Pathogenesis: P. humanus capitis affects humans and has a high propensity to infest the scalp. These lice live on the host and periodically take a blood meal from the scalp or neck area. In patients with very long hair, the blood meal may be taken from the back or any area of skin that is in contact with the hair. The lice are able to reproduce rapidly. The females, which are a bit larger than the males, lay eggs that hatch and develop into adults capable of reproducing within 4 weeks.
Histology: The histological findings on skin biopsy are similar among all forms of louse bites. Histological evaluation cannot differentiate a louse bite from any other insect bite with certainty. Skin biopsies are rarely performed in these cases, because the diagnosis is made clinically. Biopsy specimens show a nonspecific, mixed superficial and deep inflammatory infiltrate with eosinophils. This may suggest a bite reaction. Unlike tick bites or scabies, in which occasionally tick parts or scabies mites are seen in a biopsy specimen, a biopsy from a patient with a lice infestation will never show mouth parts or other elements of the louse.
Treatment: Therapeutic agents to treat lice are similar among all species of human lice. The most commonly used therapies are based on permethrin; when used appropriately, they show good cure rates. These treatments should be used in conjunction with an agent that helps remove the nits from the hair shafts, and physical removal with a lice hair comb is a must. Therapy should be repeated on a weekly basis. Bedding and clothing need to be disinfected. The use of lindane has decreased because of its potential neurotoxicity. Malathion and oral ivermectin show excellent efficacy. Oral ivermectin needs to be repeated in 1 week, because it does not kill the developing larvae within the nits.
Therapy for body lice also requires complete disinfection of the household or living areas. Overtly infested clothing should be thrown away. Professional fumigation should be considered.
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