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

BACKGROUND: Entomological surveillance provides critical information on vectors for appropriate malaria vector control and strategic decision-making. The widely documented insecticide resistance of malaria vectors in Côte d'Ivoire requires that any vector control intervention deployment be driven by entomological data to optimize its effectiveness and appropriate resource allocations. To achieve this goal, this study documents the results of monthly vector surveillance and insecticide susceptibility tests conducted in 2019 and a review of all previous entomological monitoring data used to guide vector control decision making. Furthermore, susceptibility to pirimiphos-methyl and clothianidin was assessed in addition to chlorfenapyr and pyrethroids (intensity and piperonyl butoxide (PBO) synergism) tests previously reported. Vector bionomic data were conducted monthly in four sites (Sakassou, Béoumi, Dabakala and Nassian) that were selected based on their reported high malaria incidence. Adult mosquitoes were collected using human landing catches (HLCs), pyrethrum spray catches (PSCs), and human-baited CDC light traps to assess vector density, behaviour, species composition and sporozoite infectivity. RESULTS: Pirimiphos-methyl and clothianidin susceptibility was observed in 8 and 10 sites, respectively, while previous data reported chlorfenapyr (200 µg/bottle) susceptibility in 13 of the sites, high pyrethroid resistance intensity and increased mortality with PBO pre-exposure at all 17 tested sites. Anopheles gambiae sensu lato 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 278.2 bites per person per night (b/p/n) compared to Béoumi, Dabakala and Nassian (mean of 48.5, 81.4 and 26.6 b/p/n, respectively). The mean entomological inoculation rate (EIR) was 4.44 infective bites per person per night (ib/p/n) in Sakassou, 0.34 ib/p/n in Beoumi, 1.17 ib/p/n in Dabakala and 1.02 ib/p/n in Nassian. The highest EIRs were recorded in October in Béoumi (1.71 ib/p/n) and Nassian (3.22 ib/p/n), in July in Dabakala (4.46 ib/p/n) and in May in Sakassou (15.6 ib/p/n). CONCLUSION: Based on all results and data review, the National Malaria Control Programme developed and implemented a stratified insecticide-treated net (ITN) mass distribution in 2021 considering new generation ITNs. These results also supported the selection of clothianidin-based products and an optimal spraying time for the first indoor residual spraying (IRS) campaign in Sakassou and Nassian in 2020.

Building the capacity of West African countries in Aedes surveillance: inaugural meeting of the West African Aedes Surveillance Network (WAASuN).

Arboviral diseases such as dengue, Zika and chikungunya transmitted by Aedes mosquitoes have been reported in 34 African countries. Available data indicate that in recent years there have been dengue and chikungunya outbreaks in the West Africa subregion, in countries including Côte d'Ivoire, Burkina Faso, Gabon, Senegal, and Benin. These viral diseases are causing an increased public health burden, which impedes poverty reduction and sustainable development. Aedes surveillance and control capacity, which are key to reducing the prevalence of arboviral infections, need to be strengthened in West Africa, to provide information essential for the formulation of effective vector control strategies and the prediction of arboviral disease outbreaks. In line with these objectives, the West African Aedes Surveillance Network (WAASuN) was created in 2017 at a meeting held in Sierra Leone comprising African scientists working on Aedes mosquitoes. This manuscript describes the proceedings and discusses key highlights of the meeting.

Insecticide Resistance Patterns and Mechanisms in Aedes aegypti (Diptera: Culicidae) Populations Across Abidjan, Côte d'Ivoire Reveal Emergent Pyrethroid Resistance.

From 2008 to 2017, the city of Abidjan, Côte d'Ivoire experienced several Aedes-borne disease epidemics which required control of the vector mosquito population based on the reduction of larval habitats and insecticidal sprays for adult mosquitoes. This study was undertaken to assess the insecticide susceptibility status of Aedes aegypti (Linnaeus) in the city of Abidjan. Immature Ae. aegypti were sampled from several larval habitats within seven communes of Abidjan and reared to adults. Three to five days old F1 emerged adults were tested for susceptibility using insecticide-impregnated papers and the synergist piperonyl butoxide (PBO) following WHO bioassay guidelines. The results showed that Ae. aegypti populations from Abidjan were resistant to 0.1% propoxur, and 1% fenitrothion, with mortality rates ranging from 0% to 54.2%. Reduced susceptibility (93.4-97.5% mortality) was observed to 0.05% deltamethrin, 0.75% permethrin, 0.05% lambda-cyhalothrin, 5% malathion, and 0.8% chlorpyrifos-methyl. This reduced susceptibility varied depending on the insecticide and the collection site. The restoration of mortality when the mosquitoes were pre-exposed to the synergist PBO suggests that increased activity of oxidases could be contributing to resistance. Three kdr mutations (V410L, V1016I, and F1534C) were present in populations tested, with low frequencies for the Leu410 (0.28) and Ile1016 (0.32) alleles and high frequencies for the Cys1534 allele (0.96). These findings will be used to inform future arbovirus vector control activities in Abidjan.

Susceptibility of Anopheles gambiae from Côte d'Ivoire to insecticides used on insecticide-treated nets: evaluating the additional entomological impact of piperonyl butoxide and chlorfenapyr.

BACKGROUND: Pyrethroid-treated mosquito nets are currently the mainstay of vector control in Côte d'Ivoire. However, resistance to pyrethroids has been reported across the country, limiting options for insecticide resistance management due to the paucity of alternative insecticides. Two types of insecticide-treated nets (ITNs), ITNs with pyrethroids and the synergist piperonyl butoxide (PBO), and Interceptor®G2 nets, a net treated with a combination of chlorfenapyr and alpha-cypermethrin, are believed to help in the control of pyrethroid-resistant mosquitoes. METHODS: The susceptibility of Anopheles gambiae sensu lato (s.l.) to pyrethroid insecticides with and without pre-exposure to PBO as well as to chlorfenapyr was investigated in fifteen sites across the country. Susceptibility tests were conducted on 2- to 4-day old adult female An. gambiae s.l. reared from larval collections. The resistance status, intensity, and effects of PBO on mortality after exposure to different concentrations of deltamethrin, permethrin and alpha-cypermethrin were determined using WHO susceptibility test kits. In the absence of a WHO-recommended standard protocol for chlorfenapyr, two interim doses (100 and 200 µg/bottle) were used to test the susceptibility of mosquitoes using the CDC bottle assay method. RESULTS: Pre-exposure to PBO did not result in full restoration of susceptibility to any of the three pyrethroids for the An. gambiae s.l. populations from any of the sites surveyed. However, PBO pre-exposure did increase mortality for all three pyrethroids, particularly deltamethrin (from 4.4 to 48.9%). Anopheles gambiae s.l. from only one site (Bettie) were susceptible to chlorfenapyr at the dose of 100 µg active ingredient (a.i.)/bottle. At the dose of 200 µg (a.i.)/bottle, susceptibility was only recorded in 10 of the 15 sites. CONCLUSION: Low mosquito mortality was found for pyrethroids alone, and while PBO increased mortality, it did not restore full susceptibility. The vector was not fully susceptible to chlorfenapyr in one third of the sites tested. However, vector susceptibility to chlorfenapyr seems to be considerably higher than for pyrethroids alone or with PBO. These data should be used cautiously when making ITN procurement decisions, noting that bioassays are conducted in controlled conditions and may not fully represent field efficacy where the host-seeking behaviours, which include free-flying activity are known to enhance pro-insecticide chlorfenapyr intoxication to mosquitoes.

Aedes aegypti susceptibility to insecticide from Abidjan City, Cote D'ivoire.

The susceptibility of Aedes aegypti adults of three places in Abidjan city selected for an entomological surveillance of potential arbovirus vectors to permethrin, deltamethrin, lambdacyhalothrin, and propoxur was determined using WHO standard procedures. The wild populations of A. aegypti were susceptible to permethrin, deltamethirn, and lambdacyhalothin. Resistance to propoxur was detected in strains collected at the Autonomous Port of Abidjan and at Koumassi (mortality rate: 77%) but possibly resistance to this insecticide at the national zoological park (mortality rate: 90.8%). Populations of the national zoological park were possibly resistant to propoxur whereas those of the Autonomous port of Abidjan and of Koumassi were resistant.