Scabies: Diagnosis and Treatment
Diagnosis
This clinical presentation is classic for scabies, characterized by the triad of pruritic skin lesions for 3 weeks, nocturnal worsening of itching, and progression from erythematous papules/macules to crusting with subsequent hyperpigmentation 1, 2.
Key Diagnostic Features
Intense nocturnal pruritus is virtually pathognomonic - itching that worsens at night and appears disproportionate to visible skin findings should immediately raise suspicion for scabies 1, 2.
Lesion evolution follows a characteristic pattern: initial erythematous papules or macules progress to excoriated lesions with crusting, ultimately leaving hyperpigmented areas after healing 2, 3.
Common distribution sites include finger webs, wrists, axillary folds, abdomen, buttocks, inframammary folds, and male genitalia - notably sparing the head in adults 2, 4.
Pathognomonic burrows (linear tracks up to 1 cm in length) confirm the diagnosis when present, though they may be difficult to identify due to excoriation 1, 2.
Confirmatory Testing
Microscopic examination of skin scrapings from burrows or papules demonstrating mites, eggs, or fecal pellets provides definitive diagnosis 2, 4.
Epiluminescence microscopy (dermoscopy) can visualize mites in vivo without requiring skin scraping 2.
Clinical diagnosis alone is acceptable when the history is characteristic (intense nocturnal pruritus, 3-week duration, typical distribution) and microscopy is unavailable 1.
Treatment
First-Line Pharmacologic Options
Treat with either topical permethrin 5% cream OR oral ivermectin - both have similar efficacy with clearance rates of 74% for permethrin versus 68% for ivermectin by week 2 5.
Topical Permethrin 5% Cream (Preferred for Most Cases)
Apply from neck down to entire body surface including under fingernails, leaving on for 8-14 hours before washing off 2, 5.
Repeat application after 7-14 days to eliminate newly hatched mites from eggs that survived initial treatment 2, 4.
Treatment failure occurs in 10.8% of cases, necessitating alternative therapy if symptoms persist beyond 2 weeks 5.
Oral Ivermectin (Alternative or for Special Populations)
Administer 200 mcg/kg as a single oral dose, repeated after 7 days 2, 4, 5.
Particularly indicated for: crusted scabies, immunocompromised patients, institutional outbreaks, and prophylaxis of close contacts 2.
Treatment failure rate is 11.8%, slightly higher than permethrin 5.
Adjunctive Symptomatic Management
Oral antihistamines (used in 91% of hospitalized cases) provide symptomatic relief of pruritus 3.
Topical corticosteroids may reduce inflammatory response but should not replace scabicidal therapy 3.
Oral antibiotics are indicated only when secondary bacterial superinfection is evident (present in 11% of cases) 3.
Critical Treatment Principles
All close personal contacts must be treated simultaneously, even if asymptomatic, as transmission occurs through prolonged skin-to-skin contact and the incubation period averages 10 days before symptoms appear 1, 2, 4.
Environmental decontamination is mandatory: launder all clothing, bedding, and towels used in the 3 days before treatment in hot water (≥60°C) and dry on high heat, or seal in plastic bags for 72 hours 2, 4, 5.
Pruritus may persist for 2-4 weeks after successful treatment due to ongoing hypersensitivity reaction to dead mites - this does not indicate treatment failure unless new burrows or lesions appear 2.
When to Refer
- Diagnostic uncertainty after initial evaluation 1.
- Patients under 2 months of age 1.
- Failure to respond after two courses of different scabicides 1.
- Crusted (Norwegian) scabies with hyperkeratotic plaques 1, 2.
- Institutional outbreaks requiring coordination with local health protection services 1.
- Sexually active patients should be screened for concurrent sexually transmitted infections 4.
Common Pitfalls
Do not delay treatment waiting for microscopic confirmation - the characteristic history of intense nocturnal pruritus with 3-week duration and typical lesion progression warrants empiric treatment 1, 2.
Do not confuse post-treatment pruritus with treatment failure - persistent itching for up to 4 weeks after successful eradication is expected and should be managed with antihistamines and topical corticosteroids, not repeated scabicides 2.
Do not treat the patient in isolation - failure to simultaneously treat household and intimate contacts accounts for most apparent treatment failures and reinfestations 2, 4.