Characterization and abundance of malaria vectors in Sakassou, Central Côte d'Ivoire

Before implementing vector control interventions, it is important to assess the mosquito species composition and their biting and resting behaviour. In preparation for an indoor residual spraying campaign for malaria control in Sakassou, Central Côte d'Ivoire, baseline entomological data were collected to characterize the local vector species composition and their biting behaviour.

A longitudinal entomological survey was carried out from November 2018 to July 2020 with monthly assessments, except in April and May 2020 due to lockdown restriction of the COVID-19 pandemic. Mosquitoes were collected with the human landing catch method outside and inside of four houses, with two houses located in an urban setting and two in a rural setting. Additionally, in each setting, 15 houses were sampled using the pyrethrum spray catch method and two houses with Centers for Disease Control and Prevention light traps. Mosquitoes were identified morphologically to the lowest possible taxonomic level, while a subset of Anopheles gambiae sensu lato (s.l.) specimens were further determined to species level by diagnostic polymerase chain reaction (PCR). Parity and Plasmodium sporozoite rates were estimated from a subsample of the caught specimens by dissecting the ovaries and screening heads and thoraces using enzyme-linked immunosorbent assays.

Overall, 98,346 mosquitoes, including 91,799 anophelines and 6,547 culicines, were collected with counts following the rain pattern. Anopheles gambiae s.l. was the predominant taxon (90%). Out of 826 specimens successfully screened with a diagnostic PCR, all were identified as Anopheles coluzzii. The estimated biting rates of An. coluzzii were high, peaking shortly after midnight. The overall Plasmodium falciparum infection rate in An. coluzzii was 0.02 (95% confidence interval: 0.01-0.03).

Anopheles coluzzii is the main malaria vector in Sakassou, exhibiting indoor resting and biting behaviours, with the peak biting rate after midnight when people are likely to be asleep. Given these characteristics, both insecticide-treated nets and indoor residual spraying hold promise for malaria vector control. As biting started early in the evening when people tend to be outside, outdoor interventions should also be considered.