Indoor Residual Spraying

Indoor residual spraying (IRS) includes applying residual insecticides to indoor surfaces—walls, ceilings, and others—where it is likely to come in contact with and repel and/or kill adult mosquitoes. IRS also kills mosquitoes that rest on indoor surfaces after feeding on humans, preventing them from biting again and possibly transmitting malaria (WHO 2015). To a lesser degree, IRS also reduces the number of mosquitos that enter the household.

Correct insecticide application can reduce mosquito populations, curbing malaria transmission and the contribution of malaria to anemia. In target areas, IRS coverage of 80 percent or more leads to the maximum protection for the population and can interrupt transmission in the immediate area (WHO 2016). The World Health Organization (WHO) recommends 12 insecticides for IRS, classified as pyrethroids, organochlorines, organophosphates, and carbamates. Most are effective for three to six months after they are applied (WHO 2015). Based on cost, availability, hazard classification, insecticide resistance, and length of effect per spray, one or multiple types of insecticides may be appropriate. Despite previous concerns over safety, the organochlorine—known as DDT—is still recommended for IRS because it is effective for an extended period of time (6–12 months); provides the best protection in many malaria endemic areas; and, if used in very small quantities, is not toxic to humans or the environment. Apply DDT under strict controls and regulations (WHO 2011).

IRS spray campaigns take place at least yearly, with up to two rounds per year in areas with high transmission (WHO 2015).

Often, campaigns are not done at a national level, but are targeted to high-risk areas or those with increasing epidemics, and are often closely linked to external funding.

Insecticide resistance is a growing concern that threatens the success of malaria vector control in the future. With the widespread use of IRS in recent years, more than 60 countries with endemic malaria transmission have reported resistance to one or multiple insecticides, mainly pyrethroids, although DDT resistance is also prevalent; the poor, ad-hoc monitoring in most affected countries makes it more difficult to address the problem (Roll Back Malaria and WHO 2012). You may want to include any locally relevant information on insecticide resistance in your landscape analysis.

Measurement and data sources

Population-based surveys typically report IRS information bundled with coverage of insecticide-treated bed nets (ITN), and they assess the percentage of individuals, or the percentage of households, covered by both intervention (IRS and ITNs). These surveys typically assess the percentage of residences sprayed with IRS in the 12 months before the survey, the percentage of households with at least one ITN for every two people, and the percentage of individuals who slept under an ITN prior to the survey.

Surveys that collect information related to coverage of IRS include—

  • Demographic and Health Surveys
  • Malaria Indicator Surveys
  • Multiple Indicator Cluster Surveys
  • other research or evaluation activities.

Most implementers who administer IRS, often the government or specific organizations, will have detailed reports on spray campaigns, all the way to the household level and often with global positioning system data. Information is often collected by the number (or percentage) of rooms and structures sprayed and the number (or percentage) of people protected by the structures or houses sprayed. Households will also often be given record cards to track participation in each spray round, which can be used to facilitate recall. In addition to survey or administrative data regarding coverage, you can use the World Health Organization (WHO) cone bioassays or field-collected susceptible anophelines to measure the quality of spray application.

You may also be able to access information on coverage of IRS from the President’s Malaria Initiative, which includes country profiles and other data available on its website:

Methodological issues

  • Always consider seasonality when interpreting malaria-related data, especially for population-based surveys. Generally, survey reports indicate when the survey was conducted to ensure that data is interpreted appropriately. Malaria transmission rates—and, therefore, the implementation of prevention activities at the time of data collection—will affect comparability of these estimates over time and across studies. For instance, Demographic and Health Surveys often avoid the rainy season, given the difficulties inherent with data collection during this time, while Malaria Indicator Surveys are deliberately scheduled at this time to capture indicators during a season of high malaria transmission.
  • The effectiveness of IRS programs relies on the effectiveness of the specific insecticide against the local vector populations. In addition to routine data regarding coverage and quality of the program, annual susceptibility testing should take place to ensure the insecticide used is still effective. In addition, regular collection of entomological performance indicators is necessary to track program effectiveness and plan for future spray rounds.