Mechanisms Responsible for Insecticide Resistance in Anopheles Gambiae S.L. Populations of AHERO, BUDALANGI and BUNGOMA in Western Kenya

Abstract

As malaria control interventions directed against Anopheles vectors through the distribution of insecticide-treated bednets (ITNs) and application of indoor residual sprays (IRS) increase in sub-SaharanAfrica, it is crucial to continually assess the efficacy of these main insecticide-based interventionsagainst local malaria vector populations. In western Kenya, I'fN use is widespread and the area has experienced a scale-up of IRS in targeted districts in the past few years. Resistanceoflocal vectors to the insecticides used in ITNs and IRS could lead to failure of these interventions leading to increase in malaria infections. This study investigated susceptibility ofAnopheles gambiae and Anopheles arabiensis, to two pyrethroid insecticides (permethrin and deltamethrin) currently used in ITNs and IRS in western Kenya and one carbamate insecticide (bendiocarb), which is a potential alternative to pyrethroids for IRS. The study also investigated the resistance mechanisms exploited by the vectors: biochemical resistance pathways which include elevation in detoxification enzymes (esterases, oxidases, and GSTs) and molecular pathways, in this case, the genetic acquisition, and spread of the knock-down resistance (kdr) genotype. For testing, adult mosquitoes from either larva samples collected in the field or Fls raised from blood-fed females collected from houses were used. Samples were collected from threesites in western Kenya: Ahero, Budalangi, and Bungoma. Two techniques; the CDC bottle assay and the WHO tube assays were used to determine susceptibility of vectors while microplate enzyme assays and Real-time PCR techniques were used respectively to investigate the biochemical and molecular mechanisms responsible for any observed resistance in the vectors. All samples were identified to species level by standard PCR. Out of a total of 3,315 mosquitoes used, 1124 were from Bungoma (77% A. gambiae s.s. and 23% A. arabiensisi, 885 from Budalangi (1% A. gambiae s.s. and 99% A. arabiensisi and 1102 from Ahero (100% A. arabiensisi. There was high resistance to the pyrethroids it)An. gambiae s.s. populations from Bungoma (permethrin 62.0%, deltamethrin 34.0%) and Bundalangi (permethrin 26.0%) while therewas little or no resistance to Bendiocarb in the two sites. Samples from Ahero were found to have little or no resistance to all 3 insecticides tested. Female mosquitoes showed lower susceptibility to insecticides compared to males (P=0.0127). There was no significant difference between the CDC Bottle and WHO tube bioassay methods in determining the resistance status of vector populations (P=0.2446). The kdr L1014S allele approached fixation in A.. gambiae S.s. populations of Bungoma (99.4%) and Budalangi (100%) while none of the An. arabiensis tested had the kdr genotype. Permethrin-resistant A. gambiae s.s. from Bungoma had 1.65 fold elevation in non-specific esterases but no elevation was found in resistant populations from Budalangi. Over 70% of households in all sites were covered with ITNs with coverage in Ahero being highest at 85%. The IRS was only performed in Ahero in 2010 had a coverage of 58.0% among the households sampled. This study confirms the presence of insecticide resistance to pyrethroids in western Kenya, with Bungoma having the highest level of resistance compared to the other 2 study sites. Either CDC Bottle assay or WHO tube bioassay techniques can be used to accurately diagnose and monitor insecticide resistance. The high resistance seen in Bungoma can be attributed to a combination of two resistance mechanisms, the elevated esterases, and high frequency of kdr. The elevated esterases are most likely due to the high rate of organophosphate insecticides use in the area for tobacco farming. These findings will inform selection of insecticides for IRS so as to effectively control pyrethroid resistant vectors and curb the spread of insecticide resistance.