title: Genetics of multiple insecticide resistance in Anopheles gambiae from Côte d’Ivoire
Creators: name: Edi, Ako
Publisher: name: University of Liverpool Repository
Year: 2015

description: Malaria is a major public heath disease with over 3.4 billion people at risk globally. High coverage of pyrethroid-treated long-lasting insecticide treated nets (LLINs) and indoor residual spraying (IRS) have played a key role in reducing transmission over the last decade. Unfortunately, resistance to pyrethroids is now widespread and increasingly being reported to the few other WHO-approved alternative insecticides. The problem might be critical in Côte d’Ivoire, especially in the southern rice-growing area of Tiassalé where mosquitoes have been found to be resistant to pyrethroids and DDT. In this thesis, I aimed to investigate the profile of resistance to WHO-approved insecticide classes in Anopheles gambiae from Côte d’Ivoire, with a particular emphasis on Tiassalé, where I conducted in-depth investigation resistance characterisation and investigation of the genetic basis of extreme and multiple insecticide resistance. I first demonstrated the presence of resistance to all four WHO-approved classes of insecticide in wild population of in An. coluzzii from Tiassalé. This was the first demonstration of such unprecedented multiple insecticide resistance, representing a real concern for implementation of control measures based on current insecticide classes. Target site mutations in the voltage-gated sodium channel were significantly associated with DDT, but not pyrethroids, yet a meta-analysis of published and unpublished data spanning twenty years of testing in Côte d’Ivoire suggested that significant increases in DDT and pyrethroid resistance have occurred, more strongly in the South, and are likely linked to increase in the kdr 1014F mutation in A. coluzzii. Nevertheless contemporary data suggest that overexpression of metabolic genes might be more important in pyrethroid resistance; a speculation supported by significant PBO-enhancement of Tiassalé A. coluzzii mortality to pyrethroids and other insecticides tested, suggested primary importance of P450s detoxification enzymes. In addition, using dose-response assays, females were found to exhibit an extreme level of bendiocarb resistance, with some surviving even at 8h exposure. Whole genome microarrays were used to investigate the genes potentially responsible for this extreme resistance in a stringent, multiply-replicated design, detecting overexpression of several CYP6 P450s and the ACE-1 target site genes as resistance linked. The latter association arises via duplication of ACE-1 119S resistant alleles, providing the first direct evidence in Anopheles for a link between target site duplication and insecticide resistance. Synthesis of the results from several experiments suggests that the ACE-1 G119S substitution is the primary determinant of variation in survival at 60 minutes (WHO standard) exposure to bendiocarb, whereas overexpression of ACE-1 is the primary determinant of survival at an exposure duration corresponding to the population LT50. However, at an LT80 level elevated expression of both ACE-1 (resistant alleles) and CYP6 P450s enable survival. Interestingly, this work also highlighted how specific mosquito genes such as CYP6M2 and CYP6P3 were able to contribute to resistance across insecticide classes with contrasting modes of action, providing a key novel insight into how metabolic mechanisms can lead to cross-resistance in mosquitoes. Unfortunately, results from wider testing and meta-analysis suggest that multiple resistance may be present across Southern Côte d’Ivoire. The results presented in this thesis have shed new light on the extent of multiple and cross-resistance in Anopheles and the underlying mechanisms and should help national malaria control programmes, health departments and decision-making stakeholders to better plan the resistance surveillance programmes in order to combat multiple insecticide resistant vectors in African countries.