Evaluation of a push-pull system consisting of transfluthrin-treated eave ribbons and odour-baited traps for control of indoor- and outdoor-biting malaria vectors

Abstract

Background: Push-pull strategies have been proposed as options to complement primary malaria prevention tools, indoor residual spraying (IRS) and long-lasting insecticide-treated nets (LLINs), by targeting particularly early-night biting and outdoor-biting mosquitoes. This study evaluated different configurations of a push-pull system consisting of spatial repellents [transfluthrin-treated eave ribbons (0.25 g/m² ai)] and odour-baited traps (CO₂-baited BG-Malaria traps), against indoor-biting and outdoor-biting malaria vectors inside large semi-field systems.

Methods: Two experimental huts were used to evaluate protective efficacy of the spatial repellents (push-only), traps (pull-only) or their combinations (push-pull), relative to controls. Adult volunteers sat outdoors (1830 h-2200 h) catching mosquitoes attempting to bite them (outdoor-biting risk), and then went indoors (2200 h-0630 h) to sleep under bed nets beside which CDC-light traps caught host-seeking mosquitoes (indoor-biting risk). Number of traps and their distance from huts were varied to optimize protection, and 500 laboratory-reared Anopheles arabiensis released nightly inside the semi-field chambers over 122 experimentation nights.

Results: Push-pull offered higher protection than traps alone against indoor-biting (83.4% vs. 35.0%) and outdoor-biting (79% vs. 31%), but its advantage over repellents alone was non-existent against indoor-biting (83.4% vs. 81%) and modest for outdoor-biting (79% vs. 63%). Using two traps (1 per hut) offered higher protection than either one trap (0.5 per hut) or four traps (2 per hut). Compared to original distance (5 m from huts), efficacy of push-pull against indoor-biting peaked when traps were 15 m away, while efficacy against outdoor-biting peaked when traps were 30 m away.

Conclusion: The best configuration of push-pull comprised transfluthrin-treated eave ribbons plus two traps, each at least 15 m from huts. Efficacy of push-pull was mainly due to the spatial repellent component. Adding odour-baited traps slightly improved personal protection indoors, but excessive trap densities increased exposure near users outdoors. Given the marginal efficacy gains over spatial repellents alone and complexity of push-pull, it may be prudent to promote just spatial repellents alongside existing interventions, e.g. LLINs or non-pyrethroid IRS. However, since both transfluthrin and traps also kill mosquitoes, and because transfluthrin can inhibit blood-feeding, field studies should be done to assess potential community-level benefits that push-pull or its components may offer to users and non-users.

Keywords: CO2-baited BG-malaria traps; Early-night biting; Outdoor-biting; Push–pull; Semi-field chamber; Transfluthrin treated eave-ribbons.

Fig. 1

Pictorial illustration of the semi-field chambers and the mosquito tunnel. The semi-field chambers were used to evaluate the different configuration of push-and-pull subunits (a). The 110 m long mosquito tunnel was used to evaluate the optimal distances between the eave-ribbons wrapped along the eave-space of the hut (blue structure inside the chamber) and the odour-baited BG-Malaria trap (b). A section of the empty tunnel is also shown (c). Adult male volunteers sat in the peri-domestic space of each of the huts and collected mosquitoes attempting to bite them between 1800 and 2200 h, before going indoors to sleep under intact bed nets. CDC-light traps were used to catch mosquitoes attempting to bite the sleeper between 2200 and 0630 h the next morning

Fig. 2

Illustration of the experimental setup for evaluating push–pull inside semi-field chamber. Adult male volunteers (one volunteer/hut) conducted sat outdoors from 1830 to 2200 h catching mosquitoes attempting to bite him (outdoor-biting), and thereafter moved indoors to sleep under untreated bed-nets from 2200 to 0630 h. Once the volunteer was indoors, a CDC-light trap set beside the bed net was used to collect mosquitoes indoors in each hut. The mosquitoes were always released in the chamber 30 min before volunteers moved in at 1830 h. However, whenever traps were used, they were also switched on at 1830 h. The placement of the transfluthrin-treated eave ribbons, i.e. push sub-unit and the CO₂-baited BG-Malaria trap, i.e. pull sub-unit, are shown in the peri-domestic space (a). Controls had no eave-ribbons nor traps (b)

Fig. 3

Charts representing mean numbers of mosquitoes caught per night per hut both indoors and outdoors when either push or pull was tested alone and when both push-pull tested together against Anopheles arabiensis. The figure is generated from data in Table 1, and the radii of the cycles approximate overall biting risk associated with each combination, i.e. control, push only, pull only or push-pull. Push-pull offered higher protection than traps alone against indoor-biting (83.4% vs. 35.0%) and outdoor-biting (79% vs. 31%), but its advantage over repellents alone was non-existent against indoor-biting (83.4% vs. 81.2%), and was modest for outdoor-biting (79% vs. 63%)

Fig. 4

Number of Anopheles arabiensis caught at each hut nightly outdoors (by human landing catches) and indoors (by CDC-light traps), when the push–pull system used different numbers of traps. The figure shows the actual mosquito counts per night, the medians, and also model estimated mean catches

Fig. 5

Number of Anopheles arabiensis caught per hut per night outdoors (by human landing catches) and indoors (by CDC light traps) when the push-and-pull system consisted of a trap placed at different distances from the hut. The figure shows actual mosquito counts per night, median values and model estimated mean catches