Insecticide Resistance And Its Impact On Malaria Transmission Potential Among The Major Vectors In Teso Sub – Counties, Western Kenya

Abstract

In Africa, knockdown resistance (kdr) is strongly linked to pyrethroid insecticide resistance in Anopheles gambiae s.l; which may have vital implications to the current up-scaled indoor residual spraying (IRS) and pyrethroid-treated bed net campaigns. This investigation proposed to evaluate the extent and levels of insecticide resistance among major vectors in Teso North and South sub counties, western Kenya, and its impact on entomological parameters of malaria transmission. For WHO phenotypic susceptibility tube assay, mosquito larvae were sampled using a dipper, reared into 3-5 days-old females which were exposed to 0.75% permethrin, 0.05% deltamethrin and 0.1% bendiocarb. Random sampling of adult mosquitoes [3448] was done using adult mosquito sampling methods. Abdominal statuses, parity rates, age grading and molecular species identification were recorded. Kdr East TaqMan PCR probed for the mutant allele and wild type. Sporozoite ELISA detected circum-sporozoite protein. Anopheles gambiae sensu stricto were the significant majority in terms of species composition at 78.9% as compared to Anopheles arabiensis (20.7%) [t (4, 4) = 11.1268, p < .05]. Highest proportion of SS alleles (86.1%) was also found among Anopheles gambiae sensu stricto mosquitoes. Susceptible Kengatunyi cluster had higher proportion of An. arabiensis than resistant Rwatama. Bendiocarb caused 100% mortality while deltamethrin had higher insecticidal effects (77%) on female mosquitoes than permethrin (71%). There was a significant difference in SS allele frequencies between Kengatunyi (57.8%) and Rwatama (93.4%). Of the parous vectors, 75.6% were homozygous resistant, 2.4% were heterozygous and 22% were homozygous susceptible. Mosquitoes in the resistant cluster had an average lifespan of 16 days compared to 5 days in the susceptible cluster. Resistant clusters had the largest number of infective bites per person per night at 0.4. Of the sporozoite positive xxvi mosquitoes, 71.4% were homozygous resistant and the rest homozygous susceptible. Heterozygous LS allelic vectors had significantly higher affinity for bovine blood while SS allelic vectors had significant higher affinity for human blood. All SS, LS and LL carriers were biting more at dawn than at dusk. All results were significant at p<0.05. Most likely, the higher the selection pressure exerted indoors by insecticidal nets the higher were the resistance alleles. Use of pyrethroids impregnated nets and agrochemicals may have caused female mosquitoes to select for pyrethroid resistance, consequently impacted on species composition and behaviour patterns. Confirmed phenotypic and genotypic insecticide resistance in major malaria vectors in Teso North and Teso South sub counties, impacted significantly on vector species composition, age structure, density, infectivity, feeding and resting behaviours. Insecticide resistance management practices in Kenya should be fastracked and harmonised with agricultural sector agrochemical based activities and possibly switch to carbamate usage. The implication of such high resistance levels in the mosquitoes is that resistance is likely to persist and or even increase if mono-molecules of permethrin and deltamethrin or both continue to be used in all net and non-net based mosquito control purposes. Piperonyl butoxide (PBO) that prohibit certain metabolic enzymes within malaria vectors and has been combined into pyrethroid-LLINs to create pyrethroid-PBO nets, may be an extremely viable option against the spread of insecticide resistance in Kenya. Key words: Anopheles species Insecticide resistance WHO tube assay kdr Genotypic frequency