Measuring local genetic variation in permethrin-resistant head lice (Phthiraptera: Pediculidae) from Buenos Aires, Argentina

The cosmopolitan ectoparasite human head louse, Pediculus humanus capitis (De Geer)(Phthiraptera:Pediculidae), affects mostly school-aged children, with infestations reported every year mainly due to louse resistance to pyrethroids. One of the main resistance mechanisms of pyrethroids is the target site insensitivity (kdr), which is caused by single-nucleotide point mutations (SNPs) located in the voltage-sensitive sodium channel gene. In this study, we analyzed individual head lice toxicologically via the description of their susceptibility profile to permethrin and genetically through the genotypification of their kdr alleles as well as nuclear microsatellite loci. Lice were collected from 4 schools in the city of Buenos Aires, Argentina. The resistance ratios varied from 33.3% to 71.4%, with a frequency of the T917I kdr mutation of 87.31% and with 83.6% of the head lice being homozygous resistant to pyrethroids. Microsatellite data indicated that all the louse school populations had genotype proportions that deviated from Hardy-Weinberg expectations, with FIS > 0 reflecting a deficit of heterozygotes. Bottleneck analysis suggested that all louse school populations underwent a recent reduction in population sizes, while 3 of the 4 schools had gene flow values around 1, indicating ongoing gene flow among those schools. Our study suggests that school louse populations in the city of Buenos Aires may form a metapopulation, where each school represents a small population that undergoes extinction and recolonization processes under strong permethrin selection. This is the first multilevel analysis integrating toxicological, kdr-genotyping, and microsatellite data in human louse populations.

Determination of knockdown resistance allele frequencies in global human head louse populations using the serial invasive signal amplification reaction

BACKGROUND

Pediculosis is the most prevalent parasitic infestation of humans. Resistance to pyrethrin- and pyrethroid-based pediculicides is due to knockdown (kdr)-type point mutations in the voltage-sensitive sodium channel alpha-subunit gene. Early detection of resistance is crucial for the selection of effective management strategies.

RESULTS

Kdr allele frequencies of lice from 14 countries were determined using the serial invasive signal amplification reaction. Lice collected from Uruguay, the United Kingdom and Australia had kdr allele frequencies of 100%, while lice from Ecuador, Papua New Guinea, South Korea and Thailand had kdr allele frequencies of 0%. The remaining seven countries investigated, including seven US populations, two Argentinian populations and populations from Brazil, Denmark, Czech Republic, Egypt and Israel, displayed variable kdr allele frequencies, ranging from 11 to 97%.

CONCLUSION

The newly developed and validated SISAR method is suitable for accurate monitoring of kdr allele frequencies in head lice. Proactive management is needed where kdr-type resistance is not yet saturated. Based on sodium channel insensitivity and its occurrence in louse populations resistant to pyrethrin- and pyrethroid-based pediculicides, the T917I mutation appears to be a key marker for resistance. Results from the Egyptian population, however, indicate that phenotypic resistance of lice with single or double mutations (M815I and/or L920F) should also be determined.

Three mutations identified in the voltage-sensitive sodium channel alpha-subunit gene of permethrin-resistant human head lice reduce the permethrin sensitivity of house fly Vssc1 sodium channels expressed in Xenopus oocytes

Point mutations in the para-orthologous sodium channel alpha-subunit of the head louse (M815I, T917I, and L920F) are associated with permethrin resistance and DDT resistance. These mutations were inserted in all combinations using site-directed mutagenesis at the corresponding amino acid sequence positions (M827I, T929I, and L932F) of the house fly para-orthologous voltage-sensitive sodium channel alpha-subunit (Vssc1(WT)) gene and heterologously co-expressed with the sodium channel auxiliary subunit of house fly (Vsscbeta) in Xenopus oocytes. The double mutant possessing M827I and T929I (Vssc1(MITI)/Vsscbeta) caused a approximately 4.0mV hyperpolarizing shift and the triple mutant, Vssc1(MITILF)/Vsscbeta, caused a approximately 3.2mV depolarizing shift in the voltage dependence of activation curves. Vssc1(MITI)/Vsscbeta, Vssc1(TILF)/Vsscbeta, and Vssc1(MITILF)/Vsscbeta caused depolarizing shifts ( approximately 6.6, approximately 7.6, and approximately 8.8mV, respectively) in the voltage dependence of steady-state inactivation curves. The M827I and L932F mutations reduced permethrin sensitivity when expressed alone but the T929I mutation, either alone or in combination, virtually abolished permethrin sensitivity. Thus, the T929I mutation is the principal cause of permethrin resistance in head lice. Comparison of the expression rates of channels containing single, double and triple mutations with that of Vssc1(WT)/Vsscbeta channels indicates that the M827I mutation may play a role in rescuing the decreased expression of channels containing T929I.