Sodium channel mutations associated with knockdown resistance in the human head louse, Pediculus capitis (De Geer)

Pyrethroid resistance in human head louse populations is widespread in the United States and worldwide. We previously documented that the knockdown resistance of permethrin-resistant head louse populations is associated with the T929I and L932F (T917I and L920F in the numbering of the louse amino acid sequence) mutations in the voltage-sensitive sodium channel α-subunit gene. In order to identify additional sodium channel mutations potentially associated with knockdown resistance, we cloned and sequenced full-length cDNA fragments from insecticide-susceptible (Ecuador) and permethrin-resistant (Florida) head louse populations and from an insecticide-susceptible body louse population (Israel). Sequence comparisons of the complete open reading frames of the sodium channel genes identified one additional novel mutation (M815I), which was located in the IIS1-2 extracellular loop of the α-subunit, from the permethrin-resistant head louse population. Absolute conservation of the Met815 residue at the corresponding positions within sodium channels from all known susceptible populations of insect species implied that the M815I mutation likely has a functional significance in resistance. Sequence analyses of cloned cDNA fragments and genomic DNA fragments from individual louse samples, both containing the three mutation sites, confirmed that all the mutations exist en bloc as a haplotype. Northern blot analysis identified a single 7.2 kb transcript. The comparison of complete open reading frame sequences (6156 bp) of sodium channel gene between head and body lice revealed 26 polymorphic nucleotides, of which only one resulted in a conservative amino acid substitution (glutamic versus aspartic acid at 11th amino acid position). The virtual identity in nucleotide sequences indicated that both body and head lice are conspecific, and lends justification of the use of the body louse as a surrogate organism for the head louse in biochemical and molecular biology studies. Conserved point mutations resulting in knockdown resistance to the pyrethrins, the pyrethroids, and DDT are suitable for detection by various DNA-diagnostic protocols for monitoring and resistance management.