Human body and head lice are obligatory human ectoparasites. Although both body and head lice belong to a single species, Pediculus humanus, only body lice are known to be a vector of several bacterial diseases. The higher vector competence of body lice is assumed to be due to their weaker immune response than that of head lice. To test this hypothesis, immune reactions were compared between body and head lice following infections by two model bacteria, Staphylococcus aureus and Escherichia coli, and a human pathogen, Bartonella quintana. Following dermal or oral challenge, the number of these bacteria increased both in hemocoel and alimentary tract of body lice but not in head lice and the viability of the B. quintana was significantly higher in body louse feces, the major route of infection to human. In addition, body lice showed the lower basal/induced transcription level of major immune genes, cytotoxic reactive oxygen species and phagocytosis activity compared with head lice. These findings suggest that a reduced immune response may be responsible, in part, for the increased proliferation and excretion of viable bacteria which are associated with the high level of human infectivity seen in body versus head lice.

The body and head lice (Pediculus humanus humanus and Pediculus humanus capitis, respectively) are hematophagous ectoparasites of humans and only the body louse between two is known to transmit three bacterial diseases through its feces. The proliferation profiles of Bartonella quintana, the causative agent of trench fever, inside the louse body and its excretion patterns were investigated in the two louse subspecies following oral challenge with B. quintana-infected blood meal. The initial density of B. quintana was sustained inside head lice without any noticeable proliferation for the entire period after infection. In contrast, B. quintana proliferated rapidly inside body lice and the maximum density reached at 10 days post-infection. The numbers of bacteria detected in feces from infected lice were almost the same and steadily decreased over time in both body and head lice. Nevertheless, the viability of the bacteria, as determined by fluorescence, was significantly higher in body louse feces, especially at 1 day post-infection and this tendency lasted for 11 days. These findings suggest that excretion of feces containing more viable B. quintana that is proliferated inside body lice following ingestion of infected blood meal is responsible for the higher vector competence of body lice.

The human body and head louse are ectoparasites of humans for thousands of years. Although both body and head lice belong to a single species, Pediculus humanus, only body lice are known to transmit several bacterial diseases. This different vector competence is assumed to be due to their different immune responses. Here, the immune reactions in the alimentary canal were investigated in both two louse subspecies following oral challenge of Escherichia coli as a model gram-negative bacteria. In propagation assay, head lice suppressed the proliferation of E. coli in their epithelial cells effectively at the early stage of infection, resulting in gradual reduction of E. coli number in gut tissues. In contrast, the number of E. coli steadily increased in gut tissues of body lice. No apparent alteration of transcription was observed following E. coli challenge in three important genes for the humoral immune responses, PGRP as a recognition gene and defensin1 and 2 as effector genes. Nevertheless, the basal transcription levels of these genes were higher in the gut tissues of head lice than body lice. Considering that there is no cellular immune reactions in gut tissues, these findings suggest that the higher constitutive transcription levels of major immune genes in head lice can contribute to their initial defense and immune capacity against intestinal bacterial infection.

Body and head lice (Pediculus humanus humanus and Pediculus humanus capitis, respectively) are typical ectoparasites of humans. They differ not only in the ecological habitat but also in the vector competence in spite of their conspecific nature. Only body lice transmit several bacterial pathogens to humans, including Bartonella quintana, Rickettsia prowazekii and Borrelia recurrentis. In this study, the proliferation rates of two model bacteria, a gram positive Staphylococcus aureus and a gram negative Escherichia coli, were determined following bacterial challenge by cuticular injection. Both bacteria proliferated rapidly in body lice at the early stage of bacterial challenge but not in head lice, suggesting that head lice have more sensitive immune responses to these bacteria. In vivo phagocytosis assay revealed that head lice have much higher phagocytic activity against E. coli than body lice whereas only slight differences in phagocytic activity against S. aureus were observed between the two lice species. Taken together, these findings suggest that the reduced phagocytosis activity of body lice contributes, at least in part, to their higher vector competence.

To search for hyper-variable genetic markers that can distinguish regional populations of head lice, we screened the inter simple sequence repeats (ISSR) based on the genome database of body louse, which is closely related conspecific species. An ISSR mining software, SciRoKo 3.4, was employed to excavate ISSR markers from the genome database under the MISA mode (≥ 60 bp repeats). Entire body louse genome (ca.100 Mb) was loaded to SciRoKo for ISSRs mining. A total of 5,336 ISSRs were obtained, and primers specific to individual ISSRs were designed by the Primer 3 and DesignPrimer 1.0 softwares. In order to prove the compatibility of body louse ISSRs to head lice, 31 PCR primers were randomly chosen out of a total of 613 pairs, and their appropriateness was tested by comparing the amplified PCR band patterns between body and head lice. Eleven primer pairs that resulted in poor or little amplification were excluded, and 20 primer pairs were further tested for three head louse populations (California, Panama and Chung-ju, Korea). Finally, nine primer pairs ensuring robust amplification of highly variable band patterns were selected to use for population genetic study of head lice.

A quantitative sequencing (QS) protocol that detects the frequencies of sodium channel mutations (M815I, T917I and L920F) responsible for knockdown resistance in permethrin-resistant head lice was tested as a population genotyping method. Genomic DNA fragments of the sodium channel α-subunit gene that encompass the three mutation sites were PCR-amplified from individual head lice with either resistant or susceptible genotypes, and combined together in various ratios to generate standard DNA template mixtures for QS. Following sequencing, the signal ratios between resistant and susceptible nucleotides were calculated and plotted against the corresponding resistance allele frequencies. Quadratic regression coefficients of the plots were close to 1, demonstrating that QS is highly reliable for the prediction of resistance allele frequencies. Prediction of resistance allele frequencies by QS in several globally collected lice samples including 12 Korean lice populations suggested that permethrin resistance varied substantially amongst different geographical regions. Three local populations of Korean lice were determined to have 9.8-36.7% resistance allele frequencies, indicating that an urgent resistance management is needed. QS should serve as a preliminary resistance monitoring tool for proper management strategies by allowing early resistance detection.