ETIOLOGY AND PATHOGENESISTop
1. Etiologic agent: Two species of louse, Pediculus humanus (P humanus capitis [head louse] and P humanus corporis [body louse]) and Pthirus pubis (pubic louse). Head lice are 2 to 3 mm long and dorsoventrally flattened. Numerous subspecies can be found around the world. Compared with head lice, body lice are slightly larger, often take on a lighter color, and their body proportions are different. Body lice inhabit the human skin and lay eggs in clothing seams. Pubic lice (Figure 10.9-1) are smaller than head lice, their bodies are wider and shorter, and their second and third pairs of legs have prehensile hooked claws. Lice feed on human blood and pierce the host’s skin every few hours. The parasites can survive without a host only for several days but can crawl a few meters to find a host. Females live 1 to 3 months and lay hundreds of eggs during their lifespan (Figure 10.9-2). Using a sticky secretion produced in their glands, female lice attach eggs to hair close to the scalp or to clothing. After hatching, louse nymphs reach the adult stage within 2 to 3 weeks.
2. Pathogenesis: Lice pierce the host’s skin and feed on human blood. Their saliva irritates the skin and causes pruritus and lesion formation. Each louse pierces the host’s skin 5 times a day. Its saliva contains compounds with various biologic properties, including anticoagulative and local anesthetic effects. Proteins from louse saliva can cause allergic reactions, within 3 to 4 weeks after the bite on average, which leads to persistent severe pruritus.
3. Reservoir and transmission: The reservoir of lice is humans with pediculosis. Additionally, the reservoir of body lice is the clothing in which the eggs are laid. Pediculosis is usually transmitted through close contact with an affected individual. Transmission by contact with objects from the patient’s environment is controversial. Pediculosis pubis is mainly transmitted through sexual contacts, while head lice are commonly spread by sharing brushes, combs, hair bands, and other hair accessories with an affected individual.
4. Incubation and contagious period: Pruritus may be noticed within the first few days or up to 20 days from infestation. Communicability continues throughout the infestation period and as long as living lice are present. If pediculosis is left untreated, the contagious period may be very long.
Clinical Features and Natural HistoryTop
Pruritus is the dominant symptom, which may cause irritability and therefore impair everyday activity.
Skin lesions include papules, erythema, or urticaria; severe dermatitis may also develop. Pediculosis pubis manifests with pale bluish macules on the skin of the lower abdomen, resulting from intracutaneous extravasations. In pediculosis capitis, eggs are found on hair, and the distance from the scalp where they are present indicates how long the infestation lasts (the greater the distance from the scalp, the longer the disease). Live nymphs are more difficult to spot.
Typical lesion sites:
1) Pediculosis capitis: Scalp covered with hair, particularly the area behind the ears and the neck; sometimes eyebrows, eyelashes, and facial hair.
2) Pediculosis corporis: Skin of the entire body. In neglected cases eczema, excoriations, hyperpigmentation, scars, and lichenification are observed. No eggs are found on the skin. Eggs are laid in clothing seams, but lice feed on the host’s blood.
3) Pediculosis pubis: Pubic and inguinal areas, thighs, armpits, sometimes hair on the chest, abdomen, and head, as well as eyebrows and eyelashes.
Local lymph nodes may be enlarged.
If left untreated the infestation may persist for a long time and lead to complications. In severe cases an individual may become infested by up to 20,000 lice. Sometimes spontaneous resolution of pediculosis is observed.
The diagnosis is established based on clinical features and the presence of eggs and live lice (usually microscopic examination is not necessary).
Disseminated eczema (regardless of etiology), atopic dermatitis, seborrheic dermatitis and dandruff, papular urticaria and nodular prurigo, chronic pruritus of various etiologies, folliculitis, drug-induced reactions.
1. Hair combing is the least effective method and takes great effort. It is preferred by people who do not want to use any chemicals. Wet hair is combed with a fine-tooth comb, from the scalp to hair ends, for 15 to 30 minutes, every 3 days, for 14 to 24 days.
2. An electronic comb emits a small electric charge on its teeth, which is lethal to lice. This method does not remove louse eggs. It is used on dry hair, which should be thoroughly brushed afterwards. An electronic lice comb should be used daily for 2 weeks, and prophylactically, 1 to 2 times a week.
3. A thermic comb (LouseBuster) with a hairdryer-like applicator blows warm air that kills and dries out lice. The device is particularly effective in removing louse eggs. Dry air reaches the scalp and causes dehydration and death of lice at all stages of development. Malathion cannot be used with a thermic comb, as the combination is inflammable.
4. Shaving of the affected areas, particularly in pediculosis pubis.
5. Egg removal after pharmacologic treatment (see below): Hair (preferably wet) should be thoroughly combed using a fine-tooth comb. Specific substances resolving the glue with which eggs are attached to hair are used to ease the removal of eggs from hair shafts close to the scalp, for instance, a mixture of vinegar and water diluted in a ratio of 1:2 or a ready-to-use set for egg removal.
Primary treatment involves the use of topical agents (according to preferences):
1) Cyclomethicone and dimethicone: Highly effective (97%) silicone products, which are safe for humans. The silicones cover eggs and lice with a thin, impermeable layer that deprives the parasite of air and causes its death. Ready-to-use products containing one of those substances or both are available. They can be used by individuals at any age (some of them are not approved in children aged <6 months or require special medical attention in this age group) as well as in pregnant women. Apply the product thoroughly to dry hair, from roots to ends, separating hair into smaller strands. Rub it carefully into the scalp and hair roots and wash out with a shampoo after 1 hour. Hair should be properly combed afterwards. The treatment can be repeated after a week.
2) Permethrin kills lice and eggs by paralyzing their nervous system. A shampoo containing permethrin is applied to wet hair. The foam should be left on hair for 10 minutes and then rinsed with water. After that hair should be carefully combed. The treatment can be repeated after 7 to 10 days. Permethrin can be used in individuals aged ≥3 years. Lice may become resistant to treatment. A 5% permethrin cream may be used topically in pediculosis corporis and pediculosis pubis—it is left on the affected area for 8 hours and then washed out.
3) Tincture of larkspur and acetic acid is an insecticide. Due to possible skin absorption, its use is not recommended in pregnant women and children aged <6 years. Apply a large amount of the liquid to the scalp. Then cover the scalp tightly with foil or a cap for 2 to 3 hours. Finally, wash your hair with a shampoo and comb it. It is recommended to repeat the treatment every 10 to 14 days until recovery. The tincture should not be applied to open wounds and severely inflamed skin, as it increases the absorption.
4) Tincture of wormwood, tansy, and acetic acid is an insecticide. It should not be used in children aged <7 years and in pregnant women. Apply a large amount of the liquid to the scalp. Then cover the scalp with foil for 2 to 3 hours. Finally, rinse your hair thoroughly and comb. The treatment should be repeated every 2 weeks until recovery. The tincture should not be applied to open wounds and severely inflamed skin.
5) Other drugs (unavailable in some countries or used only in veterinary medicine): 0.5% to 1% malathion, 0.9% spinosad, 1% lindan (gamma-hexachlorocyclohexane), ivermectin. If eyebrows and eyelashes are affected, multiple applications of petrolatum jelly, mechanical removal of nonviable eggs, or 0.25% physostigmine ophthalmic ointment are used.
1. Machine washing of bedding, towels, and headwear.
2. Cleaning and disinfection of brushes, combs, hair bands, and other hair accessories.
3. In the case of pediculosis corporis, clothing should be discarded or washed and ironed on high heat (particularly the seams). It is recommended not to wear clothing used during infestation for 2 weeks.
4. If stuffed toys or other objects are suspected to contain lice, they can be sealed off with foil and left unused for several days.
1. Secondary bacterial infection (impetiginization).
2. Body lice are vectors of communicable diseases and may spread and cause epidemic typhus, louse-bone relapsing fever, and trench fever.
Nonpharmacologic treatment is the safest approach in pregnant women; however, it has limited efficacy. Dimethicone and cyclomethicone are the safest drugs to be used in this population. Permethrin is also quite safe (category B drug). Tinctures containing plant extracts must be avoided.
Pediculosis is easy to treat, but nonproper therapy leads to recurrences.
1. Regular cleaning of brushes, combs, and hair bands.
2. Follow-up and potential treatment of all individuals who share the living space with the affected person.
3. Avoiding sharing headwear, pillows, combs, and hair accessories with others.
4. Avoiding direct contact with other people’s heads.
5. Avoiding crowdy places where the hygiene level is low.
6. If cases of pediculosis occur in a school or during a trip, commercially available lotions containing plant extracts and oils (rosemary, geranium, tea tree, and eucalyptus oils) can be used prophylactically every day.
Figure 10.9-1. Pubic louse eggs attached to chest hair in an immunosuppressed patient.
Head louse eggs.