Permethrin Resistance Due to Knockdown Gene Mutations is Prevalent in Human Head Louse Populations

Permethrin resistance in head lice is mainly conferred by the knockdown resistance (kdr) trait, a voltagesensitive sodium channel (VSSC) insensitivity factor. Three VSSC mutations have been identified and confirmed to reduce the sensitivity of VSSC to permethrin. A step-wise resistance monitoring system has been established based on molecular resistance detection techniques. Quantitative sequencing (QS) predicts the kdr allele frequency in head lice on a population basis. The speed, simplicity and accuracy of QS made it an ideal candidate for a routine primary resistance monitoring tool to screen a large number of wild louse populations as an alternative to conventional bioassay. As a secondary monitoring method, real-time PASA (rtPASA) provides a more precise determination of low resistance allele frequencies. To obtain more detailed information on resistance allele zygosity, as well as allele frequency, serial invasive signal amplification reaction (SISAR) is utilized as an individual genotyping method. Using this approach, kdr alleles were detected in head lice from 10 of the 14 countries examined and an overall kdr allele frequency of 73.8% determined.