Uncomplicated Scabies - Full Clinical Lecture

 

Uncomplicated Scabies - Full Clinical Lecture

Definition:

Scabies is a common parasitic skin infestation caused by the mite Sarcoptes scabiei var. hominis. It leads to an allergic hypersensitivity reaction, causing intense itching and typical cutaneous lesions. Uncomplicated scabies refers to infestations without secondary complications or crusted forms.

Etiology & Transmission

• Organism: Sarcoptes scabiei var. hominis
• Transmission: Direct prolonged skin-to-skin contact (most common), less commonly via fomites
• Risk environments: Overcrowded settings, long-term care facilities, prisons, schools, and among household or sexual contacts

Pathogenesis

Female mites burrow into the superficial stratum corneum, where they lay eggs and deposit feces (scybala). This triggers a delayed type IV hypersensitivity reaction, resulting in pruritus and skin lesions.

Clinical Manifestations

• Intense nocturnal pruritus
• Burrows: fine, wavy, thread-like lesions, often with a black dot (mite)
• Erythematous papules, vesicles, and nodules in specific areas:
  • Finger webs
  • Flexor wrists
  • Genitals and areolae
  • Waist, buttocks, axillae
  • In infants: scalp, palms, and soles may be affected
• Excoriations and secondary infection may develop

Diagnosis

• Primarily clinical based on symptoms and lesion distribution
• Definitive: Microscopic visualization of mites, eggs, or feces in skin scrapings with mineral oil or KOH
• Dermatoscopy: “Delta wing jet sign” (mite in burrow)

Differential Diagnoses

• Atopic/contact dermatitis
• Bed bug or insect bites
• Lichen planus
• Folliculitis
• Psoriasis (especially inverse type)

Treatment

First-line agents:

• Permethrin 5% cream: Apply from neck down (entire body in infants), leave for 8–14 hours, then wash off. Repeat after 7 days.
• Ivermectin oral: 200 mcg/kg in two doses, 7–14 days apart; especially useful in outbreaks, crusted scabies, or poor compliance

Alternative agents:

• Sulfur 6–10% in petrolatum: safe for infants and pregnant women
• Crotamiton 10% cream: less effective, but used when others contraindicated
• Benzyl benzoate: effective but may irritate skin

Treatment Failure

• Incorrect application (missing areas, not treating contacts)
• Re-infestation from untreated household members or fomites
• Mite resistance to topical agents (rare, but reported)
• Crusted scabies misdiagnosed as uncomplicated

Management: Repeat treatment, evaluate technique, consider oral ivermectin, and ensure all close contacts and linens are treated appropriately.

Post-Scabetic Itch

Pruritus may persist for up to 4 weeks despite mite eradication. This is due to a continued hypersensitivity reaction. Treat with:

• Topical corticosteroids
• Oral antihistamines
• Emollients

Prevention & Control

• Treat all close contacts simultaneously, even if asymptomatic
• Wash clothes, bedding, towels in hot water and dry on high heat
• Seal non-washable items in plastic bags for 3–7 days
• Environmental decontamination is minimal – mites cannot survive beyond 2–3 days off human skin

Complications

• Secondary bacterial infections (impetigo, cellulitis)
• Crusted (Norwegian) scabies: hyperkeratotic, highly contagious, requires aggressive treatment
• Sleep disruption, social stigma

Recent Research Highlights

• Mass Drug Administration (MDA): WHO supports ivermectin MDA in endemic regions
• Spinosad 0.9% cream: Emerging topical treatment with promising results
• Mite resistance monitoring: Genetic studies ongoing to understand emerging resistance patterns

Patient Education Points

• Scabies is not due to poor hygiene
• All household members and close contacts must be treated together
• Clothing, bed linens, and towels should be cleaned thoroughly
• Itching may persist even after successful treatment

References & Resources

• Harrison’s Principles of Internal Medicine, 21st Edition
• Fitzpatrick's Dermatology in General Medicine
• American Academy of Dermatology (AAD) Guidelines
• CDC Guidelines on Scabies
• WHO – Neglected Tropical Diseases: Scabies Fact Sheet
• UpToDate Clinical Reviews

For Academic & Clinical Use Only
Prepared for: Medical students, interns, dermatology learners
Contact for guidance: 01644254845