Abstract
BackgroundAt Yaokoffikro field site near Bouaké, in central Côte d'Ivoire, a group of experimental huts built in 1996 served over many years for the evaluation of insecticides against highly resistant mosquitoes. Breeding sites of mosquitoes and selection pressure in the area were maintained by local farming practices until a war broke out in September 2002. Six years after the crisis, we conducted bioassays and biochemical analysis to update the resistance status of Anopheles gambiae s.s. populations and detect other potential mechanisms of resistance that might have evolved.MethodsAn. gambiae s.s. larvae from Yaokoffikro were collected in breeding sites and reared to adults. Resistance status of this population to insecticides was assessed using WHO bioassay test kits for adult mosquitoes with seven insecticides: two pyrethroids, a pseudo-pyrethroid, an organochloride, two carbamates and an organophosphate.Molecular and biochemical assays were carried out to identify the L1014F kdr and ace-1R alleles in individual mosquitoes and to detect potential increase in mixed function oxidases (MFO), non-specific esterases (NSE) and glutathione S-transferases (GST) activity.ResultsHigh pyrethroids, DDT and carbamate resistance was confirmed in An. gambiae s.s. populations from Yaokoffikro. Mortality rates were less than 70% with pyrethroids and etofenprox, 12% with DDT, and less than 22% with the carbamates. Tolerance to fenitrothion was observed, with 95% mortality after 24 h.PCR analysis of samples from the site showed high allelic frequency of the L1014F kdr (0.94) and the ace-1R (0.50) as before the crisis. In addition, increased activity of NSE, GST and to a lesser extent MFO was found relative to the reference strain Kisumu. This was the first report detecting enhanced activity of these enzymes in An. gambiae s.s from Yaokoffikro, which could have serious implications in detoxification of insecticides. Their specific roles in resistance should be investigated using additional tools.ConclusionThe insecticide resistance profile at Yaokoffikro appears multifactorial. The site presents a unique opportunity to evaluate its impact on the protective efficacy of insecticidal products as well as new tools to manage these complex mechanisms. It calls for innovative research on the behaviour of the local vector, its biology and genetics that drive resistance.
Highlights
At Yaokoffikro field site near Bouaké, in central Côte d’Ivoire, a group of experimental huts built in 1996 served over many years for the evaluation of insecticides against highly resistant mosquitoes
Six years after the armed conflict in Côte d’Ivoire, this study was designed to update on resistance status of An gambiae s.s. at a previous WHO Pesticide Evaluation Scheme (WHOPES) field site (Yaokoffikro), where a group of experimental huts have served for the evaluation of insecticides several years prior to the crisis
In addition to high L1014F kdr and acetylcholinesterase-1 resistance (ace-1R) previously detected and confirmed in the present experiment, highly significant increase in non-specific esterases (NSE) and glutathione S-transferases (GST) activities were found in An. gambiae s.s. from Yaokoffikro field site
Summary
At Yaokoffikro field site near Bouaké, in central Côte d’Ivoire, a group of experimental huts built in 1996 served over many years for the evaluation of insecticides against highly resistant mosquitoes. During the last decade pyrethroid resistance has become widespread in Anopheles gambiae s.s. in sub-Saharan Africa [3,4,5], probably as a consequence of use of pyrethroids in agriculture [6,7] and increasingly through exposure to LLINs, as coverage is scaled up [8,9]. Even for IRS, with only four insecticide classes currently available and resistance reported to all four of these in some populations of An. gambiae s.s. Recent product development partnership has been established to stimulate the search for alternative active ingredients or improved formulations of insecticides for vector control, and several promising leads are being evaluated in laboratory and field trials [11]. Industry must continue to produce new prototype LLINs to ensure wider community coverage until new weapons are available
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