Indoor Residual Spraying (IRS) is a standardised and well-established control method for mosquitoes. It has been used widely in Asia, the Pacific and Latin America, while in Africa its use has been more limited to the margins of malaria distribution in southern Africa and to epidemic-prone countries often at higher altitudes. WHO has recently proposed extending its range in Africa.
IRS can kill a mosquito any time it enters a house for a blood meal, which it typically does every 2-3 days, so that few will survive the approximately 12 days that are required for malaria parasites to complete part of their life cycle in the vector mosquito, if all the houses they visit are properly sprayed. In practice, the effectiveness of IRS depends on adherence to application procedure, efficacy of the insecticide, public acceptance of spraying, availability of well-maintained equipment, adequately trained spraying personnel, efficient supervision and strong financial support. The size of the operational area depends on local circumstances and is influenced by the distribution of malaria and malaria vectors, the distance from important breeding sites, the flight range of the vectors and demographic features.
Generally all internal walls and ceilings of the building are treated. Residual effects depend on porosity of the surface (shorter on mud walls) and exposure to sunlight. Four classes of chemical insecticides-organochlorines, organophosphates, carbamates and pyrethroids-are still the mainstay of vector control programmes. Use of pyrethroid insecticides has, however, increased, and that of the organochlorines and some of the more toxic organophosphate compounds has decreased in recent years. The continued use of DDT for disease vector control is conditionally approved under the Stockholm Convention on Persistent Organic Pollutants, in accordance with WHO recommendations and guidelines, and when locally safe, effective and affordable alternatives are not available. Selection depends on cost, efficacy and availability. Water-dispersible powders are generally used, applied with a hand operated pressurised sprayer. Frequency of spraying will vary from once to twice yearly according to seasonality of vector breeding, transmission of malaria and residual effect of insecticide.
The Malaria Consortium is presently conducting an evaluation of IRS using new formulations of insecticide in Uganda. This is a small scale study examining the residual life of lambdacyhalothrin CS (Lambda CS) in two doses, 25 mg/m2 and 50 mg/m2 on 60 houses of three different wall types - mud, brick or plaster and paint - supported by Syngenta Professional Products. Lambda CS is a pyrethroid insectide with very low toxicity to humans. The residual life of the insecticide will be monitored over a 12 month period using light traps to look at mosquito numbers entering houses and by contact bioassays on the walls of the house.