Effects of target-site insecticide resistance on major malaria vectors’ biting patterns and entomological inoculation rates in Teso sub counties, western Kenya

Background /Introduction Malaria control is mainly based on indoor residual spraying and insecticide-treated bed nets. The efficacy of these tools depends on the feeding behaviour of mosquitoes. Objective The cross sectional study aimed at determining the biting patterns and entomological inoculation rate in Anopheles gambiae s.l in areas with and areas without insecticide resistance in Teso sub-counties, western Kenya. Materials & Methods Mosquito larvae sampled using a dipper were reared in the insectary and 3-5 days-old females exposed to 0.75%permethrin, 0.05%deltamethrin, and 0.1%bendiocarb  using WHO tube assay method. Oviposited eggs hatched into larvae which were reared and susceptibility tests done. Adult mosquitoes [605] caught using human landing catch method had species identification and kdr Eastgene PCR done on them. Results Homozygous SS allele carrying vectors had an uni-modal 5pm-7am biting pattern, heterozygous LS mosquitoes had a bi-modal while recessive LL Anopheles spp had a multi-modal one with sharp fluctuations from hour to hour. Susceptible LL vectors were earlier biters than the heterozygous LS and homozygous SS biters. All SS, LS and LL carriers were biting at dawn. Resistant clusters had the highest number of infective bites per person per night [0.4] while susceptible clusters had the lowest daily EIR [0.025]. More outdoor and lesser indoor vectors [higher biting rate before midnight] exhibited a uni-modal pattern with heightened biting attempts at 3-5pm. Dawn biting rate [>2%] was higher than at dusk [<2%] in both resistant and susceptible clusters. Discussion The guts of resistant female mosquitoes may be more receptive to gametocytes inoculation and development than guts of susceptible vectors hence higher EIR in the former. A shift in Anopheles gambiae sl highest peak hour of aggressiveness from 9-10pm to 3-5am midnight is a key change in its biting pattern. Due to development of resistance mosquitoes no longer have to compete against the time the humans enters into the formerly lethal chemical and or physical barrier. Conclusion & Recommendation Target site resistance in malaria vectors may have ensured a more consistent and successful blood meal seeking behaviour pattern hence increased daily EIRs. Insecticide resistance management should be a key component when planning for malaria vector control programmes. Key Words: Malaria Anopheles Human landing catch  Biting pattern EIR