A new insecticidal net treated with an insect growth regulator was more effective than standard insecticidal nets against pyrethroid-resistant Anopheles gambiae mosquitoes in Benin, Africa, according to study data.
“Unfortunately, the development and rapid spread of resistance to pyrethroids in mosquito vectors of the malaria parasite threaten to undermine the effectiveness of insecticidal mosquito nets,” Corine Ngufor, MSc, PhD, of the London School of Hygiene & Tropical Medicine and the Centre de Recherches Entomologiques de Cotonou, Benin, and colleagues wrote in Science Translational Medicine. “In areas of high resistance, pyrethroid-treated nets have shown reduced effectiveness in several small-scale trials, and this is driving the search for new insecticides that can maintain the utility of this once-effective means of malaria prevention.”
The researchers tested the Olyset Net — treated with either pyrethroid or pyriproxyfen — and the new Olyset Duo mosquito net (both Sumitomo Chemical) — treated with permethrin (a pyrethroid) and pyriproxyfen, an insect growth regulator that sterilizes adult mosquitoes and prevents larvae from molting — against A. gambiae mosquitoes. Testing included laboratory and field studies in southern Benin. The nets were first tested while unwashed, then tested after 5, 10, 15 or 20 washes.
In cone bioassays, both the unwashed Olyset Net and Olyset Duo killed 100% of female mosquitoes that were exposed to the net, Ngufor and colleagues reported. After five washes, the efficacy of the Olyset Net fell to 70% and remained there after 20 washes. Mortality varied with the Olyset Duo, the researchers wrote, falling to 58% after 10 washes, but rising to more than 80% after 20 washes. According to the WHO pesticide evaluation scheme, the Olyset Duo met cone test efficacy criteria of more than 95% knockdown or more than 80% mortality after 20 washes.
The Olyset Duo also was found to be more effective in WHO tunnel tests, the researchers reported. The unwashed Olyset Duo showed a mortality of 100%, compared with 84% in the unwashed Olyset Net. After 20 washes, mortality with the Olyset Duo was more than 80%, while the Olyset Net fell to 65% mortality after 20 washes. Further, Olyset Duo was more effective in inhibiting blood feeding than Olyset Net at every wash interval (P < .05).
In simulated household tests using huts typical of homes in southern Benin, the unwashed Olyset Duo net produced the highest mortality, 40%, while the unwashed Olyset Net induced 32% mortality. An unwashed pyriproxyfen net induced 30% mortality. After 20 washes, the Olyset Net’s mortality dropped to 27%, while the pyriproxyfen net fell to 22% the researchers reported. The Olyset Duo’s mortality was reduced only slightly, decreasing to 39%.
The Olyset Duo also sterilized exposed female mosquitoes. While researcher reported that 55% of mosquitoes oviposited after exposure to both an untreated control net and Olyset Net, just 12% oviposited after contact with the unwashed pyriproxyfen net (P <.001). After contact with the Olyset Duo net, 31% of exposed females oviposited (P <.05).
“These findings demonstrate that the Olyset Duo net has the potential to control pyrethroid-resistant mosquito vector populations and warrants further testing in communities of high malaria transmission due to pyrethroid-resistant mosquitoes,” Ngufor and colleagues wrote. – by Andy Polhamus
Disclosure: The researchers report no relevant financial disclosures.