title: Spatial and temporal variation in the kdr allele L1014S in Anopheles gambiae s.s. and phenotypic variability in susceptibility to insecticides in Western Kenya.
abstract: BACKGROUND
  Malaria vector control in Africa depends upon effective insecticides in bed nets and indoor residual sprays. This study investigated the extent of insecticide resistance in Anopheles gambiae s.l., Anopheles gambiae s.s. and Anopheles arabiensis in western Kenya where ownership of insecticide-treated bed nets has risen steadily from the late 1990s to 2010. Temporal and spatial variation in the frequency of a knock down resistance (kdr) allele in A. gambiae s.s. was quantified, as was variation in phenotypic resistance among geographic populations of A. gambiae s.l.
  
  
  METHODS
  To investigate temporal variation in kdr frequency, individual specimens of A. gambiae s.s. from two sentinel sites were genotyped using RT-PCR from 1996-2010. Spatial variation in kdr frequency, species composition, and resistance status were investigated in additional populations of A. gambiae s.l. sampled in western Kenya in 2009 and 2010. Specimens were genotyped for kdr as above and identified to species via conventional PCR. Field-collected larvae were reared to adulthood and tested for insecticide resistance using WHO bioassays.
  
  
  RESULTS
  Anopheles gambiae s.s. showed a dramatic increase in kdr frequency from 1996 - 2010, coincident with the scale up of insecticide-treated nets. By 2009-2010, the kdr L1014S allele was nearly fixed in the A. gambiae s.s. population, but was absent in A. arabiensis. Near Lake Victoria, A. arabiensis was dominant in samples, while at sites north of the lake A. gambiae s.s was more common but declined relative to A. arabiensis from 2009 to 2010. Bioassays demonstrated that A. gambiae s.s. had moderate phenotypic levels of resistance to DDT, permethrin and deltamethrin while A. arabiensis was susceptible to all insecticides tested.
  
  
  CONCLUSIONS
  The kdr L1014S allele has approached fixation in A. gambiae s.s. populations of western Kenya, and these same populations exhibit varying degrees of phenotypic resistance to DDT and pyrethroid insecticides. The near absence of A. gambiae s.s. from populations along the lakeshore and the apparent decline in other populations suggest that insecticide-treated nets remain effective against this mosquito despite the increase in kdr allele frequency. The persistence of A. arabiensis, despite little or no detectable insecticide resistance, is likely due to behavioural traits such as outdoor feeding and/or feeding on non-human hosts by which this species avoids interaction with insecticide-treated nets.
authors.affiliation: Centre for Global Health Research, Kenya Medical Research Institute, PO Box.1578, Kisumu, Kenya.
authors.forename: Derrick K
authors.initials: DK
authors.lastname: Mathias
authors.forename: Eric
authors.initials: E
authors.lastname: Ochomo
authors.forename: Francis
authors.initials: F
authors.lastname: Atieli
authors.forename: Maurice
authors.initials: M
authors.lastname: Ombok
authors.forename: M Nabie
authors.initials: MN
authors.lastname: Bayoh
authors.forename: George
authors.initials: G
authors.lastname: Olang
authors.forename: Damaris
authors.initials: D
authors.lastname: Muhia
authors.forename: Luna
authors.initials: L
authors.lastname: Kamau
authors.forename: John M
authors.initials: JM
authors.lastname: Vulule
authors.forename: Mary J
authors.initials: MJ
authors.lastname: Hamel
authors.forename: William A
authors.initials: WA
authors.lastname: Hawley
authors.forename: Edward D
authors.initials: ED
authors.lastname: Walker
authors.forename: John E
authors.initials: JE
authors.lastname: Gimnig
mesh_terms: D019943:Amino Acid Substitution / Q000235:genetics; D000818:Animals; D000852:Anopheles / Q000187:drug effects* / Q000235:genetics*; D002417:Cattle; D002675:Child, Preschool; D004351:Drug Resistance; D005260:Female; D005787:Gene Frequency; D005843:Geography; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D019476:Insect Proteins / Q000235:genetics*; D007306:Insecticides / Q000494:pharmacology*; D007630:Kenya; D011247:Pregnancy; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013997:Time Factors
chemical_list: D019476:Insect Proteins; D007306:Insecticides