Indoor residual spraying for malaria control in sub-Saharan Africa 1997 to 2017: an adjusted retrospective analysis

Abstract

Background: Indoor residual spraying (IRS) is a key tool for controlling and eliminating malaria by targeting vectors. To support the development of effective intervention strategies it is important to understand the impact of vector control tools on malaria incidence and on the spread of insecticide resistance. In 2006, the World Health Organization (WHO) stated that countries should report on coverage and impact of IRS, yet IRS coverage data are still sparse and unspecific. Here, the subnational coverage of IRS across sub-Saharan Africa for the four main insecticide classes from 1997 to 2017 were estimated. Methods: Data on IRS deployment were collated from a variety of sources, including the President's Malaria Initiative spray reports and National Malaria Control Programme reports, for all 46 malaria-endemic countries in sub-Saharan Africa from 1997 to 2017. The data were mapped to the applicable administrative divisions and the proportion of households sprayed for each of the four main insecticide classes; carbamates, organochlorines, organophosphates and pyrethroids was calculated. Results: The number of countries implementing IRS increased considerably over time, although the focal nature of deployment means the number of people protected remains low. From 1997 to 2010, DDT and pyrethroids were commonly used, then partly replaced by carbamates from 2011 and by organophosphates from 2013. IRS deployment since the publication of resistance management guidelines has typically avoided overlap between pyrethroid IRS and ITN use. However, annual rotations of insecticide classes with differing modes of action are not routinely used. Conclusion: This study highlights the gaps between policy and practice, emphasizing the continuing potential of IRS to drive resistance. The data presented here can improve studies on the impact of IRS on malaria incidence and help to guide future malaria control efforts.

Mapping trends in insecticide resistance phenotypes in African malaria vectors

“…Once a new variant has arisen, increases in its frequency are driven by selection pressures that differ from place to place, for example, through the spatially varying coverage of pyrethroid use in public health and agriculture [4,22,40]. The spread of these genetic variants to new locations is driven by gene flow that also varies with location [30,[41][42][43][44][45].…”

“…Until recently, pyrethroids were the only class approved for use in LLINs, and recently launched new generation nets still use pyrethroids in combination with either an insect growth regulator, a pyrrole, or a synergist that inhibits the primary metabolic mechanism of pyrethroid resistance within mosquitoes [2,3]. A wider range of options is available for indoor residual spraying (IRS), but pyrethroids are less expensive than many alternatives and are still used for IRS in malaria-endemic sub-Saharan African countries [4,5].…”

“…Sources of insecticides in the environment include the application of insecticide-based vector control interventions for public health-such as LLINs and IRS-and the application of agricultural insecticides, which include the same insecticide classes as those used in vector control [21]. LLIN coverage increased markedly across Africa from 2005 in response the global Roll Back Malaria initiative [22,23], while IRS coverage has been restricted to much smaller areas [4]. Early insecticide-treated nets (ITNs) used permethrin or deltamethrin, whereas α-cypermethrin is now the most commonly used pyrethroid in LLINs.…”

“…Over the last 20 years, deltamethrin, λ-cyhalothrin, and DDT have been used for IRS, with α-cypermethrin first used in mass campaigns in 2003. Since 2015, deltamethrin has been the only pyrethroid reported to be used in mass IRS campaigns along with DDT and other non-pyrethroid insecticides [4]. Several studies have demonstrated a local increase in insecticide resistance in field mosquito populations following the implementation of LLINs, IRS, or both [19,20,[24][25][26] although in other locations evidence of higher resistance after the introduction of these interventions was not found [24,27].…”

Household Predictors of Incidence of Malaria in Northern Uganda: Its Implication for Future Malaria Control

“…In this present study households without bed nets controlled malaria by applying IRS in combination with other preventive measures such as closing doors (with the hope of keeping mosquitos at bay), draining stagnant water pool where mosquitos lay their eggs, trimming mosquito covers around homestead (grass) and/or receiving treatment after malaria incidences. Generally, there is overall appreciation for taking measures against the spread of malaria in the study area using IRS with 84% of household using IRS to control malaria vectors which is higher than most countries in Africa [40]. This result suggests that households in the study area have achieved the successful campaign threshold of 85% of using IRS as recommended by WHO [2].…”