Scabies

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Scabies

Definition and Epidemiology

Scabies is a skin infestation by the obligate human ectoparasite Sarcoptes scabiei var. hominis (family Sarcoptidae, class Arachnida). It affects over 100 million people worldwide annually, with no predilection for age, race, or socioeconomic status. Prevalence in some underdeveloped countries ranges from 4% to 100% of the general population. Transmission increases in winter months, likely due to closer indoor contact. - Fitzpatrick's Dermatology, Vol. 1-2, p. 3303; Rosen's Emergency Medicine, p. 2411

The Mite

The female mite is 0.4 x 0.3 mm - just too small to be seen with the naked eye. It is pearl-like, translucent, white, oval, and eyeless with 4 pairs of short stubby legs. Key facts:

The mite can survive 3 days off a host in a sterile environment, or up to 7 days in mineral oil
Mites cannot fly or jump
A normal host harbors 3-50 mites; infants and the elderly may harbor 50-250; crusted scabies patients can harbor millions

Life cycle (entirely on human skin):

Female burrows into the stratum corneum at 0.5-5 mm/day, reaching the stratum granulosum boundary
She lays 0-4 eggs/day (up to 50 over her 30-day lifespan) along a burrow up to 1 cm long
Eggs hatch in 10-12 days; larvae surface and mature into nymphs (survive only 2-5 days off host), then adults

Fitzpatrick's Dermatology, p. 3304

Transmission

Primary route: Close, prolonged skin-to-skin contact (sexual or non-sexual)
Fomites: Mainly relevant in crusted scabies (6,000 mites/g have been detected in debris from sheets and nearby furniture); much less likely in ordinary scabies
All household and close contacts should be treated simultaneously, as asymptomatic mite carriers are common

Clinical Presentation

Incubation and Symptoms

First infestation: Pruritus appears 4-6 weeks after exposure (some up to 3 months; some never sensitized)
Re-infestation: Symptoms appear within 2-3 days
Cardinal symptom: Intense, nocturnal pruritus - the hallmark

Skin Lesions

Small (<5 mm) papules, pustules, vesiculopustules, with excoriations from scratching
Burrows - thread-like, serpiginous lines 1 cm long; pathognomonic when present but uncommonly observed
Indurated, crusted nodules (especially in children) at intertriginous areas

Distribution

Area	Comment
Interdigital web spaces, sides of fingers	Most characteristic location
Volar wrists, lateral palms	Very common
Elbows, axillae	Frequently involved
Scrotum, penis, labia, areolae	Highly characteristic; nodules on genitalia/axillae are strongly suggestive
Buttocks, abdomen, waist	Common
Head, neck, face, scalp, palms, soles	Spared in healthy adults; involved in infants, elderly, and immunocompromised

Scabies - characteristic scattered papules on buttocks and thighs

Scabies - Rosen's Emergency Medicine, p. 2411

Burrows visible in web spaces and on knuckles of the hand - a classic site

Thread-like burrows in web spaces and knuckles - Fitzpatrick's Dermatology, p. 3304

Crusted (Norwegian) Scabies

A distinct, highly contagious variant occurring in immunocompromised, elderly, or neurologically impaired patients (HIV, leprosy, paraplegia, dementia, neuropathy).

Crusted scabies - diffuse hyperkeratotic scaling with skin discoloration around the ear and face

Crusted scabies with hyperkeratotic plaques harboring thousands of mites - Fitzpatrick's Dermatology, p. 3304

Features: Thick, hyperkeratotic plaques diffusely over palms, soles, elbows, knees; nail thickening and dystrophy
Paradoxically minimal pruritus despite massive mite burden
Extremely contagious - mites shed freely from skin debris into the surrounding environment; anyone in the general vicinity is at risk
Requires intensive combination treatment (see below)
Rosen's Emergency Medicine, p. 2411; Textbook of Family Medicine 9e, p. 946

Diagnosis

Clinical Diagnosis

Scabies is primarily a clinical diagnosis based on:

Characteristic distribution of lesions
Intense nocturnal pruritus
Epidemiologic history (contacts with similar symptoms)

Pruritic nodules on the axillae, umbilicus, penis, or scrotum are highly suggestive.

Confirmatory Tests

Method	Notes
Skin scraping (KOH or mineral oil mount)	Scrape terminal portion of fresh burrow; identify mites (adult female 330-450 µm), eggs, or scybala (fecal pellets); definitive but low sensitivity
Dermoscopy (epiluminescence microscopy)	Identifies pigmented mite parts or air bubbles within the stratum corneum in vivo; more practical in clinical settings
Reflectance confocal microscopy	Promising, improved sensitivity/specificity
PCR on swabbed skin material	High sensitivity/specificity; not widely available

Red Book 2021, p. 1031; Fitzpatrick's Dermatology; Dermatology 2-Volume Set 5e

Differential Diagnosis

Pityriasis rosea (symmetric maculopapular rash)
Papular urticaria
Secondary syphilis
Folliculitis
Contact dermatitis / atopic dermatitis
Dermatitis herpetiformis
Lichen planus (pruritic violaceous polygonal lesions)
Psoriasis
Bullous pemphigoid
Drug eruption
Delusions of parasitosis

Treatment

All household members and close contacts must be treated simultaneously, regardless of symptoms, to prevent re-infestation. Post-treatment pruritus may persist for weeks to months due to ongoing hypersensitivity to dead mites and eggs - this does not indicate treatment failure.

Drug Summary

Drug	Dose	Notes
Permethrin 5% cream	Apply neck-down (include face/scalp in infants) for 8-14 hours, wash off; repeat in 7 days. Crusted scabies: daily x7 days, then twice weekly until cured	First-line; pregnancy category B; safe from 2 months of age; tolerance developing; no documented resistance yet
Ivermectin 200 µg/kg oral	Days 1 and 8; crusted scabies: days 1, 2, 8, 9, and 15	Highly effective; NOT ovicidal (hence two doses); not FDA-approved for scabies; avoid in children <15 kg, pregnant/lactating women
Benzyl benzoate 10% lotion	Apply for 24 hours	Not available in the US
Crotamiton 10% cream	Apply 8 hrs on days 1, 2, 3, and 8	Has antipruritic properties; considerably less effective than other options
Precipitated sulfur 5-10%	Apply 8 hrs on days 1, 2, 3	Safe in neonates and pregnancy; limited efficacy data; inexpensive
Lindane 1% lotion	Apply 8 hrs, repeat in 7 days	FDA black box warning; banned in California; avoid in premature infants, seizure disorders, children, elderly <50 kg; use only if all other options have failed or are intolerable

Fitzpatrick's Dermatology, p. 3306

Application Tips - Permethrin

Apply before bedtime; leave overnight (8-14 hours)
Apply under nails, in umbilicus, around nipples and genitals
In infants/young children: treat entire head, neck, and body
Trim fingernails and apply medication to those areas
Pregnant and breastfeeding women: limit application to 2 hours for both treatments

Ivermectin Notes

A Cochrane review found oral ivermectin is as effective as topical permethrin for typical scabies
Preferred for heavily infested or immunocompromised patients
Crusted scabies: Combination of ivermectin + topical permethrin is recommended

Symptomatic Relief

Oral antihistamines - for itching
Mid-potency topical corticosteroids - for itching; be aware they will not clear the infestation
Topical/systemic antibiotics - for secondary bacterial infections (commonly S. aureus)

Environmental Decontamination (Mandatory)

Machine-wash clothing, bed linens, and towels in hot water (≥60°C/140°F); dry in hot dryer
Items that cannot be washed: dry clean, heat in dryer for 15 minutes, or seal in plastic bags for 1-2 weeks
Stuffed animals, upholstered furniture: seal in bags or leave unoccupied for 2 weeks

Special Populations

Population	Key Points
Infants (<2 months)	Permethrin not approved; sulfur in petrolatum is safe
Children <15 kg	Ivermectin safety not established
Pregnancy/lactation	Permethrin (limit to 2 hrs); sulfur safe; avoid lindane and ivermectin
Immunocompromised/HIV	High risk for crusted scabies; combination therapy (permethrin + ivermectin)
Hospitalized patients	Contact precautions until treatment is complete

Red Book 2021, p. 1031-1032

Complications

Secondary bacterial infections (excoriation → S. aureus, streptococcal)
Post-streptococcal glomerulonephritis - a recognized complication of scabies in tropical settings
Institutional outbreaks - nursing homes, hospitals, prisons (especially from unrecognized crusted scabies cases)
Post-scabetic eczema - persistent inflammation after successful treatment

Sources: Fitzpatrick's Dermatology (Vol. 1-2), p. 3303-3307 | Rosen's Emergency Medicine, p. 2411 | Red Book 2021, p. 1031-1032 | Textbook of Family Medicine 9e, p. 946-947 | Dermatology 2-Volume Set 5e

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