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usmle step 2 ck

Scabies & Lice

Ectoparasitic infestations causing pruritus: scabies produces nocturnal itch with burrows and papules, while lice infest scalp, body, or pubic hair with nits and excoriations

Review date
reviewed 2026-05-04
Reading time
8 min read
Author
Dr Kola Tytler MBBS CertHE MBA MSt MRCGP
Dermatologycommonacute
On this page
Condition details
Dermatology
common
8 min read
reviewed 2026-05-04
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About This Page

This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.

Key points

  • Scabies = Sarcoptes mite infestation causing intense nocturnal itch, burrows, and papules in finger webs, wrists, axillae, waistline, genitalia
  • Treat scabies with permethrin 5% cream neck-down overnight and repeat in 7-14 days; treat close contacts simultaneously
  • Crusted scabies occurs in immunocompromised, elderly, or neurologically impaired patients; highly contagious and needs oral ivermectin + topical scabicide
  • Head lice = pruritic scalp with nits attached to hair shafts near scalp; first-line permethrin 1% or pyrethrins when resistance not suspected
  • Body lice live in clothing seams and can transmit typhus, trench fever, and relapsing fever; treatment is laundering/clothing replacement plus hygiene

Overview

Scabies and lice are common ectoparasitic infestations. Scabies mites burrow into the stratum corneum; symptoms reflect hypersensitivity to mites, eggs, and feces. Lice infest hair-bearing areas or clothing depending on species: Pediculus humanus capitis (head lice), Pediculus humanus corporis (body lice), and Pthirus pubis (pubic lice). The high-yield Step 2 CK principle is simultaneous treatment of contacts and decontamination of clothing/bedding to prevent reinfestation.

Epidemiology

Scabies spreads through prolonged skin-to-skin contact and outbreaks occur in households, shelters, nursing homes, prisons, childcare settings, and hospitals. Crusted scabies has a huge mite burden and can spread with minimal contact. Head lice are common in school-age children and are not a marker of poor hygiene. Body lice are associated with crowding, homelessness, limited laundering access, and can vector systemic infections. Pubic lice are usually sexually transmitted in adults and should prompt STI consideration.

Clinical Features

Symptoms
Scabies: intense generalized pruritus, worse at night; household members often itchy
Scabies symptoms may begin 2-6 weeks after first infestation, sooner with reinfestation
Crusted scabies may have minimal itch despite extensive crusting
Head lice: scalp itching, especially behind ears and occiput
Pubic lice: genital itch; blue-gray macules (maculae ceruleae) may occur
Signs
Scabies burrows: thin wavy lines in finger webs, wrists, elbows, axillae, waist, buttocks, genitalia
Papules, nodules, excoriations; nodular scabies on scrotum/penis is classic
Crusted scabies: thick hyperkeratotic crusted plaques on hands, feet, scalp, nails, widespread skin
Head lice: live lice or nits firmly attached close to scalp; dandruff moves easily, nits do not
Secondary impetigo/cellulitis from scratching

Investigations

First-line
Clinical diagnosisScabies often diagnosed by nocturnal pruritus, typical distribution, burrows, and affected contacts; lice by visualization of live lice/nits
Contact and outbreak assessmentHousehold, sexual contacts, institutional exposure, shelters, nursing homes, school/daycare contacts
Second-line
Skin scraping/mineral oil prepScabies mites, eggs, or scybala under microscopy confirms diagnosis but sensitivity is imperfect
DermoscopyScabies delta-wing/jet-with-contrail sign can support diagnosis; lice/nits may be visualized
STI testingFor pubic lice in sexually active patients; evaluate for other sexually transmitted infections
Specialist
Institutional outbreak controlPublic health/infection-control involvement for crusted scabies or outbreaks in long-term care, shelters, hospitals, or prisons

Management

CDC Scabies and Lice Clinical Care Guidance
1
Scabies
  • Permethrin 5% cream from neck down, leave 8-14 hours, wash off; repeat in 7-14 days
  • Treat household and close physical/sexual contacts simultaneously even if asymptomatic
  • Oral ivermectin 200 mcg/kg, repeat in 7-14 days, is alternative for classic scabies when appropriate; avoid in pregnancy and small children per guidance
  • Wash clothing, towels, bedding in hot water and dry high heat; seal nonwashable items for several days
  • Post-scabetic itch can persist for weeks after successful treatment; treat with topical steroids/antihistamines if no new burrows
2
Crusted scabies
  • Contact precautions and urgent dermatology/infection-control involvement
  • Combination therapy with multiple-dose oral ivermectin plus topical permethrin; keratolytics help remove crusts
  • Treat all contacts and environment aggressively because contagiousness is extreme
3
Head and pubic lice
  • Head lice: permethrin 1% lotion or pyrethrins; repeat based on product instructions because eggs may survive initial treatment
  • Wet combing/nit combing as adjunct; treat only active infestation, not empty distant nits alone
  • Pubic lice: permethrin 1% or pyrethrins; treat sexual contacts and test for STIs
  • Eyelash lice: ophthalmic petrolatum or specialist-directed removal; avoid pediculicides near eyes
4
Body lice
  • Improve hygiene and laundering; wash/dry clothing and bedding hot or replace clothing
  • Pediculicide usually unnecessary if clothing/body hygiene addressed
  • Evaluate for louse-borne infection if fever/systemic illness in high-risk setting

Complications

  • Secondary bacterial infection: Impetigo, cellulitis, post-streptococcal glomerulonephritis in some settings
  • Outbreaks: Crusted scabies can trigger large institutional outbreaks
  • Persistent pruritus: Post-scabetic itch may persist after mites eradicated
  • Louse-borne disease: Body lice can transmit epidemic typhus, trench fever, and relapsing fever
  • Psychosocial distress: Stigma, sleep loss, family/school disruption
USMLE Step 2 CK Exam Tips
  • 1Nocturnal itch + finger web burrows + itchy family members = scabies
  • 2Treat all close contacts at the same time or infestation recurs
  • 3Permethrin 5% for scabies; permethrin 1% for head lice
  • 4Crusted scabies = immunocompromised/elderly + thick crusts + extremely contagious; oral ivermectin plus topical therapy
  • 5Post-treatment itch does not necessarily mean treatment failure; look for new burrows/live mites
  • 6Nits stick to hair shafts; dandruff brushes off
  • 7Pubic lice in adult = consider STI testing and partner treatment
  • 8Body lice live in clothing seams, not primarily on the body; laundering is treatment

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Verified Sources & References

CDC Scabies Clinical Care
CDC Head Lice Treatment
CDC Body Lice Clinical Care
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