IMPACT OF HOUSE DESIGN ON THE ENTRY AND EXIT OF INSECT DISEASE VECTORS
IN THE GAMBIA

Abstract

The impact of house design on the entry and exit of insect vectors of disease in The Gambia

House design may affect the exposure of the residents to vector-borne diseases in rural areas in Africa. Improving a house by simply closing the eaves gap or by installing a ceiling could go a long way to preventing diseases such as malaria, which is killing millions of people every year. These two structural adjustments were the main focus of this thesis, which addresses three major questions:
1. What effect does eave closure have on mosquito house entry in houses that have screened doors and no other route of entry?
2. Are torn ceilings treated with insecticide as good as intact untreated ceilings at preventing mosquito house entry?
3. Is full screening and/or screened ceilings efficacious at preventing house entry by houseflies?
To determine the importance of eaves to mosquito house entry, a crossover study was conducted using 12 single-roomed houses with screened doors, in which the eaves were either open or closed for half of the study. Closing the eave gaps reduced the house entry of the malaria vector Anopheles gambiae s.l. by 65%, but no significant reduction was observed for culicine mosquitoes. To test the efficacy of insecticide-treated torn ceilings against mosquito house entry, three different insecticide treatments were compared with an intact untreated ceiling, an untreated torn ceiling, and a no ceiling control, using six experimental huts, with a man sleeping under an untreated bednet in each hut. Here treatments were rotated between different huts on different nights. The insecticide-treated ceilings failed to reduce the number of vectors entering the hut compared with the untreated torn ceilings. Finally, the number of houseflies, pests of public health importance, entering fully screened and screened-ceiling houses was estimated by sticky trap catches.
The findings indicate that anopheline mosquitoes largely enter houses through open eaves, whilst culicine mosquitoes enter through the doors. Failure to demonstrate enhanced protection with the insecticide-treated torn ceilings may have resulted from a failure of the insecticides to adhere well to the treated fabric. Fully screened houses reduced housefly entry by 24% whereas ceilings increased the houseflies by 440% compared to unscreened houses. It is likely that the increase in houseflies in houses with screened ceilings was an artefact caused by sticky traps over-estimating the number of flies in a room. Whilst ceilings failed to reduce flies entering houses, fully screened doors and windows were protective.
These results demonstrate that simple changes in house design can reduce the risk of exposure to malaria and flies. However, further work is required to determine whether different insecticide formulations on screening can increase the efficacy of this intervention. These studies demonstrate that house screening can play an important role in the control of malaria, and perhaps other diseases as well.