Lice infestation of head (capitis), body (corporis), or pubic area (pubis); intense itch and visible lice or nits.
Also known as: lice, pediculosis capitis, pediculosis corporis, pediculosis pubis, head lice, crabs
Overview
Cutaneous infestation by parasitic blood-feeding lice. Three species infect humans: Pediculus humanus capitis (head louse), P. humanus corporis (body louse), and Pthirus pubis (pubic/crab louse).
Epidemiology
Head lice affect 6-12 million US children annually, ages 3-11 most common, female > male. Body lice are a disease of poverty, homelessness, displacement, and war — vector for Bartonella quintana, Rickettsia prowazekii (epidemic typhus), Borrelia recurrentis. Pubic lice are sexually transmitted in adults; incidence declining with grooming practices.
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Question 1DermatologyEasy
A 7-year-old girl is brought in for intense scalp itching for 2 weeks after a school outbreak. Exam reveals excoriations behind the ears and on the nape of the neck. Tiny grayish-white oval structures are firmly attached to hair shafts close to the scalp, and a few small wingless insects are visible on careful inspection. Which is the most likely diagnosis?
AAtopic dermatitis
BTinea capitis
CSeborrheic dermatitis
DPediculosis capitis
Reveal answer & full explanation
Correct answer: D — Pediculosis capitis
AAtopic dermatitis
BTinea capitis
CSeborrheic dermatitis
DPediculosis capitis✓
Why Pediculosis capitis is correct
Pediculosis capitis (head lice, Pediculus humanus capitis) presents with scalp pruritus, posterior cervical lymphadenopathy, and excoriations behind the ears and at the nape of the neck.
Diagnosis is confirmed by visualizing live lice or viable nits (eggs) cemented to hair shafts within ~6 mm of the scalp.
Why the others are wrong
Tinea capitis — produces scaly patches with hair loss and may show a kerion; lacks nits cemented to hair shafts.
Seborrheic dermatitis — presents with greasy yellow scale, not nits or live insects.
Atopic dermatitis — typically spares the scalp in older children and lacks nits.
Additional high-yield points
First-line topical pediculicides include permethrin and pyrethrins.
Alternatives include ivermectin (oral or topical) and malathion, all targeting louse neuromuscular function.
Question 2DermatologyEasy
A 7-year-old girl is brought to clinic for 2 weeks of intense scalp itching that worsens at night and disturbs her sleep. Two classmates were recently treated for a similar problem. On examination, she has excoriations at the nape of the neck and tender posterior cervical lymphadenopathy. Inspection behind the ears reveals tiny moving specks on the scalp and numerous oval yellow-white concretions, each firmly cemented to a hair shaft within a few millimeters of the scalp and resistant to sliding along the hair. Which of the following is the most likely diagnosis?
ASeborrheic dermatitis
BScabies
CPediculosis capitis
DTinea capitis
Reveal answer & full explanation
Correct answer: C — Pediculosis capitis
ASeborrheic dermatitis
BScabies
CPediculosis capitis✓
DTinea capitis
Why Pediculosis capitis is correct
School-age child with a classroom outbreak, intense nocturnal scalp pruritus (delayed hypersensitivity to louse saliva), and posterior cervical/occipital lymphadenopathy is the textbook head-lice picture.
The diagnostic finding is visualization of live lice (moving specks, best seen behind the ears and at the nape) plus viable nits — oval yellow-white 0.5-1 mm eggs firmly cemented within 6 mm of the scalp that do NOT slide freely along the hair shaft.
Diagnosis is clinical by direct visualization; the wet-comb method is the most sensitive confirmatory technique.
Why the others are wrong
Seborrheic dermatitis produces greasy yellow scale and flaking (dandruff), but the scale moves freely off the hair, whereas nits are firmly attached and resist sliding, and no live lice are present.
Tinea capitis is a dermatophyte infection causing patchy alopecia with scale and broken hairs, often boggy/kerion and KOH-positive; it does not produce nits cemented to hair shafts or visible moving lice.
Scabies is a Sarcoptes scabiei infestation with burrows in finger web spaces, wrists, and the waistline; it spares the scalp in immunocompetent older children and shows no lice or nits.
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Head lice: school-age children, shared brushes/hats/helmets, sleepovers
Body lice: homelessness, poor hygiene, crowded conditions, refugee settings
Pubic lice: sexual activity (high turnover), other STIs
Long hair (slight risk increase for head lice)
Pathophysiology
Lice feed on human blood every 4-6 hours and cannot survive >24-48 hours off the host. Females lay eggs (nits) cemented to hair shafts close to scalp/skin; nits hatch in 7-10 days; adults mature in another 7-10 days and live 30 days. Itching is delayed hypersensitivity to louse saliva injected during feeding.
Clinical presentation
Symptoms
Intense pruritus of scalp (capitis), trunk (corporis), or pubic area (pubis)
Sleep disturbance, irritability in children
Posterior cervical or occipital lymphadenopathy (head lice)
Signs / physical exam
Head lice: live lice on scalp (best seen behind ears and at nape); nits — oval yellow-white 0.5-1 mm — firmly attached to hair shaft within 6 mm of scalp; viable nits closer to scalp, dead/hatched nits further out; excoriations and impetigo from scratching
Body lice: lice and nits in seams of clothing (not on body); excoriations, papules, hyperpigmentation, lichenification on shoulders, trunk, waistline ('vagabond's disease')
Pubic lice: lice and nits attached to pubic hairs; may infest eyelashes (especially in children — pediatric phthiriasis palpebrarum is a sentinel for sexual abuse), eyebrows, axillae, beard, perianal hair; maculae ceruleae — bluish macules at feeding sites
Classic findings
Live louse or viable nit close to scalp = active infestation; nits >6 mm from scalp may be dead/hatched (not necessarily active).
Differential diagnosis
Seborrheic dermatitis / dandruff (vs nits) — Scale moves freely; nits are firmly attached to hair shaft and slide along it only with effort
Hair casts / hair sleeves — White cylindrical concretions slide easily; no lice
Scabies — Burrows in web spaces; not localized to scalp
Eczema / contact dermatitis — No live lice or nits on inspection
Tinea capitis — Patchy alopecia with scale and broken hairs; KOH positive
Folliculitis — Pustules around hair follicles; no lice
Diagnostic workup
Diagnostic criteria
Visualization of live louse or viable nit confirms infestation.
Labs
Clinical diagnosis by direct visualization of live lice or viable nits
Wet-comb method (lice comb through wet conditioned hair) is most sensitive
Dermoscopy or microscopy to confirm nits and viability
Pubic lice → screen for other STIs (gonorrhea, chlamydia, syphilis, HIV)
Body lice → consider screening for Bartonella, typhus, relapsing fever in appropriate epidemiologic settings
Imaging
Not indicated
Diagnostic algorithm
Variant
Site
First-Line Therapy
Pediculosis capitis
Scalp — nits within 6 mm of scalp
Permethrin 1% or pyrethrin (repeat day 7-10); resistance → ivermectin 0.5% / spinosad / malathion; wet combing
Pediculosis corporis
Lice in clothing seams; trunk/waist papules
Improve hygiene, launder clothing at 149°F; pediculicide rarely needed
Pediculosis pubis
Pubic/perianal/axillary/beard hair; eyelashes (children = abuse red flag)
Head lice: topical pediculicide — permethrin 1% lotion or pyrethrin/piperonyl butoxide shampoo (OTC); apply 10 min, rinse; REPEAT in 7-10 days; in areas with documented resistance, use first-line alternative — topical ivermectin 0.5% lotion (single application, no nit combing required), spinosad 0.9% topical suspension, or malathion 0.5% lotion
Wet combing with fine-toothed nit comb (e.g., LiceMeister) every 3-4 days for 2 weeks — adjunct to pediculicide or stand-alone
Decontamination: wash bedding/clothing in hot water (≥130°F) and dry on high heat; non-washable items bagged for 2 weeks (lice die in 1-2 days off host); soak combs/brushes in hot water 10 min
Body lice: improve hygiene — bathe, change into clean clothes; launder all clothing/bedding at 149°F (65°C); pediculicide rarely needed; treat secondary bacterial infections
Pubic lice: topical permethrin 1% or pyrethrin to affected areas; repeat in 7-10 days; treat sexual contacts; eyelash involvement — petrolatum (Vaseline) BID × 8 days to suffocate lice, manual removal of nits; physostigmine and oral ivermectin alternatives
Pediculicide-resistant or refractory
Oral ivermectin 400 mcg/kg on day 0 and day 7 (off-label) — effective for resistant head lice
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