Evaluation of dual-active ingredient long-lasting insecticidal nets against wild resistant malaria vectors in Tanzania: a series of experimental hut trials

The effectiveness of malaria vector control is increasingly threatened by widespread insecticide resistance, particularly to pyrethroids. In response, the World Health Organization recommends dual-active ingredients (dual-A.I.) insecticide-treated nets (ITNs) in resistant settings. This study evaluated the efficacy of several dual-A.I. ITNs in two experimental hut trial sites an area with high insecticide resistant malaria vectors in Tanzania with differing vector species composition and bionomics. Five ITNs were evaluated: a pyrethroid-only net (MAGNet(R)), three pyrethroid-piperonyl butoxide (PBO) nets (OlysetTM Plus, Veeralin(R), and PermaNet(R) 3.0), one dual-A.I. net (Interceptor(R) G2; chlorfenapyr + alpha-cypermethrin), and an untreated control net. Mosquitoes were collected from huts, morphologically identified, and monitored for mortality up to 72 h. WHO cone and tunnel tests using susceptible An. gambiae s.s. were conducted for quality control, while field-collected mosquitoes were used for bio-efficacy monitoring. Molecular species identification was performed using PCR assays, and genomic DNA was extracted from An. funestus s.l. and An. gambiae s.l. collected from the trials. All ITNs met WHO bio-efficacy criteria in cone and tunnel tests. An. gambiae s.l. predominated in Mwagagala (77%) but was rare in Magu (4%), where An. funestus complex was dominant. In Magu, Interceptor(R) G2 and Veeralin(R) achieved significantly higher mortality than MAGNet(R) at 24 h (53% and 45% vs 24%) and 72 h (61% and 53% vs 24%). In contrast, no significant mortality differences were observed among ITNs in Mwagagala. High resistance intensity was detected in both vector groups (mortality <20% in bioassays). In An. funestus from Magu, the 4.3 kb-SV resistant genotype was fixed, while G454A-CYP9K1 resistance was highly prevalent (RR: 0.70-1.00). Interceptor(R) G2 and Veeralin(R) outperformed the pyrethroid-only reference net against An. funestus complex but not against predominantly An. arabiensis populations.