title: Pyrethroid susceptibility of malaria vectors in four Districts of western Kenya.
abstract: BACKGROUND
  Increasing pyrethroid resistance in malaria vectors has been reported in western Kenya where long lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) are the mainstays of vector control. To ensure the sustainability of insecticide-based malaria vector control, monitoring programs need to be implemented. This study was designed to investigate the extent and distribution of pyrethroid resistance in 4 Districts of western Kenya (Nyando, Rachuonyo, Bondo and Teso). All four Districts have received LLINs while Nyando and Rachuonyo Districts have had IRS campaigns for 3-5 years using pyrethroids. This study is part of a programme aimed at determining the impact of insecticide resistance on malaria epidemiology.
  
  
  METHODS
  Three day old adult mosquitoes from larval samples collected in the field, were used for bioassays using the WHO tube bioassay, and mortality recorded 24 hours post exposure. Resistance level was assigned based on the 2013 WHO guidelines where populations with <90% mortality were considered resistant. Once exposed, samples were identified to species using PCR.
  
  
  RESULTS
  An. arabiensis comprised at least 94% of all An. gambiae s.l. in Bondo, Rachuonyo and Nyando. Teso was a marked contrast case with 77% of all samples being An. gambiae s.s. Mortality to insecticides varied widely between clusters even in one District with mortality to deltamethrin ranging from 45-100%, while to permethrin the range was 30-100%. Mortality to deltamethrin in Teso District was < 90% in 4 of 6 clusters tested in An arabiensis and <90% in An. gambiae s.s in 5 of 6 clusters tested. To permethrin, mortality ranged between 5.9-95%, with <90% mortality in 9 of 13 and 8 of 13 in An. arabiensis and An. gambiae s.s. respectively. Cluster specific mortality of An. arabiensis between permethin and deltamethrin were not correlated (Z = 2.9505, P = 0.2483).
  
  
  CONCLUSIONS
  High levels of pyrethroid resistance were observed in western Kenya. This resistance does not seem to be associated with either species or location. Insecticide resistance can vary within small geographical areas and such heterogeneity may make it possible to evaluate the impact of resistance on malaria and mosquito parameters within similar eco-epidemiological zones.
authors.affiliation: School of Public Health and Community Development, Maseno University, Maseno, Kenya. eochomo@kemricdc.org.
authors.forename: Eric
authors.initials: E
authors.lastname: Ochomo
authors.forename: Nabie M
authors.initials: NM
authors.lastname: Bayoh
authors.forename: Luna
authors.initials: L
authors.lastname: Kamau
authors.forename: Francis
authors.initials: F
authors.lastname: Atieli
authors.forename: John
authors.initials: J
authors.lastname: Vulule
authors.forename: Collins
authors.initials: C
authors.lastname: Ouma
authors.forename: Maurice
authors.initials: M
authors.lastname: Ombok
authors.forename: Kiambo
authors.initials: K
authors.lastname: Njagi
authors.forename: David
authors.initials: D
authors.lastname: Soti
authors.forename: Evan
authors.initials: E
authors.lastname: Mathenge
authors.forename: Lawrence
authors.initials: L
authors.lastname: Muthami
authors.forename: Teresa
authors.initials: T
authors.lastname: Kinyari
authors.forename: Krishanthi
authors.initials: K
authors.lastname: Subramaniam
authors.forename: Immo
authors.initials: I
authors.lastname: Kleinschmidt
authors.forename: Martin James
authors.initials: MJ
authors.lastname: Donnelly
authors.forename: Charles
authors.initials: C
authors.lastname: Mbogo
mesh_terms: D000818:Animals; D000852:Anopheles / Q000187:drug effects*; D007303:Insect Vectors / Q000187:drug effects*; D007305:Insecticide Resistance / Q000187:drug effects*; D007306:Insecticides / Q000494:pharmacology; D007630:Kenya / Q000453:epidemiology; D007814:Larva / Q000187:drug effects; D008288:Malaria / Q000453:epidemiology / Q000635:transmission*; D009570:Nitriles / Q000494:pharmacology*; D026023:Permethrin / Q000494:pharmacology*; D011722:Pyrethrins / Q000494:pharmacology*
chemical_list: D007306:Insecticides; D009570:Nitriles; D011722:Pyrethrins; C017180:decamethrin; D026023:Permethrin