The World Health Organization (WHO) launched the Roll Back Malaria Initiative in 1998, a global partnership with the goal to halve the burden of malaria. Since 2000, worldwide, the number of annual malaria infections has decreased by 26 % (173–128 million) with a concomitant 47 % reduction in mortality . To continue this progress, proven interventions, such as rapid diagnostic testing (RDT), artemisinin-based combination therapy, intermittent preventive therapy for pregnant women, long-lasting insecticidal-treated nets (LLINs), and indoor residual spraying (IRS) are recommended [2, 3]. Universal coverage with LLINs is defined as one net per two people, and is recommended by the WHO for all people at risk of malaria . To complement LLIN use, IRS has been scaled up in many African countries with the aim of supporting malaria control or elimination, depending on the underlying transmission. In 2014, a total of 90 countries, 42 in the African region, recommended IRS for vector control as a primary intervention for malaria .
IRS operates by either repelling mosquitoes from entering sprayed houses or by killing female mosquitoes that are resting inside houses after having taken a blood meal [5, 6]. IRS is most effective for endophilic and endophagic vectors, with maximum killing potency achieved when malaria vectors rest on IRS-treated inside walls [6, 7]. A ‘mass effect’ of IRS is thought to be obtained with high, e.g., >85 %, coverage of structures in a sprayed area . Scientific evidence supporting this threshold is, however, limited and the combined impact in areas of high LLIN coverage is unclear from the few rigorous studies that have been conducted [8–10]. Furthermore, impact may be modified by transmission intensity and length of the malaria transmission season . The WHO currently encourages full coverage of LLINs plus supplemental IRS, but more evidence is needed .
Historically, IRS has generally been implemented at district level or other similar, large-scale geopolitical unit. This approach is largely due to limited availability of data on the exact geographic distribution of households and IRS coverage at sub-district levels. This status quo approach presumably developed as most countries adopted a ‘blanket spraying’ strategy to target all eligible structures. The considerable challenges of delivering IRS, however, mean achievement of 100 % coverage is often unrealistic due to logistics, refusals, absent residents, and other factors, such that 85 % coverage is recommended by the WHO .
Increasing levels of insecticide resistance have forced IRS programmes to adopt insecticides costing more than triple the price of pyrethroids. For example, pyrethroid lambda-cyhalothrin costs ~$2–$3 per unit (sachet/bottle equivalent), whereas carbamate bendiocarb costs ~$12 per unit, while pirimiphos-methyl, a long-lasting organophosphate, costs ~$23 per unit. With one unit able to cover ~ three houses depending on size, the need to target resources to where they will have maximum impact becomes increasingly necessary in resource-constrained settings . Considering that malaria transmission is highly heterogeneous within districts and is often focalized into hotspots (<1km2) [14–16], the strategy of blanket spraying in areas of universal LLIN coverage may be unnecessary and even cost-ineffective to achieve maximum gains in the reduction of malaria transmission, particularly in resource-constrained environments . Unfortunately, limited policy and little data exist to inform the best strategies for targeted IRS to achieve maximum reduction in malaria transmission, particularly in areas of documented pyrethroid resistance and universal LLIN coverage. At this time, the WHO recommends only focal IRS in elimination settings to target remaining clusters or outbreaks of transmission . However, sub-district targeting of non-pyrethroid IRS in low- to medium-transmission areas with universal LLIN coverage might be considered to mitigate pyrethroid resistance and drive down transmission in ‘hot spots’ .
Tools sufficient to manage targeted IRS campaigns must address three issues. First, the spatial location of all structures in a district must be mapped and the structures enumerated. Second, a robust targeting strategy must be developed to determine the size of the geographical units for which targeting is feasible or desirable to achieve the greatest impact with limited resources. Third, spray operators in the field must be guided to deliver IRS to targeted structures and record spray activities structure-by-structure to determine target area coverage. Other papers outline the use of freely available satellite imagery to determine the spatial location of all eligible structures , and forthcoming work will outline the development of a tool to guide spray operators in the field . This paper focuses on the second aspect of targeted IRS campaigns: the need to develop robust targeting methodologies for IRS. Critical issues that remain to ensure effective and efficient IRS planning and implementation are outlined.