Abstract
BackgroundThe emergence of mosquitoes that can avoid indoor-deployed interventions, such as treated bed nets and indoor residual spraying, threatens the mainstay of malaria control in Zambia. Furthermore, the requirement for high coverage of these tools poses operational challenges. Spatial repellents are being assessed to supplement these vector control tools, but limitations exist in the residual effect of the repellent and the need for external power or heat for diffusion of the volatiles.MethodsA semi-field evaluation of a novel controlled release spatial repellent device (CRD) was conducted in Macha, Zambia. These devices emanate metofluthrin with no need for external power. Devices were deployed in huts within the semi-field system (SFS). Female Anopheles gambiae sensu stricto released within the SFS were trapped overnight by light traps and collected by aspiration the next morning inside and outside of huts to determine the extent of mosquito repellency and the impact on host-seeking and survival. Experiments studied the impact of number of devices as well as the presence of hut occupants. The study was complemented with numerical methods based on computational fluid dynamics to simulate spatial distribution of metofluthrin.ResultsPresence of CRDs was associated with significant reductions in indoor counts of mosquitoes, regardless of whether huts were occupied or not. Repellency ranged from 15 to 60% compared to huts with no devices. Reducing the number of devices from 16 to 4 had little impact on repellency. When huts were occupied, indoor mosquito host-seeking was higher in the presence of CRDs, whilst survival was significantly reduced.ConclusionsThis study demonstrated that deployment of as few as four CRDs within a hut was associated with reduced indoor mosquito densities. As would be expected, presence of occupants within huts, resulted in greater indoor catches (both with and without devices). The increased indoor mosquito host-seeking and mortality in huts when devices were present may be explained by the excito-repellency activity of metofluthrin. These semi-field experiments provide preliminary data on the utility of CRD spatial repellents to reduce indoor densities of An. gambiae mosquitoes. Studies will further investigate the impact of CRDs on mosquito behaviour as well as epidemiological protective efficacy.
Highlights
The emergence of mosquitoes that can avoid indoor-deployed interventions, such as treated bed nets and indoor residual spraying, threatens the mainstay of malaria control in Zambia
When calculating the percentage difference in proportions caught when the devices were in place across all rotations, the presence of the controlled release spatial repellent device (CRD) resulted in a 24% reduction in indoor host-seeking, and a 62% reduction in the proportion of total mosquitoes found indoors
After controlling for temperature, humidity, moonlight intensity, wind, selected chamber and the date of the experiment, the presence of devices was significantly associated with a reduction in indoor total catch [Odds Ratio (OR) = 0.32, 95% confidence interval (CI) = 0.25, 0.40, p < 0.001], but the association with reduction in indoor host-seeking was not significant (OR = 0.58, 95% CI = 0.28, 1.20, p = 0.144)
Summary
The emergence of mosquitoes that can avoid indoor-deployed interventions, such as treated bed nets and indoor residual spraying, threatens the mainstay of malaria control in Zambia. Despite intensive scale-up of long-lasting insecticide-treated bed nets (LLINs) and annual IRS since the early 2000s, [4] malaria remains one of the primary causes of morbidity and mortality in children under 5 years in Zambia [5,6,7]. These mainstays of vector control are threatened by mosquito resistance to insecticides and changes in mosquito behaviour that may result in increased outdoor foraging [8, 9]. Elimination of malaria will require additional new vector control approaches [1, 11, 12]
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