Elucidating factors that contribute to insecticide resistance in the M and S molecular forms of Anopheles gambiae s.s. in West Africa

Insecticide resistance genes in Anopheles gambiae Giles sensu stricto threaten to compromise vector-based malaria control programs. Two mutations at the same locus in the voltage-gated sodium channel gene are known to confer knockdown resistance (kdr) to pyrethroids and DDT. Kdr-e is associated with resistance in East Africa, while kdr-w is associated with resistance in West Africa. In West Africa, An. gambiae exists in discrete subpopulations known as M and S molecular forms. These forms occur in sympatry but exhibit very limited gene flow. On the island of Bioko, Equatorial Guinea, kdr was not detectable in either form prior to a large-scale pyrethroid spray campaign. Following the campaign we detected kdr at high frequencies in the M form. Kdr was absent in the S form on the island and absent in the nearest continental populations. Through sequence comparisons of the polymorphic intron preceding kdr, we determined that kdr arose independently in the M form, most likely in response to intensive insecticide use. The kdr-e allele is no longer restricted to East Africa and has been detected in areas of West Africa co-occurring with kdr-w. We analyzed the frequency and relationship between kdr genotypes and resistance to Type I and Type II pyrethroids and DDT. Kdr-e and kdr-w were detected in both forms but were predominant in the S form. Both kdr-e and kdr-w were closely associated with resistance to DDT and weakly associated with resistance to Type II pyrethroids. Kdr-w conferred greater resistance to permethrin than kdr-e . Implications of allele spread on insecticide efficacy are discussed. In Mali, kdr is limited to the S molecular form, even in sympatric populations with high insecticide use. We examine the role of alternative resistance mechanisms in conferring resistance in both forms. This family level study analyzed the role of kdr and enzyme activity by P450 monooxygenase, esterase and glutathione S-transferase in conferring resistance in populations from villages with high and low insecticide use. Overall this study suggests that kdr is the primary mechanism of resistance in An. gambiae and illustrates the need for multiple methods of control to support integrated vector management.