What is Pediculosis?
Nymphs and adults of all three kinds of human lice feed at least once a day, ingesting human blood exclusively. Head lice (Pediculus capitis) infest mainly the hair of the scalp, body lice (Pediculus humanus) the clothing, and crab or pubic lice (Phthirus pubis) mainly the hair of the pubis. The saliva of lice produces an intensely irritating maculopapular or urticarial rash in sensitized persons.
Female head and pubic lice cement their eggs firmly to hair and female body lice to clothing. A nymph hatches after ~10 days of development. The empty egg (nit) may remain affixed for months thereafter.
Head lice infest ~1% of elementary school–aged children. Head lice are transmitted mainly by direct head-to-head contact rather than by fomites (shared headgear, grooming implements, bedding).
Infestations by head lice tend to be asymptomatic. Pruritus, due mainly to hypersensitivity to the louse’s saliva, generally is transient and mild. Head lice removed from a person succumb to desiccation and starvation within ~1 day. Head lice are unimportant as vectors of pathogenic agents.
Body lice remain on clothing except when feeding and generally succumb in 2 days if separated from their host. These lice mainly infest disaster victims or indigent people who are in close contact with other infested individuals. Body lice are acquired by direct contact or by sharing of clothing and bedding.
These lice are vectors for the agents of louse-borne typhus, louse-borne relapsing fever, and trench fever. Pruritic lesions are particularly common around the neckline. Chronic infestations result in a postinflammatory hyperpigmentation and thickening of skin known as vagabonds’ disease.
The crab or pubic louse is transmitted mainly by sexual contact. These lice occur mainly on pubic hair and less frequently on hair of the axillae and the face, including the eyelashes.
Children and adults may acquire pubic lice by sexual or close nonsexual contact. Intensely pruritic lesions and blue macules ~3 mm in diameter (maculae ceruleae) develop at the site of bites. Blepharitis commonly accompanies infestations of the eyelashes.
Pediculosis may be suspected upon the detection of nits on hairs or in clothing, but confirmation should be based upon discovery of a live louse.
Generally, treatment is warranted only if live lice are discovered. The presence of nits alone is evidence of former—but not current—infestation. Mechanical removal of lice and their eggs by means of a fine-toothed louse or nit comb often fails to eliminate infestations.
Treatment of newly identified active infestations generally relies upon a 10-min application of ~1% permethrin or pyrethrins, with a second application 10 days later.
Lice persisting after this treatment may be resistant to pyrethroids (see below). Chronic infestations may be treated for 12 hours with 0.5% malathion.
Lindane is applied for just 4 min but seems less effective and may pose a greater risk of adverse reaction, particularly when misused. Resistance of head lice to permethrin, malathion, and lindane has been reported.
Ivermectin may be useful in cases of resistance to malathion and permethrin but has not been approved for this purpose by the FDA.
Although children infested by head lice are frequently isolated or excluded from school, this practice is increasingly seen as unjustified.
Body lice are usually eliminated by bathing and by changing to laundered clothes. Application of topical pediculicides from head to foot may be necessary for hirsute patients. Clothes and bedding are effectively deloused by heating in a clothes dryer at 55°C for 30 min or by heat-pressing.
Emergency mass delousing of persons and clothing may be warranted during periods of civil strife and after natural disasters to reduce the risk of pathogen transmission by body lice. Pubic lice infestations are treated with topical pediculicides except for eyelid infestations (phthiriasis palpebrum), which generally respond to a coating of petrolatum applied for 3–4 days.
Harrisson’s Principles of Internal Medicine