Impact of indoor residual spraying with pirimiphos-methyl (Actellic 300CS) on entomological indicators of transmission and malaria case burden in Migori County, western Kenya

Abstract

Indoor residual spraying (IRS) of insecticides is a major vector control strategy for malaria prevention. We evaluated the impact of a single round of IRS with the organophosphate, pirimiphos-methyl (Actellic 300CS), on entomological and parasitological parameters of malaria in Migori County, western Kenya in 2017, in an area where primary vectors are resistant to pyrethroids but susceptible to the IRS compound. Entomological monitoring was conducted by indoor CDC light trap, pyrethrum spray catches (PSC) and human landing collection (HLC) before and after IRS. The residual effect of the insecticide was assessed monthly by exposing susceptible An. gambiae s.s. Kisumu strain to sprayed surfaces in cone assays and measuring mortality at 24 hours. Malaria case burden data were extracted from laboratory records of four health facilities within the sprayed area and two adjacent unsprayed areas. IRS was associated with reductions in An. funestus numbers in the intervention areas compared to non-intervention areas by 88% with light traps (risk ratio [RR] 0.12, 95% CI 0.07-0.21, p < 0.001) and 93% with PSC collections (RR = 0.07, 0.03-0.17, p < 0.001). The corresponding reductions in the numbers of An. arabiensis collected by PSC were 69% in the intervention compared to the non-intervention areas (RR = 0.31, 0.14-0.68, p = 0.006), but there was no significant difference with light traps (RR = 0.45, 0.21-0.96, p = 0.05). Before IRS, An. funestus accounted for over 80% of Anopheles mosquitoes collected by light trap and PSC in all sites. After IRS, An. arabiensis accounted for 86% of Anopheles collected by PSC and 66% by CDC light trap in the sprayed sites while the proportion in non-intervention sites remained unchanged. No sporozoite infections were detected in intervention areas after IRS and biting rates by An. funestus were reduced to near zero. Anopheles funestus and An. arabiensis were fully susceptible to pirimiphos-methyl and resistant to pyrethroids. The residual effect of Actellic 300CS lasted ten months on mud and concrete walls. Malaria case counts among febrile patients within IRS areas was lower post- compared to pre-IRS by 44%, 65% and 47% in Rongo, Uriri and Nyatike health facilities respectively. A single application of IRS with Actellic 300CS in Migori County provided ten months protection and resulted in the near elimination of the primary malaria vector An. funestus and a corresponding reduction of malaria case count among out-patients. The impact was less on An. arabiensis, most likely due to their exophilic nature.

Figure 1

Map of Kenya, showing study sites in western Kenya with the names of sub-counties. Yellow shading represents non-intervention sites and red dots represent sampled houses. The green shading is the intervention site with the blue dots representing sampled houses (The map was created on ArcGIS 10.2.2).

Figure 2

Mean number of observed female Anopheles (means ± std error) per trap-night per month in indoor CDC light trap and PSC before and after IRS in sprayed and unsprayed areas. The grey shaded area is the period post-spray with residual efficacy above 80%. The primary scale shows Anopheles density while the secondary scale shows rainfall in milliliters. Sixty trap-nights for CDC-LT and 30 for PSC per study arm per month. Standard errors were calculated by dividing the standard deviation by the square root of the sample size, a function was developed in R that generated the standard error around each mean.

Figure 3

Proportions of Anopheles species collected by CDC-LT and PSC before and after IRS in sprayed and unsprayed regions.

Figure 4

Sporozoite rates in An. funestus and An. arabiensis in sprayed and unsprayed areas, pre- and post-IRS.

Figure 5

Profiles of biting by An. funestus experienced by the human population in intervention and non-intervention sites before and after IRS. The black area represents biting that occurs outdoors, the dark red represents biting that occurs indoors when people are away from their bed nets and the blue represents biting that occurs while people are asleep.

Figure 6

Comparison of mammalian host blood meal type between An. funestus and An. arabiensis (Numbers tested; An. funestus- 61 cow, 5 goats, 79 humans, 1 pig and 5 mixed and An. arabiensis – 60 cows, 4 goats, 8, humans, 11 pigs and 3 mixed).

Figure 7

Health facility laboratory test positivity rates among febrile out-patients in Homa Bay (non-intervention) and Migori (Intervention) covering two years pre-IRS and over one-year post-IRS.