Semi-field studies to better understand the impact of eave tubes on mosquito mortality and behaviour

Abstract

Background: Eave tubes are a type of housing modification that provide a novel way of delivering insecticides to mosquitoes as they attempt to enter the house. The current study reports on a series of semi-field studies aimed at improving the understanding of how eave tubes might impact mosquito mortality and behaviour.

Methods: Experiments were conducted using West African style experimental huts at a field site in M'be, Côte d'Ivoire. Huts were modified in various ways to determine: (i) whether mosquitoes in this field setting naturally recruit to eave tubes; (ii) whether eave tubes can reduce house entry even in the absence of screening; (iii) whether mosquitoes suffer mortality if they attempt to exit a house via treated eave tubes; and, (iv) whether screening and eave tubes might deflect mosquitoes into neighbouring houses without the intervention.

Results: Ninety percent more mosquitoes (Anopheles gambiae sensu lato, and other species) entered huts through open eaves tubes compared to window slits. The addition of insecticide-treated eave tubes reduced mosquito entry by 60%, even when windows remained open. Those mosquitoes that managed to enter the huts exhibited a 64% reduction in blood feeding and a tendency for increased mortality, suggesting contact with insecticide-treated inserts prior to hut entry. When An. gambiae mosquitoes were deliberately introduced into huts with treated eave tubes, there was evidence of six times increase in overnight mortality, suggesting mosquitoes can contact treated eave tube inserts when trying to exit the hut. There was no evidence for deflection of mosquitoes from huts with screening, or screening plus eave tubes, to adjacent unmodified huts.

Conclusions: Eave tubes are a potentially effective way to target Anopheles mosquitoes with insecticides. That treated eave tubes can reduce mosquito entry even when windows are open is a potentially important result as it suggests that eave tubes might not need to be combined with household screening to have an impact on malaria transmission. The absence of deflection is also a potentially important result as coverage of eave tubes and/or screening is unlikely to be 100% and it is important that households that do not have the technology are not disadvantaged by those that do.

Keywords: Anopheles gambiae; Blood-feeding inhibition; Deflection; Housing improvement; Malaria; Mosquito entry; Vector control.

Figures

Fig. 1

West African experimental hut in M’be, Côte d’Ivoire, and modifications with addition of eave tubes. a Is the schematic from the experimental hut (modified from Djènontin et al. [14]). b Represents the front of the hut, c the left side of the hut. The huts were modified to include multiple tubes (12) for use in other experiments but for the current study, half the tubes were blocked so that each experimental hut had 6 functioning eave tubes (2 on each side and 2 at the front)

Fig. 2

Eave tube and eave tube insert. a An insert inside an eave tube (view from outside); b a treated insert with visible insecticide powder

Fig. 3

Semi-field enclosure for release-recapture studies. a Metallic framework of the enclosure built around 2 experimental huts; b white tarpaulin floor to facilitate collection of dead mosquitoes; c netting walls and door, and tarpaulin roof; d water gutter to reduce entry of ants

Fig. 4

Mean (± SE) number of mosquitoes (all species) and of Anopheles gambiae s.l. captured per hut per night with open eaves tubes or open windows. The approach used 2 experimental huts that were assigned 1 of 2 treatments: (i) open eaves, in which eave tubes were open and windows closed; or (ii) open windows, in which eaves were closed and windows open. Means are based on a total of 20 nights of capture per treatment

Fig. 5

Mean (± SE) number of mosquitoes (all species) and of Anopheles gambiae s.l. captured per hut per night, comparing huts fitted with treated eave tubes or with untreated eave tubes. Both huts have open windows. Means are based on 24 nights of capture per treatment

Fig. 6

Mean (± SE) proportion of blood-fed Anopheles gambiae s.l. per hut and night comparing huts fitted with treated eave tubes or with untreated eave tubes. Both huts have open windows. Means are based on 24 nights of capture per treatment

Fig. 7

Effect of treated eave tubes on exit mortality. Adult An. gambiae s.l. were released into experimental huts with closed windows and door in the evening and recovered the following morning. The Figure shows the mean (± SE) proportion of dead mosquitoes at recapture or 24 h post recapture, comparing huts fitted with treated eave tubes, with huts fitted with untreated eave tubes. Treatments were replicated over 8 nights

Fig. 8

Mean (± SE) proportion of adult Anopheles gambiae captured inside a ‘control’ hut (i.e., a hut with open windows and open eaves) when paired with adjacent huts in a semi-field enclosure. Adult An. gambiae s.l. mosquitoes were released in the semi-field enclosure in the evening and recovered the following morning. Treatment pairings were control + control, control + screened hut (i.e., hut with untreated eave tube inserts and closed windows), and control + eave tubes (hut with treated eave tubes and closed windows). Means are based on 8 nights of release-recapture per treatment combination

Fig. 9

Mean (± SE) proportion of dead mosquitoes recovered from the semi-field enclosure for different treatment combinations. Adult An. gambiae (s.l.) mosquitoes were released in the semi-field enclosure in the evening and recovered the following morning. Here, ‘control’ indicates both huts had open eaves and open windows, ‘screening’ means 1 control hut and 1 with untreated eave tube inserts and closed windows, and ‘screening + eave tubes’ means 1 control hut and 1 with treated eave tubes and closed windows. Means are based on 8 nights of release-recapture per treatment combination