House design modifications reduce indoor resting malaria vector densities in rice irrigation scheme area in western Kenya

Abstract

Background: Simple modifications of typical rural house design can be an effective and relatively inexpensive method of reducing indoor mosquito vector densities and consequently decreasing malaria transmission. Public health scientists have shown the potential for house design to protect people against malaria, yet this type of intervention remains virtually ignored. A randomized-controlled study was, therefore, undertaken to determine the effects of this method of vector control on the density of indoor resting malaria vectors in a rice irrigation scheme area in lowlands of western Kenya.

Methods: Ten treatment houses were modified with ceilings of papyrus mats and insecticide-treated netting (ITN) and tested against ten control houses without papyrus ceilings. To determine densities of mosquitoes resting in homes, the pyrethrum spray method was used to simultaneously collect indoor resting malaria vectors in intervention and control houses. Each house was sampled a total of eight times over a period of four months, resulting in a total of 80 sampling efforts for each treatment. Community response to such intervention was investigated by discussions with residents.

Results: Papyrus mats ceiling modification reduced house entry by Anopheles gambiae s.l and Anopheles funestus densities by between 78-80% and 86% respectively compared to unmodified houses. Geometric mean density of Anopheles gambiae s.l. and Anopheles funestus in modified houses were significantly lower (t(18) = 7.174, P < 0.0001 and t(18) = 2.52, P = 0.02, respectively) compared to controls. Unmodified houses were associated with relatively higher densities of malaria vectors. There was a 84% (OR 0.16, 95% CI 0.07-0.39, P < 0.0001) and 87% (OR 0.13, 95% CI 0.03-0.5, P = 0.0004) reduction in the odds of Anopheles gambiae s.l. and Anopheles funestus presence in modified houses, respectively, compared with unmodified houses. Residents responded favourably to this mode of vector control.

Conclusion: House modifications involving insect screen ceilings made from locally available materials and small ITN incorporated in house construction have the potential to reduce human exposure to malaria vectors, and thus parasite infection, in a rice irrigation scheme area of western Kenya. Ceiling modification is likely to be acceptable and is expected to be of greatest benefit when used in combination with other malaria control strategies.

Figure 1

Map of western Kenya showing location of study site in Ombeyi division, Nyando district.

Figure 2

Study houses and modifications. A) Typical village house with funnel shaped roof; B) Open eave between roof and wall; C) Closed eaves with papyrus mats ceiling; D) Sleeping room with mat ceiling, small ceiling ITN and bed net; E) Papyrus reeds in nearby swampy farm; F) Mats being sold at nearby Ahero market.

Figure 3

Mean number of mosquitoes entering houses, a) Mean density of female Anopheles gambiae s.l/house; b) Mean density of fully fed Anopheles gambiae s.l/house; c) Mean density of Anopheles funestus/house; d) Mean density of Culicines/house.

Figure 4

a) Twenty four hour average indoor temperatures, b) twenty four hour average relative humidity.