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

BackgroundMalaria 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.MethodsTo 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.ResultsAnopheles 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.ConclusionsThe 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.

Early biting of the Anopheles gambiae s.s. and its challenges to vector control using insecticide treated nets in western Kenya highlands

“…Of concern is the possibility of such shifts in A. gambiae s.l. continuing thereby undermining the efficacy of ITNs, a situation that could further deteriorate with the vector reported increase in resistance towards operational pyrethoids (Vulule et al, 1994;Stump et al, 2004;Mathias et al, 2011).…”

“…According to Stump et al (2004) significant differences in kdr gene frequency observed between the large-scale insecticide treated net trial in Asembo area and other areas in western Kenya. Mathias et al, (2011) reported a causal relationship between high ITNs coverage levels and increasing kdr frequency in A. gambiae s.s. in western Kenya.…”

Insecticide Resistance Allele Frequencies in Anopheles gambiae before and after Anti-Vector Interventions in Continental Equatorial Guinea

“…They reported "a sharp increase in homozygous frequencies from complete absence in both locations initially to 80.5% in Seme in 2008 and 91.7% for Asembo in 2010." 17 In a recent study on the impact of a large scale IRS and ITN campaign in southern Benin, kdr frequencies were also found to have increased, although this was also true for areas in which no planned interventions were implemented, underscoring the effect of the agricultural and household insecticide use on resistance. 18 In east Africa, a similar increase has been observed.…”

Reemergence of Anopheles funestus as a Vector of Plasmodium falciparum in Western Kenya after Long-Term Implementation of Insecticide-Treated Bed Nets

“…gambiae sensu lato (s.l. ), [14][15][16] a complex of at least eight closely related species including An. gambiae s.s. and An.…”