Increased frequency of the T929I and L932F mutations associated with knockdown resistance in permethrin-resistant populations of the human head louse, Pediculus capitis, from California, Florida, and Texas

The resistance levels of different human head louse populations from the USA to 1% permethrin were evaluated using permethrin-impregnated, filter paper disk-contact bioassay. Populations from southern California, south Florida and south central Texas showed 1.5-, 3.1-, and 1.5- to 5.1-fold resistance compared to insecticide-susceptible head louse populations from Panama or Ecuador. Permethrin-resistant or permethrin-susceptible homozygous or heterozygous genotypes were determined from sequences of PCR-amplified genomic DNA fragments of the voltage-sensitive sodium channel a-subunit gene by the presence of a T or C, or both, respectively, at nucleotide positions 36 and 44 in the sequence. The presence of a T at both these positions resulted in the amino acid substitutions, T929I and L932F, respectively. Of the 424 louse samples examined that had the T929I mutation, all also possessed the L932F mutation, indicating that the two mutations were tightly linked. The southern California population was phenotypically determined by bioassay to be comprised of 45% resistant individuals and had a resistant allele frequency of 0.53 by DNA sequence analysis. The south Florida population was phenotypically determined to consist of 87% resistant individuals and had a resistant allele frequency of 0.97. The four Texas populations varied in the level of resistance and in resistant allele frequency. The Mathis population was phenotypically determined to consist of 15% resistant individuals and had a resistant allele frequency of 0.33. However, the populations from San Antonio, Mansfield, and Corpus Christi were likewise phenotyped to have 91%, 94%, and 100%, respectively, resistant individuals and a 0.98, 1.00 and 1.00, respectively, resistant allele frequency. The log survival time versus logit mortality regression lines of susceptible-homozygotes, resistant-homozygotes, and heterozygotes determined that the resistance trait was complete recessive.