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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.

scientific reports

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