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Scabies: Complete Guide

1. The Organism

• Can survive 3 days off a host in a sterile container, 7 days in mineral oil
• Life cycle is completed entirely on human skin
• The female excavates a burrow at 0.5-5 mm/day in the stratum corneum down to the stratum granulosum boundary
• She lays 0-4 eggs/day, up to 50 eggs over her 30-day lifespan
• Eggs hatch in 10-14 days; larvae molt to nymphs then adults; full cycle: ~10-17 days

• Fitzpatrick's Dermatology, p. 3304

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2. Epidemiology & Transmission

• Worldwide, all ages, races, and socioeconomic classes
• Developing countries: 4-100% prevalence depending on region
• Primary route: prolonged close skin-to-skin contact (sexual and non-sexual)
• Fomite transmission: much less common in classic scabies, but live mites have been recovered from bedroom floors, chairs, and couches in every patient's home in one study
• Crusted scabies is highly contagious even from brief contact - up to 6,000 mites/g of skin debris detected on sheets, floors, and curtains
• Incubation: 4-6 weeks in first exposure (no prior immunity); 1-4 days on re-exposure (sensitized host)
• Can be transmitted throughout the entire presymptomatic period
• Red Book (AAP), p. 1030

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3. Pathogenesis

• First infection: itch starts only after 4-6 weeks (sensitization phase)
• Re-infection: itch begins within 1-3 days
• Dead mites and debris continue to cause itch for weeks after successful treatment - this is NOT treatment failure

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4. Classic Clinical Presentation

• Intense nocturnal pruritus - the defining symptom
• Burrows - thin, grey/white, serpiginous, thread-like lines 1-10 mm long (pathognomonic but often obliterated by scratching)
• Erythematous or brown papules
• Secondary excoriations and eczematous changes

• Interdigital web spaces (fingers and toes)
• Flexor wrists
• Extensor elbows
• Anterior axillary folds
• Periumbilical area
• Waistband area
• Lower abdomen and thighs
• Genitalia (penis, scrotum in men; areolae in women)
• Intergluteal cleft
• Buttocks

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5. Unusual / Non-Classic Variants

A. Crusted (Norwegian) Scabies

• Immunocompromised patients (HIV, transplant, biologic therapy)
• Patients with developmental disabilities or dementia
• Debilitated/institutionalized patients
• Patients on long-term topical corticosteroids (even otherwise healthy children)
• Leprosy, paraplegia, elderly

• Millions of mites (vs. 3-50 in classic)
• Thick, hyperkeratotic, crusted, confluent plaques - especially on hands, feet, scalp, elbows, buttocks
• Pruritus is minimal or absent (paradoxically)
• Psoriasiform scaling under the nails; swollen, crusted fingertips
• Generalized distribution including face and scalp
• Extremely contagious even from brief contact or fomites

B. Nodular Scabies

• Persistent red-brown nodules, 2-5 mm, particularly on covered areas: genitalia, groin, axillae
• Represents a granulomatous reaction to dead mite antigens and feces
• Mites are usually absent from nodules
• Can persist for weeks to months after successful eradication
• Does NOT indicate treatment failure
• Management: topical corticosteroids, intralesional corticosteroid injections

C. Scabies in Infants (< 2 years)

• Eruption is vesicular or pustular on palms and soles (unlike adults)
• Face, scalp, and neck are commonly involved (spared in adults)
• Bullous lesions may occur
• Classic web-space burrows may be absent

D. Scabies in the Elderly

• Atypical distribution - may involve scalp, face
• Intermediate mite burden (50-250)
• May be misdiagnosed as eczema or pruritus of aging

E. Bullous Scabies

• Vesicles and bullae, particularly in infants and the elderly
• Can mimic bullous pemphigoid

F. Scabietic "Id" Reaction (Autosensitization)

• Widespread urticarial papules predominantly on trunk and proximal extremities
• Represents a systemic hypersensitivity reaction
• Does NOT require additional acaricide treatment; manages like id reaction

G. Animal Scabies (Canine Mange)

• Sarcoptes scabiei var. canis from dogs
• Causes a self-limited, mild infestation - usually only the area in direct contact with the animal
• Mites cannot replicate in humans; resolves spontaneously when contact is removed

H. Postscabetic Pustulosis

• Reactive phenomenon after successful treatment - especially in infants and young children
• Episodic crops of sterile, pruritic papules and pustules - acral distribution predominantly
• Does NOT require re-treatment with scabicide

I. "7-Year Itch" (Chronic Undiagnosed Scabies)

• Repeated sensitization with multiple infestations producing an eczematized, lichenified appearance
• Tintinalli's Emergency Medicine, p. 1684; Red Book (AAP), p. 1030; Fitzpatrick's, p. 3305

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6. Diagnosis

Clinical

• History + clinical pattern is usually sufficient
• Ask about nocturnal itch, household members with similar symptoms, recent close contact

Dermoscopy

• "Delta-wing jet" or "hang-glider" sign: triangular brown structure (mite's body) at end of burrow

Skin Scraping with Microscopy

• Scrape a burrow with a scalpel blade, transfer to glass slide with mineral oil and coverslip
• Look for mites, eggs, or fecal pellets (scybala)
• Low sensitivity - negative result does NOT rule out scabies
• In classic scabies only 3-50 mites present; easy to miss

PCR (in research/outbreak settings)

• High sensitivity, not widely available clinically

Differential Diagnosis

• Atopic dermatitis / eczema
• Contact dermatitis
• Dermatitis herpetiformis
• Bullous pemphigoid
• Lichen planus
• Urticaria
• Drug eruption
• Linear IgA bullous dermatosis
• Psoriasis
• Pediculosis infestation
• Systemic causes of pruritus (uremia, hypo/hyperparathyroidism)
• Delusions of parasitosis

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7. Complications

1. Secondary bacterial infection (most common complication) - typically Staphylococcus aureus and/or Streptococcus pyogenes
   • Can lead to: impetigo, cellulitis, invasive bacterial infections
   • Poststreptococcal glomerulonephritis (rare but documented correlation with scabies-associated strep)
   • Possibly acute rheumatic fever
2. Post-treatment persistent itch - dead mite debris continues causing hypersensitivity for weeks
3. Nodular scabies - persistent granulomatous nodules
4. Institutional outbreaks - especially with unrecognized crusted scabies index case

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8. Treatment - Full Guide

General Principles

• Treat the patient AND all household members/intimate contacts simultaneously - even if asymptomatic, to prevent ping-pong reinfection
• Within 1 day of properly administered treatment, patient is considered non-communicable
• Dead mites continue to cause itch for 2-4 weeks post-treatment - do NOT retreat immediately

Environmental Decontamination

• Wash all clothing, bedding, and towels in hot water (60°C/140°F) and dry on high heat
• Items that cannot be washed: seal in a plastic bag for 2 weeks (mites starve off-host)
• Clean household environmental surfaces

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Treatment Table (from Fitzpatrick's, p. 3306)

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First-Line Treatments in Detail

Permethrin 5% Cream (Gold Standard)

• Synthetic pyrethroid; disrupts mite nerve cell membrane
• Cure rate >90% with single application
• Active against mites and eggs for several days post-application
• How to apply:
  • Apply to cool, dry skin (not after a hot bath - hot skin increases absorption)
  • Massage thoroughly into entire body from neck/jawline down to soles of feet
  • Special attention: interdigital spaces, under nails, wrists, axillae, navel, buttocks, external genitalia
  • Leave on 8-14 hours (typically overnight)
  • Wash off with soap and water
  • Reapply to hands if you wash them during the application period
  • Repeat in 1 week (second application)
• Special populations:
  • Infants (<2 years): apply to entire body including face, scalp, neck, and ears (avoid around mouth and eyes)
  • Pregnant women, breastfeeding mothers, children <2 years: limit contact time to 2 hours per application
  • Elderly/scalp involvement: include hairline, neck, temples, forehead

Oral Ivermectin 200 µg/kg

• Two doses: day 1 and day 8-14 (take with food/meal to increase bioavailability - contrary to strongyloidiasis instructions)
• Similar efficacy to permethrin at 2 weeks; may clear slightly slower in first week
• 2026 meta-analysis (BMC Infectious Diseases, 2026) confirms comparable effectiveness
• NOT FDA-approved for scabies (approved for strongyloidiasis), but widely used off-label
• Avoid: children <15 kg, pregnancy, lactation
• Preferred when: topical application is difficult, patient compliance issues, crusted scabies, institutional outbreaks

Crusted Scabies - Requires Combination Therapy

• Must combine topical permethrin + oral ivermectin
• Pre-treat with keratolytic (6% salicylic acid or 12% lactic acid) to debulk crusts before permethrin application
• Permethrin applied to entire body surface including scalp, face, and ears
• Permethrin schedule: daily × 7 days, then twice weekly until cured (Harrison's, p. 3773)
• Ivermectin: 3-7 doses over 8-30 days (days 1, 2, 8, 9, 15 minimum per Fitzpatrick's)
• Isolate patient; use gown and gloves when handling

Adjunctive Symptom Relief

• Oral antihistamines (cetirizine, hydroxyzine)
• Topical emollients
• Menthol 1-2% preparations, calamine lotion
• Topical corticosteroids (after treatment, for residual itch or nodular lesions)
• Intralesional triamcinolone for persistent nodular scabies
• Treat secondary bacterial infection with appropriate antibiotics if present

Treatment Failure

• A 2024 British Journal of Dermatology systematic review and meta-analysis (PMID 37625798) analyzed causes of treatment failure - most common causes: inadequate application, lack of simultaneous household treatment, re-exposure, and (emerging) reduced permethrin sensitivity

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9. The Direct Answer: Can You Apply Permethrin to the Penis Shaft?

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10. Quick Reference Summary