Influence of testing modality on bioefficacy for the evaluation of Interceptor® G2 mosquito nets to combat malaria mosquitoes in Tanzania

BACKGROUND: Insecticide-treated net (ITN) durability is evaluated using longitudinal bioefficacy and fabric integrity sampling post-distribution. Interceptor((R)) G2 was developed for resistance management and contains two adulticides: alpha-cypermethrin and chlorfenapyr; it is a pro-insecticide that is metabolized into its active form by mosquito-detoxifying enzymes and may be enhanced when the mosquito is physiologically active. To elucidate the impact of bioassay modality, mosquito exposures of the alphacypermethrin ITN Interceptor((R)) and dual adulticide Interceptor((R)) G2 were investigated. METHODS: This study evaluated the performance of Interceptor((R)) G2 compared to Interceptor((R)) against local strains of mosquitoes in Tanzania. Unwashed and 20x times washed nets were tested. Efficacy of ITNs was measured by four bioassay types: (1) World Health Organisation (WHO) cone test (cone), (2) WHO tunnel test (tunnel), (3) Ifakara ambient chamber test (I-ACT) and (4) the WHO gold standard experimental hut test (hut). Hut tests were conducted against free-flying wild pyrethroid metabolically resistant Anopheles arabiensis and Culex quinquefasciatus. Cone, tunnel and I-ACT bioassays used laboratory-reared metabolically resistant An. arabiensis and Cx. quinquefasciatus and pyrethroid susceptible Anopheles gambiae sensu stricto and Aedes aegypti. RESULTS: Against resistant strains, superiority of Interceptor((R)) G2 over Interceptor((R)) was observed in all "free-flying bioassays". In cone tests (which restrict mosquito flight), superiority of Interceptor((R)) over Interceptor((R)) G2 was recorded. Mortality of unwashed Interceptor((R)) G2 among An. arabiensis was lowest in hut tests at 42.9% (95% CI: 37.3-48.5), although this increased to 66.7% (95% CI: 47.1-86.3) by blocking hut exit traps so mosquitoes presumably increased frequencies of contact with ITNs. Higher odds of mortality were consistently observed in Interceptor((R)) G2 compared to Interceptor((R)) in "free-flying" bioassays using An. arabiensis: tunnel (OR = 1.42 [95% CI:1.19-1.70], p < 0.001), I-ACT (OR = 1.61 [95% CI: 1.05-2.49], p = 0.031) and hut (OR = 2.53 [95% CI: 1.96-3.26], p < 0.001). Interceptor((R)) and Interceptor((R)) G2 showed high blood-feeding inhibition against all strains. CONCLUSION: Both free-flying laboratory bioassays (WHO Tunnel and I-ACT) consistently measured similarly, and both predicted the results of the experimental hut test. For bioefficacy monitoring and upstream product evaluation of ITNs in situ, the I-ACT may provide an alternative bioassay modality with improved statistical power. Interceptor G2((R)) outperformed Interceptor ((R)) against pyrethroid-resistant strains, demonstrating the usefulness of chlorfenapyr in mitigation of malaria.