Remarks WHO recommends that products from the insecticide classes indicated below be used for IRS. Selection of the product should be made from the WHO list of prequalified products, and with due consideration of the insecticide susceptibility of the local malaria vector(s). IRS is considered to be 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 Surfaces (indoors and outdoors) could potentially be treated with residual insecticides or other residual active ingredients against mosquitoes in ways other than spraying, for example by painting. The systematic review aimed to gather evidence on alternative methods of applying insecticides and outdoor treatments. However, no studies were identified that met the inclusion criteria. Furthermore, surfaces may be fully or partially treated (such as treating the lower or upper sections of walls or specific rooms). The latter approach may be more cost-effective. However, there is currently insufficient evidence to determine whether partial surface treatments are as effective or as cost-effective as full surface treatments. The practical guidance provided here, therefore, refers to the implementation of IRS treating all indoor surfaces of a structure. IRS is considered to be 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. IRS may not be suitable for some structures, such as open-sided structures, but, in general, insecticides can be applied to several different wall types (e.g. cement, painted surfaces, brick, wood, mud). However, it is important to consider whether the surface material compromises the residual nature of the insecticide (e.g. some plastic sheeting materials). The longevity of the insecticide also varies with the insecticide used and its formulation. Residual efficacy, i.e. the insecticide’s ability to still kill mosquitoes that are exposed to sprayed surfaces, needs to continue for at least the duration of the malaria transmission season following the application of the insecticide to the substrate. If treatment of certain surfaces or use of a particular insecticide and/or formulation reduces its residual life, additional spray rounds may be needed to provide population protection throughout the transmission season(s). More information is provided in the WHO publication Operational manual on indoor residual spraying: control of vectors of malaria, Aedes-borne diseases, Chagas disease, leishmaniases and lymphatic filariasis [66]. Insecticide formulations for IRS currently recommended by WHO fall into fall into several major insecticide classes with different modes of action on the vector species. The insecticide classes with products that have been prequalified, as listed below. Acetylcholinesterase inhibitors
• Organophosphates: pirimiphos-methyl
• Carbamates: bendiocarb Nicotinic acetylcholine receptor competitive modulators
• Neonicotinoids: clothianidin GABA-gated chloride channel allosteric modulators
• Meta-diamides: broflanilide
• Isoxazolines: isocycloseram Uncouplers of oxidative phosphorylation via disruption of the proton gradient
• Pyrroles: chlorfenapyr Sodium channel modulators
• Pyrethroids: alphacypermethrin, deltamethrin, lambda-cyhalothrin, etofenprox, bifenthrin
• Organochlorines (e.g. DDT): no prequalified products available While the organochlorine DDT has a long history of use for IRS, there are no organochlorine IRS formulations prequalified. 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 insecticide 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 [79]. 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. However, the expansion of products available for IRS and overall innovation in vector control interventions have provided multiple viable options as alternatives to using DDT in IRS. 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 the use of alternative insecticides or other vector control methods. With the availability of multiple classes of insecticides for IRS, there are very few cases in which the use of DDT may still be warranted. Any 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 [66]. 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. When selecting insecticides for IRS, it is important to investigate the resistance profile of the local vectors to select insecticides that are effective against the local dominant vectors. Continuous use of the same product in the same area for multiple seasons is not recommended, as this may select for resistance in mosquitoes. Switching to other insecticides to which mosquitoes are susceptible should therefore be planned proactively. Furthermore, in deciding which products and formulations to procure, residual efficacy must be considered. Insecticides should remain efficacious throughout the transmission season after application and must do so when applied to a variety of surfaces (cement, mud or wood) [80]. 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 because 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 and may cause decolourization of painted 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 [81]. It is therefore important to develop information, education and communication (IEC) strategies to keep the community informed and engage with communities to ensure full support and cooperation. IRS is generally conducted campaign-style across a large geographical area or higher risk area prior to the beginning of a malaria transmission season (i.e. proactive spraying). However, IRS can be deployed in a much smaller, more targeted way in the likely location of infection of an index case and its neighbours. This is termed reactive IRS; further information and guidance is provided under the “Interventions in the final phase of elimination and prevention of re-establishment” section of these Guidelines. When IRS is deployed proactively in wider areas of ongoing malaria transmission, it is important to maintain optimal coverage (see Section 4.1.1 interventions recommended for large scale deployment and the glossary for further details on how optimal coverage is determined). Following application of the insecticide(s), it is important to determine the quality of the application and to subsequently monitor the residual activity by using wall cone bioassays. It is also important to evaluate the impact of IRS through entomological surveillance activities and assess any impact on the environment. Further detailed information is provided in the WHO publication Operational manual on indoor residual spraying: control of vectors of malaria, Aedes-borne diseases, Chagas disease, leishmaniases and lymphatic filariasis [66]. This manual is designed to assist malaria programme managers, entomologists and public health officers in designing, implementing, and monitoring and evaluating high-quality IRS programmes.