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Lice

(Pediculosis)

by James G. H. Dinulos, MD

Lice (pediculosis) can infect the scalp, body, pubis, and eyelashes. Head lice are transmitted by close contact; body lice are transmitted in cramped, crowded conditions; and pubic lice are transmitted by sexual contact. Symptoms, signs, diagnosis, and treatment differ by location of infestation.

Lice are wingless, blood-sucking insects that infest the head ( Pediculus humanusvar. capitis), body ( P. humanus var. corporis), or pubis ( Phthirus pubis). The 3 kinds of lice differ substantially in morphology and clinical features. Head lice and pubic lice live directly on the host; body lice live in garments. All types occur worldwide.

Head lice

Head lice are most common among girls aged 5 to 11 but can affect almost anyone; infestations are rare in blacks. Head lice are easily transmitted from person to person with close contact (as occurs within households and classrooms) and may be ejected from hair by static electricity or wind; transmission by these routes (or by sharing of combs, brushes, and hats) is likely but unproved. There is no association between head lice and poor hygiene or low socioeconomic status.

Infestation typically involves the hair and scalp but may involve other hair-bearing areas. Active infection usually involves 20 lice and causes severe pruritus. Examination is most often normal but may reveal scalp excoriations and posterior cervical adenopathy.

Diagnosis depends on demonstration of living lice. Lice are detected by a thorough combing-through of wet hair from the scalp with a fine-tooth comb (teeth of comb about 0.2 mm apart); lice are usually found at the back of the head or behind the ears. Nits are more commonly seen and are ovoid, grayish white eggs fixed to the base of hair shafts. Each adult female louse lays 3 to 5 eggs/day, so nits typically vastly outnumber lice and are not a measure of severity of infestation.

Treatment is outlined in Treatment Options for Lice. Drug resistance is common and should be managed with use of oral ivermectin and by attempting to rotate pediculicides. After applying a topical pediculicide, nits are removed by using a fine-tooth comb on wet hair (wet combing). Termination or removal of live (viable) nits is important in preventing reinfestation; live nits fluoresce on illumination with a Wood lamp. Most pediculicides also kill nits. Dead nits remain after successful treatment and do not signify active infection; they do not have to be removed. Nits grow away from the scalp with time; the absence of nits less than one fourth of an inch from the scalp rules out current active infection. Hot air has been shown to kill > 88% of nits but has been variably effective in killing hatched lice. Thirty minutes of hot air, slightly cooler than a blow drier, may be an effective adjunctive measure to treat head lice.

Controversy surrounds the need to clean the personal items of people with lice or nits and the need to exclude children with head lice or nits from school; there are no conclusive data supporting either approach. However, some experts recommend replacement of personal items or thorough cleaning, followed by drying at 130° F for 30 min. Items that cannot be washed may be placed in airtight plastic bags for 2 wk to kill the lice, which live only about 10 days.

Body lice

Body lice primarily live on bedding and clothing, not people, and are most frequently found in cramped, crowded conditions (eg, military barracks) and in people of low socioeconomic status. Transmission is by sharing of contaminated clothing and bedding. Body lice are main vectors of epidemic typhus (see Epidemic Typhus), trench fever (see Trench Fever), and relapsing fever (see Relapsing Fever).

Body lice cause pruritus; signs are small red puncta caused by bites, usually associated with linear scratch marks, urticaria, or superficial bacterial infection. These findings are especially common on the shoulders, buttocks, and abdomen. Nits may be present on body hairs.

Diagnosis is by demonstration of lice and nits in clothing, especially at the seams.

Primary treatment is thorough cleaning (eg, cleaning, followed by drying at 149° F) or replacement of clothing and bedding, which is often difficult because affected people often have few resources and little control over their environment.

Pubic lice

Pubic lice (“crabs”) are sexually transmitted in adolescents and adults and may be transmitted to children by close parental contact. They may also be transmitted by fomites (eg, towels, bedding, clothing). They most commonly infest pubic and perianal hairs but may spread to thighs, trunk, and facial hair (beard, mustache, and eyelashes).

Pubic lice cause pruritus. Physical signs are few, but some patients have excoriations and regional lymphadenopathy and/or lymphadenitis. Pale, bluish gray skin macules (maculae caeruleae) on the trunk, buttocks, and thighs are caused by anticoagulant activity of louse saliva while feeding; they are unusual but characteristic of infestation. Eyelash infestation manifests as eye itching, burning, and irritation.

Diagnosis is by demonstration of nits, lice, or both by close inspection (Wood light) or microscopic analysis. A supporting sign of infestation is scattering of dark brown specks (louse excreta) on skin or undergarments.

Treatment is outlined in Treatment Options for Lice. Treatment of eyelid and eyelash infestation is often difficult and involves use of petrolatum, physostigmine ointment, oral ivermectin, or physical removal of lice with forceps. Sex partners should also be treated.

Treatment Options for Lice

Therapy

Instructions

Comments

Lice, head

Malathion 0.5%

Apply to dry hair and scalp, wash and rinse in 8–12 h, shampoo scalp, and remove nits

May repeat in 7–9 days if live nits (nits closer than a 1/4 inch away from the scalp) are seen

Highly effective but not 1st-line treatment because of flammability and unpleasant odor

Permethrin, other pyrethroids, pyrethrins*

Wash hair and apply to wet hair, behind ears and on nape; wash off in 10 min

May repeat in 7 days if live nits (closer than a 1/4 inch away from the scalp) are seen

Contraindicated in patients sensitive to the chrysanthemum family of plants

Wet combing with a metal nit comb

Should be combined with all of the therapies

Lindane 1% shampoo

Apply shampoo, lather for 4–5 min, rinse, and comb with fine-tooth comb

Repeat in 1 wk

Increasing resistance to drug

Toxicity (eg, seizures) not typical with treatment of head lice but not recommended for children < 2 yr, people with an uncontrolled seizure disorder, or for pregnant or lactating women

Cannot be used on eyelashes

Ivermectin

200 mcg/kg po for 1 dose

Repeat in 7–10 days

Useful for resistant lice

Cetaphil ® cleanser

Apply, wait 2 min, comb out excess, blow dry hair, wait 8 h, then shampoo and remove nits with wet combing

Repeat weekly as necessary

Lice, body

Treatment of pruritus and secondary infection

Topical measures not used because body lice are found in clothing

Wash clothes and linens and dry them at at least 149° F

Dry cleaning or ironing clothes

Lice, pubic

Lindane 1% (60 mL) shampoo

Same as for head lice

Same as for head lice

Pyrethrins* with piperonyl butoxide (60 mL) shampoo

Apply to dry hair and skin, leave on for 10 min, rinse, and repeat in 7–10 days

Cannot be applied more than twice in 24 h

Permethrin 1% (60 mL) cream

Same as for head lice

Must repeat in 10 days

Lice, eyelashes

Petrolatum ointment

Apply 3–4 times/day for 8–10 days

Fluorescein drops 10–20%

Applied to the eyelids

Provides immediate pediculicidal effect

*Pyrethrins are natural components of chrysanthemum flowers, with strong insecticidal activity, pyrethroids are synthetic and natural relatives of pyrethrin, and permethrin is a commonly used synthetic pyrethroid. Pyrethrins are combined with a piperic acid derivative (piperonyl butoxide) to enhance efficacy.

Key Points

  • Head and pubic lice live on people, whereas body lice live in garments.

  • Confirm the diagnosis of lice by finding live lice or live nits.

  • Treat head or pubic lice with a topical drug (eg, a pyrethroid) or oral ivermectin.

  • Treat body lice symptomatically and by eliminating the source of lice.

Resources In This Article

Drugs Mentioned In This Article

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  • STROMECTOL
  • No US brand names
  • No US brand name
  • ELIMITE, NIX

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