The non-inferiority of piperonyl-butoxide Yorkool® G3 insecticide-treated nets compared to Olyset®Plus measured by Anopheles arabiensis mortality in experimental huts in Tanzania.

BACKGROUND: Non-inferiority trials are recommended by the World Health Organization (WHO) to demonstrate that health products show comparable efficacy to that of existing standard of care. As part of the WHO Global Malaria Programme (GMP) process of assessment of malaria vector control products, a second-in-class insecticide-treated net (ITN) must be shown to be non-inferior to a first-in-class product based on mosquito mortality. The public health impact of the first-in-class pyrethroid-piperonyl butoxide (PBO) ITN, Olyset® Plus, has been demonstrated in epidemiological trials in areas with insecticide-resistant mosquitoes, but there is a need to determine the efficacy of other pyrethroid-PBO nets to ensure timely market availability of nets in order to increase access to ITNs. The non-inferiority of a deltamethrin-PBO ITN Yorkool® G3 was evaluated entomologically against Olyset® Plus in experimental huts in Tanzania, following WHO guidelines for non-inferiority trials. METHODS: The trial of the two pyrethroid-PBO ITNs was conducted in experimental huts in Lupiro, Tanzania, using a randomized 7 × 7 Latin square block design. The study ran for 49 nights in 14 huts assessing the mosquito mortality and blood-feeding of wild, free-flying, pyrethroid-resistant Anopheles arabiensis. Using the non-inferiority approach, the comparative efficacy (primary endpoint was mosquito mortality at 24 h and secondary endpoint was blood-feeding) of unwashed and 20 times field-washed pyrethroid-PBO Yorkool® G3 ITNs, were compared with the first-in-class product Olyset® Plus and against a pyrethroid-only ITN, PermaNet® 2.0 ITNs, as a standard comparator. RESULTS: The experimental hut trial demonstrated non-inferiority and superiority of Yorkool® G3 to Olyset® Plus based on mosquito mortality [51% vs. 39%, OR 1.68 (95% CI 1.50-1.88)], given that lower 95% CI exceeded 0.74 (delta of 39%) and the margin of no difference (1). Blood-feeding inhibition was high for all treated ITNs (> 90%) and Yorkool® G3 was non-inferior to Olyset® Plus [4% vs. 2%, OR 1.81 (95% CI 1.46-2.39)], given that upper 95% CI was less than 4.85 (delta of 4%). The pyrethroid-PBO ITNs were superior to the pyrethroid-only net, PermaNet® 2.0, as determined by both the proportion of mortality and blood-feeding of mosquitoes (p-value < 0.05). CONCLUSION: Yorkool® G3 ITNs demonstrated non-inferiority to the first-in-class Olyset® Plus and superiority over the standard pyrethroid-only ITN, PermaNet® 2.0 as measured by mortality and blood-feeding inhibition of wild pyrethroid-resistant An. arabiensis mosquitoes. Yorkool® G3 ITNs are potential tools for the control of metabolic insecticide-resistant malaria vectors, and their market availability will contribute to the cost-effective selection of ITNs by malaria control programmes to improve population access to ITNs.

Time of exposure and assessment influence the mortality induced by insecticides against metabolic resistant mosquitoes.

BACKGROUND: Increasing metabolic resistance in malaria vector mosquitoes resulted in the development of insecticide-treated nets (ITNs) with active ingredients (AI) that target them. Bioassays that accurately measure the mortality induced by these AIs on ITNs are needed. Mosquito metabolic enzyme expression follows a circadian rhythm. Thus, this study assessed (i) influence of the time of day of mosquito exposure and (ii) timing of assessment of mortality post exposure (24 and 72 h) to ITNs against vectors that are susceptible to pyrethroids and those with metabolic and knockdown resistance mechanisms. METHODS: Two cone bioassay experiments were conducted following World Health Organization (WHO) guidelines. Firstly, on ITNs incorporated with 2 g AI/kg of deltamethrin (DM) alone, or combined with 8 g AI/kg piperonyl butoxide (PBO) synergist, during the day (9:00-14:00 h) and repeated in the evening (18:00-20:00 h). This was followed by a confirmatory experiment during the afternoon (12:00-14:00 h) and repeated in the night (22:00-24:00 h) using mosquitoes unexposed or pre-exposed to PBO for 1 h before exposure to DM ITNs. Each net piece was tested with a minimum of eight cones per time (N = 24). The outcome was mortality after 24 h (M24) or 72 h (M72) of holding. RESULTS: The cone bioassays performed using metabolic resistant mosquitoes during the evening showed significantly lower M24 than those performed in the day for DM: odds ratio (OR) 0.14 [95% confidence interval (CI) 0.06-0.30, p < 0.0001] and DM PBO [OR 0.29 (95% CI 0.18-0.49, p < 0.0001). M72 was higher than M24 for metabolic resistant mosquitoes exposed to DM [OR 1.44 (95% CI 1.09-1.88), p = 0.009] and DM PBO [OR 1.82 (95% CI 1.42-2.34), p < 0.0001]. An influence of hour of experiment and time of assessment was not observed for mosquitoes that had knockdown resistance or that were pyrethroid-susceptible. CONCLUSIONS: Time of day of experiment and hour of assessment of delayed mortality after exposure of mosquitoes are important considerations in evaluating insecticides that interact with mosquito metabolism to counter metabolic resistant mosquitoes. This is important when evaluating field-aged ITNs that may have lower concentrations of AI.