Abstract
BackgroundSeveral studies were carried out in experimental hut station in areas surrounding the city of Bouaké, after the crisis in Côte d’Ivoire. They reported increasing resistance levels to insecticide for malaria transmiting mosquitoes. The present work aims to evaluate the current resistance level of An. gambiae(s.l.) in rural and urban areas in the city of Bouaké.MethodsLarvae of Anopheles gambiae (s.l.) were collected from five different study sites and reared to adult stages. The resistance status was assessed using the WHO bioassay test kits for adult mosquitoes, with eight insecticides belonging to pyrethroids, organochlorines, carbamates and organophosphates classes. Molecular assays were performed to identify the molecular forms of An. gambiae (s.l.), the L1014F kdr and the ace-1R alleles in individual mosquitoes. The synergist PBO was used to investigate the role of enzymes in resistance. Biochemical assays were performed to detect potential increased activities in mixed function oxidase (MFO) levels, non-specific esterases (NSE) and glutathione S-transferases (GST).ResultsHigh resistance levels to pyrethroids, organochlorines, and carbamates were observed in Anopheles gambiae (s.l.) from Bouaké. Mortalities ranged between 0 and 73% for the eight tested insecticides. The pre-exposure to PBO restored full or partial susceptibility to pyrethroids in the different sites. The same trend was observed with the carbamates in five sites, but to a lesser extent. With DDT, pre-exposure to PBO did not increase the mortality rate of An. gambiae (s.l.) from the same sites. Tolerance to organophosphates was observed. An increased activity of NSE and higher level of MFO were found compared to the Kisumu susceptible reference strain. Two molecular forms, S form [(An. gambiae (s.s)] and M form (An. coluzzi) were identified. The kdr allele frequencies vary from 85.9 to 99.8% for An. gambiae (s.s.) and from 81.7 to 99.6% for An. coluzzii. The ace-1R frequencies vary between 25.6 and 38.8% for An. gambiae (s.s.) and from 28.6 to 36.7% for An. coluzzii.ConclusionResistance to insecticides is widespread within both An. gambiae (s.s.) and An. coluzzii. Two mechanisms of resistance, i.e. metabolic and target-site mutation seemed to largely explain the high resistance level of mosquitoes in Bouaké. Pyrethroid resistance was found exclusively due to the metabolic mechanism.
Highlights
Several studies were carried out in experimental hut station in areas surrounding the city of Bouaké, after the crisis in Côte d’Ivoire
Resistance to these four classes of chemical insecticides is widespread among An. gambiae (s.l.) in subSaharan Africa [5, 6] and represents a major threat to the effectiveness of malaria vector control strategies based on long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) which were the major contributors to the drastic reduction of Plasmodium falciparum infection prevalence over the past decade [7]
The mortality rates were less than 43% for permethrin, 73% for deltamethrin and 33.5% for α-cypermethrin
Summary
Several studies were carried out in experimental hut station in areas surrounding the city of Bouaké, after the crisis in Côte d’Ivoire. Four classes of insecticides (pyrethroids, organophosphates, carbamates and organochlorines) are the cornerstone of vector control programs [3], but pyrethroids are the only class of insecticide currently recommended by the WHOPES for the treatment of nets because of their irritant and fastacting properties and their safety for humans [4] Resistance to these four classes of chemical insecticides is widespread among An. gambiae (s.l.) in subSaharan Africa [5, 6] and represents a major threat to the effectiveness of malaria vector control strategies based on long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) which were the major contributors to the drastic reduction of Plasmodium falciparum infection prevalence over the past decade [7]. Insecticide resistance in malaria vectors is a dynamic process in which the resistance level might change quickly and strongly by the mean of selection pressure from both public health and agricultural practices [14,15,16]
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