Entomological monitoring data driving decision making for appropriate and sustainable malaria vector control in Côte d’Ivoire

Entomological surveillance provides critical information on vectors for timely and appropriate malaria vector control and strategy decision making. Owing to the widely documented insecticide resistance of malaria vectors in Côte d’Ivoire, it is necessary that any vector control intervention deployment be driven by entomological data to optimize the effectiveness and allow appropriate allocation of resources. To achieve this goal, bioassays to evaluate insecticide susceptibility, resistance intensity, and piperonyl butoxide (PBO) synergism were conducted using World Health Organization (WHO) susceptibility test kits and Centers for Disease Control and Prevention (CDC) bottles against wild collected An. gambiae s.l. from 17 sites in 2019. In addition, monthly vector bionomics were conducted in four of the sites (Sakassou, Béoumi, Dabakala and Nassian) selected based on their reported high malaria incidence. Adult mosquito collections were conducted using human landing catches (HLCs), pyrethrum spray catches (PSCs), and CDC light traps to assess vector density, species composition, sporozoite infectivity, and behavior. High pyrethroid resistance was observed in all 17 sites. Pre-exposure to PBO before bioassays with pyrethroids yielded substantial increases in mortality, particularly with deltamethrin. When CDC bottle bioassays were conducted with chlorfenapyr (200µg/bottle), susceptibility was observed in 13 of the sites while clothianidin and pirimiphos-methyl susceptibility was observed in 8 and 10 sites, respectively. Anopheles gambiae s.l. was the predominant malaria vector collected in all four bionomic sites. Vector density was relatively higher in Sakassou throughout the year with mean biting rates of 340.1 bites per person per night (b/p/n) compared to Béoumi, Dabakala and Nassian (mean of 48.5, 81.4 and 26.7 b/p/n, respectively). The mean entomological inoculation rate (EIR) was 7.1 infective bites per person per night (ib/p/n) in Sakassou, 0.5 ib/p/n in Beoumi, 1.5 ib/p/n in both Nassian and Dabakala. The highest EIRs were recorded in October in Béoumi (3.2 ib/p/n) and Nassian (1.7 ib/p/n), in July in Dabakala (4.5 ib/p/n) and in May in Sakassou (15.6 ib/p/n). Based on these results and previously reported data, the National Malaria Control Program developed and implemented a stratified insecticide treated net (ITN) mass distribution in 2021 with the inclusion of new generation ITNs in areas of high pyrethroid resistance. These results also supported the selection of clothianidin-based products for the first indoor residual spraying (IRS) campaign in Sakassou and Nassian in 2020, as well as the optimal spray timed before transmission peaks to maximize the potential impact of the intervention.