can only prevent malaria if it is indeed spread by mosquitos. I do not believe that this hypothesis has been proven.