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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.

malaria journal

Spatial and temporal variation in the kdr allele L1014S in Anopheles gambiae s.s. and phenotypic variability in susceptibility to insecticides in Western Kenya