Monitoring pyrethroid resistance in the house mosquito, Culex pipiens, in Lebanon -

Abstract

Pyrethroid insecticides have been extensively used against mosquitoes in Lebanon and worldwide for public health control. However, mosquitoes are known to develop resistance with time against chemicals. In this study, we evaluate percent resistance to pyrethroids in populations of the house mosquito, Culex pipiens, in selected cities in Lebanon and we determine the mechanisms of resistance, target site-resistance and-or metabolic resistance. Target site resistance consists of mutations in the sodium channel, which is the target of pyrethoids and DDT; while metabolic resistance involves elevated levels of detoxifying enzymes, which can be mixed function oxidases and non-specific esterases. For this study a sample of 672 mosquitoes was collected from six sites from May 2014-2016 and screened for resistance to pyrethroids, type I and type II using the WHO susceptibility test, then analyzed for the L1014F mutation in the sodium channel gene and for the over production of mixed function oxidases and non-specific esterases. Our results show that all sampled mosquito populations were resistant to pyrethroids, type I and type II. Only the population from Beirut showed high resistance to pyrethroids type II but borderline susceptibility to pyrethroids type I. This was because only in Beirut the municipality has been spraying type II pyrethroids. Moreover, resistance in the Lebanese C.pipiens populations to pyrethroids was caused by two mechanisms involving an over production in the detoxifying enzymes and the L1014F mutation. It was found that 66.96percent of the sampled mosquitoes at the national level were homozygote for the mutation L1014F having the genotype [RR], 16.22percent are heterozygote for the mutation exhibiting [RS] genotype and 16.82percent were susceptible [SS]. High levels of oxidases and esterases were also detected in C.pipiens populations in most of the sites in Lebanon, mainly agricultural regions.