Entomological impact of three years of clothianidin-based indoor residual spraying in two high malaria endemic districts in Côte d'Ivoire.

BACKGROUND: Indoor residual spraying (IRS) represents a major malaria vector control strategy to reduce malaria burden. The first major IRS campaign for malaria control in Côte d'Ivoire was implemented by the US Presidents Malaria Initiative in two malaria endemic districts between 2020 and 2022. This study report on the effect of clothianidin-based IRS on key entomological indices of malaria transmission in the IRS districts following three rounds of IRS with clothianidin-based insecticide formulations. METHODS: Pre- and post-IRS adult mosquito collections were conducted monthly in two IRS districts (Sakassou and Nassian) and two control districts (Beoumi and Dabakala) using human landing catches (HLCs) and pyrethrum spray catches (PSC) from April 2019 through December 2022. Mosquitoes collected were morphologically identified, further analysed for sibling species using PCR, and tested for sporozoite infection using the enzyme-linked immunosorbent assay (ELISA). A proportion of the Anopheles gambiae sensu lato mosquitoes collected from HLCs were also dissected to assess their parity rates. RESULTS: Anopheles coluzzii and An. gambiae were the main malaria vectors collected in the study area. In the IRS arm HBR declined progressively across spray rounds, with a statistically significant reduction of about 60% by the third round (IRR = 0.40; p = 0.002) but not in the controls (all p > 0.5). The odds of sporozoite infection decreased by 49% after the first (OR = 0.51; p = 0.023) and second (OR = 0.51, p = 0.043) at the IRS sites but did not show any further decline by the third round (p = 0.51). These reductions resulted in four-fold decrease in EIR from 3.92 infective/bites/person/night pre-IRS to 1.25 ib/p/n post-IRS. EIR in control declined only marginally from 0.91ib/p/n pre-IRS to 0.58 ib/p/n post-IRS. CONCLUSIONS: Clothianidin-based IRS was associated with a clear reduction in An. gambiae and An. coluzzii biting densities in the IRS districts and, more importantly, a significant reduction in EIR after accounting for changes over the same period in the neighbouring control areas. These findings are consistent with the epidemiological findings reported in the same sites and demonstrates the utility of IRS as an important tool for malaria control in a highly endemic setting like Côte d'Ivoire.