Studies aimed at monitoring the spread of knockdown resistance to pyrethroids (kdr) in time and space are particularly useful for detecting barriers to gene flow among the chromosomal and molecular forms of Anopheles gambiae. We used a recently developed polymerase chain reaction assay to estimate changes in kdr frequency that occurred in several mixed-form populations from Mali, West Africa, in the past decade. We found that the kdr allele significantly increased in frequency in most populations but was still absent from the M molecular form. Importantly, within the S molecular form, kdr was detected for the first time in the Bamako chromosomal form. These results provide important insights on the patterns of spread and emergence of pyrethroid knockdown resistance in West Africa.

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Monitoring the spread of the knockdown resistance allele kdr in areas of extensive pyrethroid use is critical to vector-control projects. Currently available methods for detecting kdr from DNA samples are characterized by poor amplification, time-consuming steps, and primers that exhibit frequent null alleles. We describe a new PCR diagnostic that uses fluorescent primers based on conserved priming sites and enables simple detection of the kdr allele on a sequencer. Using samples from a West African Anopheles gambiae population, we show that the new PCR yielded significantly higher rates of amplification and more accurate estimates of kdr frequency. The method works equally well for the leucine to phenylalanine substitution found in West Africa and the East African leucine to serine substitution.

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