Remarks WHO recommends that WHO-prequalified insecticidal products be selected for IRS use and that these be selected based on the insecticide susceptibility of the local malaria vector(s). IRS is considered an appropriate intervention where:

• the majority of the vector population feeds and rests indoors;
• people mainly sleep indoors at night;
• the malaria transmission pattern is such that the population can be protected by one or two rounds of IRS per year; and
• the majority of structures are suitable for spraying.

Practical Info IRS is considered an appropriate intervention where:

• the majority of the vector population tends to feed and rest indoors;
• people mainly sleep indoors at night;
• the malaria transmission pattern is such that the population can be protected by one or two rounds of IRS per year;
• the majority of structures are suitable for spraying. When selecting insecticides to be used for IRS, it is important to investigate the resistance profile of the local vectors in order to select insecticides to which the vectors are susceptible. Insecticide formulations currently used for IRS [26] fall into five major insecticide classes with three modes of action, based on their primary target site in the vector. WHOprequalified products have been assessed for their safety, quality and entomological efficacy, which includes evaluation of their mortality effect on mosquitoes when applied to a range of interior surfaces of dwellings found in malariaendemic areas. Sodium channel modulators
• Pyrethroids: alphacypermethrin, deltamethrin, lambdacyhalothrin, etofenprox, bifenthrin
• Organochlorines (e.g. DDT): No prequalified product available Acetylcholinesterase inhibitors
• Organophosphates: malathion, fenitrothion, pirimiphosmethyl
• Carbamates: bendiocarb, propoxur Nicotinic acetylcholine receptor competitive modulators
• Neonicotinoids: clothianidin IRS products using four of these insecticide classes have been prequalified by WHO; as of August 2020, there were no organochlorine IRS formulations prequalified [26], including DDT. This means that no DDT product has been assessed by WHO for its efficacy, safety and quality for vector control, and no inspection of manufacturing sites has been conducted. Unlike the other four classes covered by WHO’s recommendation for IRS, DDT has been classified as a persistent organic pollutant. As such, its production and use are strictly restricted by an international agreement known as the Stockholm Convention on Persistent Organic Pollutants [64]. The Convention’s objective is to protect both human health and the environment from persistent organic pollutants. When the Stockholm Convention was established in 2004, it provided an exemption for the production and use of DDT for disease vector control, mainly because of the absence of equally effective and efficient alternatives at the time. The recent expansion of products available for IRS and overall expansion of vector control interventions has provided additional options. WHO actively supports the promotion of chemical safety and, together with the United Nations Environment Programme, shares a common commitment to the global goal of reducing and eventually eliminating the use of DDT, while minimizing the burden of vector-borne diseases. DDT use for malaria vector control has declined over the years and WHO supports continuation of this trend. In some areas, the use of DDT may be warranted. The decision to use DDT for malaria vector control needs to be based on a detailed analysis that considers all other potential options for vector control and provides clear reasoning for choosing DDT over the other options. WHO considers DDT to be a last resort, not a first choice. If DDT is selected, it should be used under strict control measures and only for the intended purpose. Its use requires that the conditions set by the Stockholm Convention be met. Effective use and safe storage of DDT rely on compliance with well-established and well-enforced rules and regulations in accordance with national guidelines and following WHO technical guidance provided in the WHO Operational manual for IRS for malaria transmission, control and elimination [65]. Where DDT is deployed, it is essential for adequate resources and technical support to be in place to ensure the sound management of this persistent organic pollutant. Countries that are using DDT for malaria vector control need to regularly (at least once every two years) reassess whether there is a justified continued need for DDT. The outcome of such assessment should be reported to the WHO Global Malaria Programme and to the Secretariat of the Stockholm Convention as part of the formal reporting process [64]. When selecting products and formulations, residual efficacy needs to continue for at least three months after the application of the insecticide to the substrate (usually cement, mud or wood) [66]. Insecticides are available in various formulations to increase their longevity on different surfaces. Community acceptance of IRS is critical to the programme’s success, particularly as it requires householders to grant permission for spray teams to enter their house. It also involves disruption to the household, requiring householders to remove personal items from their house prior to spraying. Furthermore, some insecticide formulations leave unsightly residue on sprayed surfaces. Repeated, frequent spraying of houses over extended periods can lead to refusal by householders. Reduced acceptance has been an impediment to effective IRS implementation in various parts of the world [67]. It is therefore important to develop information, education and communication (IEC) strategies to keep the community informed and to ensure full support and cooperation. In areas with ongoing malaria transmission, optimal coverage of IRS should be maintained. Implementation of the intervention should take place prior to the onset of the peak transmission season. Following application of the insecticide(s), it is important to monitor the residual activity. The WHO Operational manual for IRS for malaria transmission, control and elimination [65] aims to assist malaria programme managers, entomologists and public health officers in designing, implementing and sustaining high-quality IRS programmes.