An observational analysis of the impact of indoor residual spraying with non-pyrethroid insecticides on the incidence of malaria in Ségou Region, Mali: 2012-2015

Abstract

Background: Ségou Region in Central Mali is an area of high malaria burden with seasonal transmission, high access to and use of long-lasting insecticidal nets (LLINs), and resistance to pyrethroids and DDT well documented in Anopheles gambiae s.l. (the principal vector of malaria in Mali). Ségou has recently received indoor residual spraying (IRS) supported by Mali's collaboration with the US President's Malaria Initiative/Africa Indoor Residual Spraying programme. From 2012 to 2015, two different non-pyrethroid insecticides: bendiocarb in 2012 and 2013 and pirimiphos-methyl in 2014 and 2015, were used for IRS in two districts. This report summarizes the results of observational analyses carried out to assess the impact of these IRS campaigns on malaria incidence rates reported through local and district health systems before and after spraying.

Methods: A series of retrospective time series analyses were performed on 1,382,202 rapid diagnostic test-confirmed cases of malaria reported by district routine health systems in Ségou Region from January 2012 to January 2016. Malaria testing, treatment, surveillance and reporting activities remained consistent across districts and years during the study period, as did LLIN access and use estimates as well as An. gambiae s.l. insecticide resistance patterns. Districts were stratified by IRS implementation status and all-age monthly incidence rates were calculated and compared across strata from 2012 to 2014. In 2015 a regional but variable scale-up of seasonal malaria chemoprevention complicated the region-wide analysis; however IRS operations were suspended in Bla District that year so a difference in differences approach was used to compare 2014 to 2015 changes in malaria incidence at the health facility level in children under 5-years-old from Bla relative to changes observed in Barouéli, where IRS operations were consistent.

Results: During 2012-2014, rapid reductions in malaria incidence were observed during the 6 months following each IRS campaign, though most of the reduction in cases (70% of the total) was concentrated in the first 2 months after each campaign was completed. Compared to non-IRS districts, in which normal seasonal patterns of malaria incidence were observed, an estimated 286,745 total fewer cases of all-age malaria were observed in IRS districts. The total cost of IRS in Ségou was around 9.68 million USD, or roughly 33.75 USD per case averted. Further analysis suggests that the timing of the 2012-2014 IRS campaigns (spraying in July and August) was well positioned to maximize public health impact. Suspension of IRS in Bla District after the 2014 campaign resulted in a 70% increase in under-5-years-old malaria incidence rates from 2014 to 2015, significantly greater (p = 0.0003) than the change reported from Barouéli District, where incidence rates remained the same.

Conclusions: From 2012 to 2015, the annual IRS campaigns in Ségou are associated with several hundred thousand fewer cases of malaria. This work supports the growing evidence that shows that IRS with non-pyrethroid insecticides is a wise public health investment in areas with documented pyrethroid resistance, high rates of LLIN coverage, and where house structures and population densities are appropriate. Additionally, this work highlights the utility of quality-assured and validated routine surveillance and well defined observational analyses to rapidly assess the impact of malaria control interventions in operational settings, helping to empower evidence-based decision making and to further grow the evidence base needed to better understand when and where to utilize new vector control tools as they become available.

Keywords: Indoor residual spraying; Malaria incidence; Mali; Next generation IRS; Observational analysis.

Fig. 1

Study site. a The location of Mali in West Africa (left), with Ségou Region highlighted. b The locations of the community health facilities in Ségou that reported malaria RDT results during the months analysed here (Jan 2012 to Jan 2016). The IRS districts of Barouéli and Bla are shown in dark and light green, respectively; non-IRS comparator districts are shown in blue

Fig. 2

Reduction in malaria incidence following seasonal IRS campaigns in Ségou Region, Mali. Epidemiological curves showing the monthly incidence of RDT+ confirmed malaria cases in Ségou, stratified by district IRS status (inlayed maps). The area of the tan curve illustrates the difference between the incidence rates observed in the IRS districts (green) and the non-IRS districts (blue). The duration of each year’s IRS campaign is also illustrated

Fig. 3

Model outputs showing potential impact of having changed the IRS campaign start date. The model estimates what the average impact of the IRS campaigns from 2012 to 2014 might have been if the campaign start dates had shifted a 2 months earlier, to May; b 1 month earlier, to June; c no shift, illustrating the actual impact of the campaigns that started in July; d 1 month later, to August; and e 2 months later, to September. The table shown in f shows the estimated number of cases per 10,000 person-months that were averted in each scenario, *based on the cumulative incidence from the 6 months after each modelled campaign

Fig. 4

Changes in u5 malaria incidence (2014–2015) by health facility in Barouéli and Bla Districts. At each health facility location on the map, the colour blue represents a decrease in malaria rates from year to year while the colour red represents an increase in malaria rates. The magnitude of the change is represented by the size of the marker. Aggregated by district, malaria rates were the same from year to year in Barouéli, where IRS with operations were consistent, but increased by 202 cases/10,000 (1386 total extra cases) in Bla, where IRS operations were suspended after 2014

Fig. 5

Comparing the entomological and epidemiological trends showing the impact of IRS in Ségou Region, 2012–2014. The monthly malaria incidence curves are presented with the results of PMI/AIRS entomological surveillance activities, including fluctuations in mosquito densities as measured by pyrethrum spray catches and human landing collections and the declining residual insecticidal efficacy as measured by standard WHO wall bioassay tests. Bendiocarb was sprayed in 2012 and 2013, while pirimiphos methyl was sprayed in 2014