Insecticide-Treated Screening (ITS) of windows for household protection against insecticide resistant Anopheles gambiae s.l.: a semi-field trial at Kolongonouan village, central Bouaké, Côte d'Ivoire

Background Despite the significant impact of long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) on malaria, 597,000 deaths from malaria were still recorded in 2023. Additional measures are clearly needed to complement current tools. This study assessed the efficacy of various household modifications comprising different combinations of insecticide treated and untreated screening on the eaves and windows, to protect households against malaria vectors. Method The following interventions were evaluated in experimental houses in the village of Kolongonouan, within central Cote d’Ivoire: (i) Standard control house with small openings in the eaves and no window screens (SCH); ( ii) Insecticide Treated Screening (ITS) of windows with small openings in the eaves (ITS op); (iii) ITS of windows with eaves blocked to prevent mosquito entry via eaves (ITS blq); (i v) Insecticide treatment applied to the eaves using In2Care eave tubes, without window screening (ET); ( v) Untreated window screening together with insecticide treated eave tubes (SET). The efficacy of these treatments on indoor and outdoor density of naturally recruiting Anopheles gambiae mosquitoes was assessed using human landing catches. These impact of these treatments on mortality and blood feeding rate was further assessed using release-recapture experiments in which large enclosures were built around individual houses and a known number of mosquitoes released per night. In addition, for the ITS blq treatment, the importance of physical integrity was assessed by evaluating the effect of damaging the screening with two or four 4x4cm holes in each screened window. Results Adding treated window screening and blocking eaves (ITS blq) reduced the number of naturally recruiting An. gambiae entering houses overnight by 31.8% relative to standard control houses (SCH). The use of untreated screening + eave tubes (SET) produced a significantly similar reduction (27.0%). Eave tubes alone (ET) reduced indoor populations by 23.5% (significantly less impact than ITS blq or SET, but still a significant reduction relative to controls), while insecticide treated window screening with open eaves (ITS op) resulted in a marginally non-significant reduction of 18.1%. There was no significant difference in house entry between ET and ITS blq or ITS op. Maximum capture rates occurred between 11pm and 3am. There were no significant impacts of the treatments on the numbers of mosquitoes captured adjacent to the houses outdoors. In the release-recapture experiments, all treatments significantly reduced blood feeding rate and increased 24h mortality of An. gambiae , with again similar impact for ITS blq and SET. Damaging the ITS with two holes still led to a significant reduction in blood feeding rate, but with more extensive damage of four holes, the protective effect was lost. Mortality rate also declined with increasing levels of damage, although remained significantly higher relative to untreated window screening. Conclusion This study demonstrated that adding insecticide treated screening to windows and blocking access points in the eaves reduced mosquito entry, mortality and blood-feeding rate at household level to similar levels as insecticide treated eaves tubes combined with untreated window screening (SET). A previous cluster randomised controlled trial in this location demonstrated a significant reduction in malaria incidence and prevalence when SET was implemented at village scale. Insecticide-treated window screens could, therefore, yield similar epidemiological impact, while potentially being simpler and less expensive to implement. Larger-scale epidemiogical trials involving communities are needed to test this assumption and further optimize the approach.