Definition and epidemiology
Scabies is a parasitic skin infestation caused by the mite Sarcoptes scabiei var. hominis, an obligate human ectoparasite that burrows into the stratum corneum. It is one of the most common dermatologic conditions worldwide and is included in the WHO list of neglected tropical diseases because of its high burden in resource-limited settings.
- Estimated to affect over 200 million people globally at any time, with highest prevalence in tropical, overcrowded, and low-resource regions.
- No age, sex, or socioeconomic predilection, though outbreaks cluster in close-contact settings: households, schools, prisons, refugee camps, and long-term care facilities.
- Transmission is primarily by prolonged, direct skin-to-skin contact; fomite transmission (shared bedding, clothing) is less efficient except in crusted scabies, where mite burden is extremely high.
- Classic (non-crusted) scabies typically involves a mite burden of only 10-15 mites per person.
- Itch worsens at night — a classic board clue for scabies
- Pruritus is due to delayed type IV hypersensitivity, not direct mite damage
- Primary infestation itching begins 3-6 weeks after exposure; reinfestation itches within 1-2 days
- Burrows classically in finger webs, wrists, and genital/periumbilical areas
- Dermoscopy shows the "delta-wing jet" sign at the end of a burrow
- Crusted (Norwegian) scabies occurs in immunosuppressed patients and is highly contagious
- Topical permethrin is first-line; oral ivermectin is key for outbreaks and crusted scabies
- Treat all household/close contacts simultaneously, regardless of symptoms
Etiology and risk factors
The causative organism is Sarcoptes scabiei, an eight-legged mite roughly 0.3-0.4 mm long. The female mite burrows into the epidermis to lay eggs, which hatch into larvae that mature over about 10-14 days.
Risk factors
- Close physical contact with an infested person (sexual contact, caregiving, bed-sharing)
- Institutional or crowded living conditions (nursing homes, prisons, refugee camps, dormitories)
- Immunosuppression (HIV, chemotherapy, chronic corticosteroid use, organ transplant) — predisposes to crusted (Norwegian) scabies, a hyperinfestation variant with thousands to millions of mites
- Low socioeconomic status and limited access to water/sanitation
- Delayed diagnosis due to resemblance to other dermatoses
Pathophysiology
The gravid female mite burrows into the stratum corneum, creating a linear tunnel where she deposits eggs and fecal pellets (scybala) as she moves. Larvae hatch within 2-3 days and migrate to the skin surface, maturing into adults.
The intense pruritus is not from the burrowing itself but from a delayed-type (type IV) hypersensitivity reaction to mite proteins, saliva, eggs, and feces. This explains the characteristic 3-6 week delay in symptom onset after a primary infestation, whereas reinfestation causes itching within 1-2 days because the immune system is already sensitized.
In crusted scabies, impaired cell-mediated immunity prevents the normal itch-driven scratching that limits mite numbers, allowing massive proliferation and hyperkeratotic crust formation — this form is highly contagious even through brief contact or fomites.
Clinical features
- Pruritus: intense, classically worse at night and after hot showers; the hallmark symptom
- Burrows: thin, serpiginous, grayish-white lines, pathognomonic but often few in number and hard to see
- Distribution: finger webs, flexor wrists, axillae, periumbilical area, belt line, buttocks, genitalia (men), areolae (women); in infants and the elderly, the face, scalp, palms, and soles may be involved
- Secondary lesions: excoriations, erythematous papules, eczematization, and secondary bacterial infection from scratching
- Nodular scabies: persistent reddish-brown pruritic nodules (hypersensitivity response), especially on genitals and axillae, that may persist for weeks after successful treatment
- Crusted scabies: widespread thick, scaly, hyperkeratotic plaques, often minimal itching, seen in immunosuppressed or institutionalized patients
- Household members or close contacts often report similar itching ("itchy family" clue)
Diagnosis
Diagnosis is primarily clinical but can be confirmed with bedside tools.
Clinical criteria
- Typical distribution and morphology of lesions plus nocturnal pruritus
- History of itching in close contacts
Confirmatory tests
- Dermoscopy: visualizes the mite as a dark triangular structure at the end of a burrow ("delta-wing jet" sign) — fast, noninvasive, increasingly used as first-line confirmation
- Skin scraping with microscopy: mineral oil preparation of a burrow scraping examined for mites, eggs, or fecal pellets — the traditional gold standard but has low sensitivity if few burrows sampled
- Adhesive tape test: alternative to scraping in children
- Skin biopsy rarely needed, reserved for atypical or crusted presentations
IFSD (International Alliance for the Control of Scabies) consensus criteria combine clinical, dermoscopic, and microscopic findings to classify confirmed, clinical, and suspected scabies for research and outbreak settings.
Differential diagnosis
| Condition | Distinguishing feature |
|---|---|
| Atopic dermatitis | Flexural distribution, personal/family atopy history, no burrows |
| Contact dermatitis | Clear exposure history, geometric/localized pattern |
| Pediculosis (lice) | Nits/lice visible, scalp or pubic hair involvement |
| Papular urticaria (insect bites) | Grouped papules, seasonal, pets/exposure history |
| Dermatitis herpetiformis | Grouped vesicles, celiac association, symmetric extensor distribution |
| Psoriasis (crusted scabies mimic) | Well-demarcated plaques with silvery scale, nail pitting |
| Bullous pemphigoid | Tense bullae, elderly patients, linear IgG/C3 on biopsy |
Management
Treatment goals are to eradicate the mite, treat all close contacts simultaneously, and decontaminate fomites.
First-line topical therapy
- Permethrin cream: applied to the entire body from neck down (including scalp and face in infants/elderly), left on for the recommended duration, then washed off; repeated as directed to kill newly hatched larvae. Considered first-line by CDC and most guidelines.
Alternatives
- Oral ivermectin: effective and convenient, especially for outbreaks, crusted scabies, or when topical application is impractical; given as multiple doses. WHO and many national guidelines now favor ivermectin for mass drug administration and institutional outbreaks.
- Crotamiton, sulfur ointment, benzyl benzoate, lindane: second-line or used where permethrin/ivermectin are unavailable; lindane is avoided in young children and pregnant women due to neurotoxicity risk.
Crusted scabies
- Requires combination therapy: topical permethrin or benzyl benzoate plus oral ivermectin (multiple doses), often with a keratolytic agent; strict contact isolation given high contagiousness.
Adjunctive measures
- Treat all household and close contacts simultaneously, even if asymptomatic
- Wash bedding, clothing, and towels used recently in hot water and dry on high heat, or seal unwashable items in a plastic bag for several days
- Antihistamines or low-potency topical corticosteroids for post-treatment pruritus (itching may persist for weeks after successful eradication — this does not indicate treatment failure)
- Treat secondary bacterial infection with appropriate antibiotics if present
Guideline note: CDC emphasizes permethrin as first-line with ivermectin as an alternative, while WHO neglected tropical disease programs favor ivermectin-based mass treatment for community outbreaks given ease of administration.
Complications and prognosis
- Secondary bacterial infection: Staphylococcus aureus or Streptococcus pyogenes superinfection from excoriation; can rarely lead to post-streptococcal glomerulonephritis in endemic settings
- Nodular scabies: persistent pruritic nodules that may take weeks to months to resolve even after successful mite eradication
- Crusted scabies: can be fatal if untreated in immunosuppressed patients due to overwhelming secondary sepsis
- Psychosocial impact: stigma, sleep disturbance from chronic pruritus, and school/work absenteeism
Prognosis is excellent with appropriate treatment and contact decontamination; most patients clear the infestation after one or two treatment cycles. Persistent symptoms beyond 4-6 weeks should prompt reassessment for treatment failure, reinfestation, or an alternate diagnosis (e.g., post-scabetic dermatitis).
SCABIES: Spreads by contact, Crusted form in immunocompromised, Axillae/web spaces affected, Burrows pathognomonic, Itch worse at night, Everyone (contacts) treated, Sarcoptes scabiei causative mite
Practice questions
1. A 24-year-old woman presents with intense itching that is worse at night, for the past 4 weeks. She lives in a shared dormitory, and two roommates report similar symptoms. Examination reveals thin, grayish linear lesions in the finger web spaces and excoriated papules around the wrists and waistline. Which of the following best explains the delayed onset of her symptoms after initial exposure?
2. A 68-year-old man with poorly controlled HIV (CD4 count 45 cells/microL) is admitted with widespread thick, scaly, hyperkeratotic plaques covering his trunk, scalp, and extremities. He reports minimal itching. Skin scraping reveals thousands of mites. Which of the following is the most appropriate treatment approach?
3. A mother brings her 6-month-old infant for evaluation of fussiness and poor feeding. Examination reveals vesicles and pustules on the palms and soles, along with excoriated papules on the scalp and face. Older siblings have been treated recently for an itchy rash. Which feature distinguishes scabies presentation in this infant from that typically seen in adults?
Frequently asked questions
What causes scabies?
Scabies is caused by infestation with the mite Sarcoptes scabiei var. hominis, which burrows into the top layer of skin to lay eggs. The resulting itch and rash come from a delayed allergic reaction to mite proteins and waste rather than from the burrowing itself. It spreads through prolonged, direct skin-to-skin contact, and less often through shared bedding or clothing.
What are the first symptoms of scabies?
The first and most prominent symptom is intense itching, typically worse at night and after a hot shower. Small red bumps or thin, thread-like burrow lines appear in finger webs, wrists, armpits, waistline, or genital area. In a first-time infestation, symptoms often take 3 to 6 weeks to appear after exposure, while reinfestation causes itching within a day or two.
How is scabies diagnosed?
Scabies is usually diagnosed clinically based on the pattern and distribution of itchy lesions plus a history of itching in close contacts. Dermoscopy can confirm the diagnosis at the bedside by visualizing the mite as a dark triangular shape at the burrow's end. A skin scraping examined under a microscope for mites, eggs, or fecal pellets remains the traditional confirmatory test.
Is scabies curable?
Yes, scabies is fully curable with appropriate treatment. Topical permethrin cream or oral ivermectin eliminates the mites, usually requiring an initial treatment plus a repeat application to kill newly hatched larvae. Itching can persist for a few weeks after successful treatment due to lingering allergic inflammation, which does not mean the treatment failed.
Scabies vs eczema — what is the difference?
Scabies is caused by a contagious mite infestation and classically affects finger webs, wrists, and genital areas with visible burrows, while eczema (atopic dermatitis) is a non-contagious inflammatory skin condition that favors flexural areas like the elbow and knee creases. Scabies often causes itching in multiple household members simultaneously, whereas eczema has a personal or family history of allergies, asthma, or hay fever rather than clustering among close contacts.
Key drugs
Related conditions and differentials
Sources and further reading
StatPearls (NCBI Bookshelf)MedlinePlusWHO ICD-10: B86
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.