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. Which is the most likely diagnosis?
ASeborrheic dermatitis
BPeripilar hair casts
CTrichorrhexis nodosa
DTinea capitis
EPediculosis capitis
Reveal answer & full explanation
Correct answer: E — Pediculosis capitis
ASeborrheic dermatitis
BPeripilar hair casts
CTrichorrhexis nodosa
DTinea capitis
EPediculosis 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
Peripilar hair casts — pseudonits: keratin sleeves that encircle the hair shaft and slide freely along it, whereas nits are cemented to one side of the shaft; casts are usually asymptomatic and not transmissible, so they explain neither intense itching nor a school outbreak.
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.
Trichorrhexis nodosa — like nits, the small grayish-white nodes of trichorrhexis nodosa sit on the hair shaft and do not slide along it, but they are fracture points in the shaft itself, with frayed fibers and easy breakage; the condition does not itch and is not contagious, so it explains neither the intense pruritus nor a school outbreak.
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 2DermatologyMedium
A 23-year-old man has had 10 days of intense itching in the groin. Examination shows tiny crab-shaped lice clinging to the pubic hairs, nits cemented to the hair shafts, and several blue-gray macules on the inner thighs. He shares an apartment with two roommates. He began a relationship with a new sexual partner 3 weeks ago, and that partner has already been notified and treated for pubic lice. Permethrin 1% cream rinse is prescribed today. Which of the following is the most appropriate additional measure at this visit?
AHIV, syphilis, gonorrhea, and chlamydia testing
BInsecticide fumigation of the shared apartment
CTrichomonas vaginalis NAAT on a urine sample
DOral ivermectin for both roommates in the apartment
EEmpiric ceftriaxone and doxycycline treatment
Reveal answer & full explanation
Correct answer: A — HIV, syphilis, gonorrhea, and chlamydia testing
AHIV, syphilis, gonorrhea, and chlamydia testing✓
BInsecticide fumigation of the shared apartment
CTrichomonas vaginalis NAAT on a urine sample
DOral ivermectin for both roommates in the apartment
EEmpiric ceftriaxone and doxycycline treatment
Why HIV, syphilis, gonorrhea, and chlamydia testing is correct
Crab-shaped lice and nits on the pubic hairs, with maculae ceruleae (blue-gray macules from louse bites), point to pediculosis pubis from Phthirus pubis.
Pubic lice spread mainly through sexual contact, so the infestation marks a sexual exposure that could also have transmitted other STIs.
The CDC 2021 STI Treatment Guidelines say patients with pubic lice should be evaluated for other STIs: HIV, syphilis, and gonorrhea/chlamydia NAAT at the exposed sites.
Permethrin has already been prescribed and his partner has already been treated, so STI testing is the measure still needed at this visit, along with advice to launder his bedding, towels, and clothing.
Why the others are wrong
Oral ivermectin for both roommates in the apartment - Oral ivermectin is an alternative regimen for the patient himself, but roommates are not treated just for sharing an apartment. Phthirus pubis spreads mainly by close sexual contact, and ordinary shared living rarely transmits it; laundering his bedding, towels, and clothing covers the small fomite risk (the trap is carrying over the scabies rule of treating all household contacts).
Empiric ceftriaxone and doxycycline treatment - Presumptive gonorrhea/chlamydia treatment is reserved for a clinical syndrome (urethritis, cervicitis, PID, proctitis, epididymitis), a recent partner of someone diagnosed with those infections, or a sexual assault survivor. None applies here: his complaint is pubic itching from lice, and his partner was treated for lice, not for a bacterial STI. Pubic lice signal a sexual exposure, not a confirmed infection, so the CDC approach is to test. Empiric antibiotics also would not replace the HIV and syphilis testing he still needs, and they could partially treat and mask incubating syphilis (the trap is treating presumptively instead of screening).
Insecticide fumigation of the shared apartment - The CDC says fumigating living areas is not needed; fumigant sprays or fogs add no benefit and can be toxic if inhaled or absorbed through the skin. Pubic lice die within about 1-2 days off a human host, so machine-washing and hot-drying the bedding, towels, and clothing used in the last 3 days is enough (the trap is the bed-bug reflex, where professional pest-control treatment usually is needed).
Trichomonas vaginalis NAAT on a urine sample - trichomoniasis is a common curable STI, so testing for it can look thorough, but Trichomonas is not part of routine STI screening in asymptomatic men, and this test would not replace the HIV, syphilis, and gonorrhea/chlamydia evaluation that pubic lice call for (the trap is testing for the wrong STIs).
Additional high-yield points
CDC-recommended treatment: permethrin 1% cream rinse or pyrethrins with piperonyl butoxide, washed off after 10 minutes. Alternatives are malathion 0.5% lotion or oral ivermectin 250 mcg/kg repeated in 7-14 days.
Sex partners from the previous month should be treated, and sexual contact should be avoided until both partners have been treated and re-evaluated. Reassess at 1 week if symptoms persist.
Because the new exposure was only 3 weeks ago, HIV and syphilis tests drawn today may still fall within the window period, so negative results should be repeated later.
Phthirus pubis favors coarse hair (pubic, perianal, axillary, chest, beard, eyebrows, eyelashes) and seldom infests the scalp, so examine every hair-bearing site, including the eyelashes, before settling on a regimen.
Eyelash involvement (pediculosis ciliaris) is treated with an occlusive ophthalmic ointment applied to the lid margins twice daily for 10 days, because pediculicides should not be used near the eyes.
Pubic lice in a child should prompt an evaluation for sexual abuse.
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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 (CDC 2021): permethrin 1% cream rinse or pyrethrins with piperonyl butoxide to affected areas, washed off after 10 min; alternatives are malathion 0.5% lotion (washed off after 8-12 h) or oral ivermectin 250 mcg/kg repeated in 7-14 days; re-evaluate at 1 week if symptoms persist and retreat if lice or eggs at the hair-skin junction are found; treat sex partners from the previous month; eyelash involvement — occlusive ophthalmic ointment or petrolatum to the lid margins BID × 10 days (no pediculicides near the eyes) plus manual removal of nits
Pediculicide-resistant or refractory
Oral ivermectin 400 mcg/kg on day 0 and day 7 (off-label) — effective for resistant head lice
Wet combing every 3-4 days × 4 weeks as monotherapy
Second-line / adjunct
Lindane is no longer recommended (AAP 2022; CDC 2021 STI guidelines) because of neurotoxicity (seizures; FDA boxed warning) and is contraindicated in children — do not use
DO NOT use kerosene, gasoline, or dog/cat shampoos (toxic, ineffective)
No-nit school policies should be abandoned (AAP 2022); a child with nits or live lice should not be excluded or sent home early — notify the caregiver and treat promptly
Complications
Secondary bacterial infection (impetigo, folliculitis, cellulitis) from scratching — most common complication
Cervical/occipital lymphadenopathy
Body lice as vectors: epidemic typhus (R. prowazekii), trench fever (B. quintana), louse-borne relapsing fever (B. recurrentis)
Pubic lice in children — sentinel for sexual abuse (eyelash involvement)
Sleep disturbance, school absences, social stigma
Coexisting STIs in adults with pubic lice
PANCE pearls
Nits >6 mm from scalp are usually empty shells (dead/hatched); only viable nits within 6 mm represent active infestation.
Persistent live lice after permethrin: first check for misdiagnosis, nonadherence or improper application, a missed retreatment, and reinfestation; if live lice persist after a correctly used full course, suspect resistance and switch to a different class (ivermectin 0.5% lotion, spinosad, or malathion).
Body lice live in clothing seams, not on the body — examine the clothes.
Pubic lice in a child — especially eyelash involvement — is a red flag for sexual abuse; report appropriately.
DO NOT exclude children from school for nits alone — current pediatric guidance no longer supports 'no-nit' policies.
References
AAP 2022 — Nolt D, Moore S, Yan AC, Melnick L; AAP Committee on Infectious Diseases, Committee on Practice and Ambulatory Medicine, Section on Dermatology. Head Lice. Pediatrics 2022;150(4):e2022059282
CDC 2024 — CDC Parasites — Lice (Pediculosis) Health Professional Resources
CDC STI 2021 — Workowski KA, Bachmann LH, Chan PA, et al. Sexually Transmitted Infections Treatment Guidelines, 2021 — Pediculosis Pubis. MMWR Recomm Rep 2021;70(No. RR-4):1-187
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