mass effect has been well documented in a number of IRS trials. Specific examples include three demonstration projects in Africa: an observational study in the Pare-Taveta Malaria Scheme in Tanzania where dieldrin reduced malaria transmission from an annual EIR of 10-50 to <1 (Pringle, 1969; Bradley, 1991); a trial in Kisumu, Kenya, where IRS with fenitrothion reduced malaria transmission by 96 percent compared to baseline over 2 years (Payne et al., 1976); and the Garki project in northern Nigeria where IRS with propoxur also substantially decreased transmission and improved infant and child mortality (Molineaux, 1985) (Box 8-1).

Over the last 30 years, DDT-based IRS has declined, in part, because of DDT resistance among malaria vectors. A lack of sustained government support and financing as well as general disapproval of DDT by the international community also have contributed to IRS’s restricted use in sub-Saharan Africa. In parts of Asia, Latin America, and southern and northeastern Africa where IRS is still used, it is typically organized and paid for by governments (for example, government-funded DDT house spraying was recently reinstated in KwaZulu Natal, South Africa, and the Madagascar highlands because of rising prevalence of pyrethroid-resistant A. funestus vectors and human malaria cases [Hargreaves et al., 2000]). IRS also is used in urban epidemics and refugee camps worldwide, and sometimes provided by foreign and multinational companies for the protection of employees and local communities in malaria-endemic areas (Sharp et al., 2002a).

The implementation of IRS is not trivial, and, if incorrectly performed, may be quite ineffective (Shiff, 2002). Houses and animal shelters within a target area should receive IRS before the start of the transmission season and at regular intervals thereafter. Before application of insecticide, all furniture, hanging clothing, cooking utensils, food and other items should be removed from human habitations, and left covered outside. Emulsions or solutions of insecticide are often preferred over suspensions of water-dispersible powders, which leave whitish deposits. Mud and porous plaster walls retain less IRS insecticide than wood or non-absorptive surfaces.

ITNs and IRS both require user cooperation, albeit in different ways. Current-generation ITNs must be properly installed, faithfully used, and retreated with insecticide every 6 to 12 months in order to maintain extended efficacy. IRS, in comparison, is passive but intrusive. Some residents of endemic areas forfeit IRS benefits by painting or replastering sprayed walls, while other families evade IRS altogether by locking their houses during the spraying round (Mnzava et al., 2001; Goodman et al., 2001). In addition, some housing or shelter materials such as plastic sheeting are not amenable to residual spraying.