Merck Manual

Please confirm that you are a health care professional

honeypot link

Lice

(Pediculosis)

By

James G. H. Dinulos

, MD, Geisel School of Medicine at Dartmouth

Last full review/revision May 2020| Content last modified May 2020
Click here for Patient Education
Topic Resources

Lice 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 humanus var. capitis), body (P. humanus var. corporis), or pubis (Phthirus pubis). The 3 kinds of lice differ substantially in morphology and clinical features (see Figure: A close-up look at lice A close-up look at lice Lice 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... read more A close-up look at lice ). Head lice and pubic lice live directly on the host; body lice live in garments. All types occur worldwide.

Head lice

A close-up look at lice

A close-up look at lice

Head lice are most common among girls aged 5 to 11 but can affect almost anyone; infestations are less common 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 the table Treatment Options for Lice Treatment Options for Lice Lice 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... read more 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 Wood light Diagnostic tests are indicated when the cause of a skin lesion or disease is not obvious from history and physical examination alone. These include Patch testing Biopsy Scrapings Examination... read more Wood light . 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 dryer, 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 54° C (130° F) for 30 minutes. Items that cannot be washed may be placed in airtight plastic bags for 2 weeks to kill the lice, which live only about 10 days.

Body lice

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 65° C [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).

Images of Pubic Lice

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 ceruleae) 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 lamp) or microscopic analysis. A supporting sign of infestation is scattering of dark brown specks (louse excreta) on skin or undergarments.

Treatment is outlined in the table Treatment Options for Lice Treatment Options for Lice Lice 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... read more 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.

Table
icon

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.

Drugs Mentioned In This Article

Drug Name Select Trade
No US brand name
STROMECTOL
ELIMITE, NIX
No US brand names
Click here for Patient Education
NOTE: This is the Professional Version. CONSUMERS: Click here for the Consumer Version
Professionals also read

Test your knowledge

Lichen Planus
A 65-year-old man comes to the office because he has had an itchy rash for the past 6 weeks that is purple in color on the flexor aspect of each wrist. On physical examination, the lesions are flat and smooth. Biopsy is performed, and lichen planus is confirmed. Which of the following laboratory studies should be done? 
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
Download the Manuals App iOS ANDROID
 

Also of Interest

 
TOP