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 nonintervention 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 1 Abt Associates, PMI VectorLink Project, White House, Milimani, Ojijo Oteko Road,

High level of resistance in Anopheles arabiensis mosquito to pyrethroid insecticides from low malaria transmission zone of Moroto district, Karamoja region, Uganda: Implication for malaria vector control

“…The exophagic and exophilic nature of An. arabiensis similarly can affect the performance of IRS since these mosquitoes tend to rest and feed outdoors [53].…”

Biting and resting preferences of malaria vectors in The Gambia

“…LLINs program in the study sites which seems to successfully limit mosquito access to human blood meal indoors and consequently reducing transmission, as previously reported [39,40]. The observed choice of animal blood by majority of the vectors could lead to increase in vector population that may eventually resort to biting humans in the long run and become difficult to control.…”

The impact of stopping and starting indoor residual spraying on malaria burden in 14 districts of Uganda

“…In Northern Zambia, implementation of IRS with Actellic 300CS® targeting only high burden areas over a three year period was associated with a 25% decline in parasite prevalence during the rainy season, but no decline during the dry season 37 . In Western Kenya, the introduction of a single round of IRS with Actellic 300CS® was associated with a 44-65% decrease in district level malaria case counts over a 10 month period compared to pre-IRS levels 38 . In addition, several recent reports have documented dramatic resurgences following the withdrawal of IRS with bendiocarb in Benin 39 , and the withdrawal of IRS with Actellic 300CS® in Mali and Ghana 34,36 .…”