Indoor Residual Spraying

Indoor residual spraying (IRS) is one of the two cornerstone chemical vector-control interventions in global public health and medical entomology (alongside insecticide-treated nets), consisting of the uniform application of a long-acting, residual insecticide formulation (wettable powder, suspension concentrate, or microencapsulated suspension) onto the interior walls, eaves, and ceilings of human dwellings and domestic animal shelters. Unlike outdoor space-spraying (thermal fogging or ultra-low-volume aerial misting), which targets flying mosquitoes transiently and leaves no lasting deposit, indoor residual spraying exploits a specific behavioral vulnerability of major disease vectors—endophily (the habit of blood-fed female mosquitoes, sandflies, and triatomine bugs to rest motionless for 12 to 48 hours on dark interior wall surfaces while digesting a blood meal and maturing their eggs via anautogeny). First demonstrated with short-acting pyrethrum sprays in South Africa and India by Botha De Meillon (1936) and Paul F. Russell (1942), revolutionized by the discovery of the persistent organochlorine DDT by Swiss chemist Paul Hermann Müller in 1939 (Nobel Prize in Physiology or Medicine 1948), and mathematically theorized by British malariologist George Macdonald in 1952–1957, IRS served as the primary operational engine of the WHO Global Malaria Eradication Programme (1955–1969)—eliminating malaria from 37 countries across Europe, North America, the Caribbean, and parts of Asia—and today protects over 100 million people annually against malaria (Anopheles gambiae, An. funestus), visceral leishmaniasis (Phlebotomus argentipes), and Chagas disease (Triatoma infestans) using next-generation Next-Gen IRS (NGRS) chemistries designed to overcome pyrethroid resistance (Macdonald, 1957; Pluess et al., 2010; Bhatt et al., 2015; Oxborough, 2016).

Why is killing an adult female Anopheles mosquito after she has already bitten a human inside a house and rested on the bedroom wall so much more effective at collapsing malaria transmission than draining 90% of larval breeding puddles outdoors?

In 1952–1957, George Macdonald extended Sir Ronald Ross's malaria equations into the Ross–Macdonald model (later formulated as Vectorial Capacity, V, by C. Garrett-Jones in 1964), which gives the expected number of potentially infectious bites that will arise per day from all the mosquitoes biting a single infectious human host on a single day:

where:

• m is the mosquito density per human host (m = M / H, governed linearly by larval breeding habitats).
• a is the daily human-biting rate per female mosquito (the inverse of the gonotrophic cycle duration, ~0.33 to 0.5 day⁻¹, multiplied by the human blood index; a appears squared because a mosquito must bite once to acquire Plasmodium gametocytes and bite a second time to transmit sporozoites).
• p is the probability of daily adult mosquito survival (typically p ≈ 0.85 to 0.90 per day in untreated tropical villages, corresponding to a mean adult lifespan of 1 / [−ln p] ≈ 6.2 to 9.5 days).
• n is the extrinsic incubation period (EIP) of the parasite inside the mosquito vector—the number of days required for Plasmodium falciparum ookinetes to traverse the peritrophic matrix, mature into oocysts on the midgut wall, and migrate as infectious sporozoites into the salivary glands (typically n = 10 to 14 days at 25–28 °C).

Because the fraction of mosquitoes surviving long enough to become infectious scales exponentially as pⁿ divided by (−ln p), daily adult survival p is by far the most sensitive parameter in the entire malaria transmission cycle (Macdonald, 1957; Smith et al., 2012):

• In an Unsprayed Village (p = 0.90, n = 12 days): A female mosquito has a p¹² = (0.90)¹² = 28.2% chance of surviving the 12-day extrinsic incubation period, and her expected infectious life expectancy after day 12 is p¹² / (−ln p) = 0.282 / 0.105 = 2.68 infectious days.
• In an IRS-Sprayed Village (p Reduced by Just 22%, from 0.90 to 0.70): Because she must rest on a treated interior wall after each blood meal every 2–3 days, reducing her daily survival probability p from 0.90 to 0.70 slashes her probability of surviving the 12-day EIP by 20-fold, down to p¹² = (0.70)¹² = 1.38%, and reduces her expected infectious lifespan (p¹² / [−ln p]) from 2.68 days to 0.039 days—a 69-fold (98.5%) collapse in Vectorial Capacity (V) without needing to kill a single mosquito larva.

Indoor residual spraying is maximally effective when the target vector exhibits endophagy (biting humans indoors) and endophily (resting indoors on walls, ceilings, or undersides of roofs after feeding). When a newly blood-fed female Anopheles gambiae s.s. or Anopheles funestus (which takes up to 2 to 3 times her unfed body weight in blood) flies heavily from the sleeping host to the nearest dark vertical wall within 1 to 2 meters, she absorbs a lethal dose of insecticide through her tarsal pads (feet) while resting to diurese excess plasma water via her Malpighian tubules.

Across insecticide classes, tarsal contact on a treated wall produces three distinct physiological and behavioral effects (Grieco et al., 2007):

1. Direct Contact Toxicity (Mortality): The insect absorbs a lethal dose through the tarsal cuticle before leaving the wall and dies within 30 minutes to 72 hours.
2. Contact Irritancy (Excito-Repellency): Upon touching a DDT- or α-cypermethrin-treated wall, peripheral sensory neurons in the mosquito's tarsi fire rapidly, agitating her to take flight and exit the house prematurely through open eaves or windows before absorbing a lethal dose (reducing indoor biting and indoor resting, though potentially diverting bites outdoors).
3. Non-Contact Spatial Deterrency: Volatile molecules emitted by DDT deposits deter host-seeking females from entering treated huts altogether.

To achieve a community-wide "mass effect" that suppresses vector longevity across an entire district, IRS campaigns must spray at least 80% of all targeted structures prior to the onset of the seasonal rains, using compression hand-pumps (such as the Hudson X-Pert sprayer fitted with an 8002E flat-fan nozzle held 45 cm from the wall) to deposit a precise target dose on mud, brick, cement, wood, or thatch surfaces.

Because widespread reliance on a single cheap insecticide class selects rapidly for target-site mutations (kdr L1014F/S in the para voltage-gated sodium channel; Ace-1 G119S in acetylcholinesterase) and metabolic cytochrome P450 monooxygenase upregulation (CYP6P3, CYP6M2), the World Health Organization (WHO) prequalifies five distinct biochemical classes for IRS rotation and mosaic deployment (Oxborough, 2016; Sherrard-Smith et al., 2018):

1. The Global Malaria Eradication Programme (GMEP, 1955–1969) and the Sardinia / Sri Lanka Classics: Armed with DDT IRS and George Macdonald's mathematical threshold theory, the WHO launched the GMEP in 1955. In Sri Lanka (Ceylon), national DDT spraying cut annual malaria cases from 2.8 million in 1946 to just 17 cases in 1963 (before premature withdrawal of spraying triggered a 1968 resurgence of 500,000 cases, followed by ultimate malaria elimination in 2016). Globally, IRS eliminated malaria from southern Europe, the southern United States, Taiwan, and the Caribbean, though sub-Saharan Africa was excluded from the 1955 campaign due to extreme baseline entomological inoculation rates (EIR > 100–500 infectious bites per person per year).
2. 21st-Century Resurgence, Next-Gen IRS, and Chagas / Visceral Leishmaniasis Elimination: Between 2000 and 2015, the scale-up of insecticide-treated nets and IRS averted an estimated 663 million clinical cases of malaria in sub-Saharan Africa (with IRS contributing ~10% of total cases averted while protecting high-burden epidemic fringes and urban/mining zones; Bhatt et al., 2015). Beyond malaria, pyrethroid and organophosphate IRS of domestic adobe/mud walls in the Southern Cone Initiative of South America (launched 1991) interrupted domestic transmission of Chagas disease (Trypanosoma cruzi) by eliminating domiciliated Triatoma infestans kissing bugs across Uruguay, Chile, Brazil, and Paraguay, while alpha-cypermethrin IRS of village houses across Bihar, India, and Bangladesh has driven visceral leishmaniasis (kala-azar, transmitted by endophilic Phlebotomus argentipes sandflies) to the brink of regional elimination (<1 case per 10,000 population).
3. Behavioral Resistance and Residual Outdoor Transmission: In regions subjected to decades of intensive IRS and bed-net coverage, vector populations undergo species replacement and behavioral adaptation: in East and Southern Africa, strictly anthropophilic, indoor-resting Anopheles gambiae s.s. has frequently been replaced by its sibling species Anopheles arabiensis, which feeds heavily outdoors in the early evening (exophagy), feeds opportunistically on cattle (zoophagy), and exits houses immediately after feeding to rest outdoors in vegetation or granaries (exophily)—creating "residual malaria transmission" that requires complementary larval source management, attractive targeted sugar baits (ATSBs), or systemic livestock/human endectocides (ivermectin) (Killeen, 2014).

In 2020 and 2021, multi-country indoor residual spraying programs supported by the Next Generation IRS (NGRS) initiative transitioned away from pyrethroids and short-acting carbamates toward third-generation neonicotinoid formulations—including clothianidin wettable granules (SumiShield 50WG) and clothianidin–deltamethrin mixtures (Fludora Fusion)—while experimental-hut evaluations demonstrated prolonged residual control of pyrethroid-resistant Anopheles gambiae with the novel meta-diamide insecticide broflanilide (Ngufor et al., 2021).

In 2022, community-scale cluster-randomized and non-inferiority trials in Benin and Tanzania confirmed that wettable-powder broflanilide (VECTRON T500, targeting a distinct allosteric site on the insect GABA-gated chloride channel) achieved non-inferior and more durable mortality of wild pyrethroid- and carbamate-resistant Anopheles mosquitoes across mud and cement household walls for 9 to 12 months compared with microencapsulated pirimiphos-methyl (Ngufor et al., 2022).

In 2023 and 2024, the World Health Organization prequalified broflanilide (VECTRON T500) as a new IRAC Group 30 mode-of-action class for public health vector control, enabling rotational deployment alongside neonicotinoids, organophosphates, and pyrroles to forestall multi-insecticide resistance across endemic African malaria districts (Syme et al., 2024).

1. Macdonald, G. (1957). The Epidemiology and Control of Malaria. Oxford University Press, London.
2. Garrett-Jones, C. (1964). Prognosis for interruption of malaria transmission through assessment of the mosquito's vectorial capacity. Nature, 204(4964), 1173–1175. https://doi.org/10.1038/2041173a0
3. Grieco, J. P., Achee, N. L., Chareonviriyaphap, T., Suwonkerd, W., Chauhan, K., Sardelis, M. R., & Roberts, D. R. (2007). A new classification system for the actions of IRS chemicals traditionally used for malaria control. PLOS ONE, 2(8), e716. https://doi.org/10.1371/journal.pone.0000716
4. Pluess, B., Tanser, F. C., Lengeler, C., & Sharp, B. L. (2010). Indoor residual spraying for preventing malaria. Cochrane Database of Systematic Reviews, 2010(4), CD006657. https://doi.org/10.1002/14651858.CD006657.pub2
5. Killeen, G. F. (2014). Characterizing, controlling and eliminating residual malaria transmission. Malaria Journal, 13, 330. https://doi.org/10.1186/1475-2875-13-330
6. Bhatt, S., Weiss, D. J., Cameron, E., Bisanzio, D., Mappin, B., Dalrymple, U., Battle, K., Moyes, C. L., Henry, A., Eckhoff, P. A., Wenger, E. A., Briët, O., Penny, M. A., Smith, T. A., Bennett, A., Yukich, J., Eisele, T. P., Griffin, J. T., Fergus, C. A., … Gething, P. W. (2015). The effect of malaria control on Plasmodium falciparum in Africa between 2000 and 2015. Nature, 526(7572), 207–211. https://doi.org/10.1038/nature15535
7. Oxborough, R. M. (2016). Trends in US President's Malaria Initiative-funded indoor residual spraying coverage and insecticide choice in sub-Saharan Africa (2008–2015): Urgent need for affordable, long-lasting insecticides. Malaria Journal, 15, 146. https://doi.org/10.1186/s12936-016-1201-1
8. Ngufor, C., Govoetchan, R., Fongnikin, A., Vigninou, E., Syme, T., Akogbeto, M., & Rowland, M. (2021). Efficacy of broflanilide (VECTRON T500), a new meta-diamide insecticide, for indoor residual spraying against pyrethroid-resistant malaria vectors. Scientific Reports, 11(1), 7976. https://doi.org/10.1038/s41598-021-86935-3
9. Ngufor, C., Fongnikin, A., Todjinou, D., Odjo, A., Fagbohoun, J., Cochennec, N., ... & Rowland, M. (2022). Broflanilide (VECTRON T500) is non-inferior to pirimiphos-methyl (Actellic 300CS) for indoor residual spraying against pyrethroid-resistant malaria vectors in community trials in Benin. Malaria Journal, 21(1), 320. https://doi.org/10.1186/s12936-022-04340-3
10. Syme, T., Gbegbo, M., Obuobi, D., Fongnikin, A., Agbevo, A., Todjinou, D., & Ngufor, C. (2024). Rotational and mosaic deployment of next-generation indoor residual spraying insecticides against resistant Anopheles vectors. Parasites & Vectors, 17(1), 112. https://doi.org/10.1186/s13071-024-06198-x