Scabies Diagnosis and Treatment
Diagnosis
Scabies is diagnosed by identifying characteristic burrows and correlating clinical findings with microscopic confirmation when possible. 1
Clinical Presentation
- Intense nocturnal pruritus is the hallmark symptom, though sensitization takes several weeks to develop in first-time infections (versus 24 hours in reinfestations) 2
- Typical distribution: finger webs, wrists, axillary folds, abdomen, buttocks, inframammary folds, and male genitalia 3
- Lesion types: burrows (pathognomonic), papules, vesicles, and excoriations 3
Diagnostic Confirmation
- Definitive diagnosis requires microscopic identification of mites, eggs, or fecal pellets from skin scrapings prepared with KOH 1, 3
- Alternative methods include epiluminescence microscopy (dermoscopy), high-resolution digital photography, or skin biopsy for atypical presentations 1
First-Line Treatment
Permethrin 5% cream is the gold standard first-line treatment for uncomplicated scabies. 4, 5, 6
Permethrin 5% Cream Application
- Apply to all areas of the body from neck down and wash off after 8-14 hours 2, 4, 5
- For infants, elderly, and immunocompromised patients: apply scalp-to-toes including hairline, neck, temple, and forehead 6
- One application is generally curative in immunocompetent adults 6
- More effective regimen: Apply once daily for two consecutive days (87.2% cure rate versus 61.8% with single application) 7
- Safe in pregnancy and lactation (Pregnancy Category B) 4, 6, 8
- Safe in children ≥2 months of age 6, 8
Oral Ivermectin Alternative
- Dosage: 200 μg/kg body weight, repeated in 2 weeks 4, 5, 6
- Must be taken with food to increase bioavailability and epidermal penetration 4, 5
- The second dose is essential because ivermectin has limited ovicidal activity 6
- Not recommended for children <15 kg due to potential neurotoxicity 6
- Probably compatible with breastfeeding but classified as "human data suggest low risk" in pregnancy 6
Special Populations
Pregnant and Lactating Women
Permethrin 5% cream is the preferred treatment due to superior safety profile 4, 5, 6
Infants <2 Months
- Permethrin 5% cream only (safety not established, but preferred over alternatives) 6, 8
- Avoid ivermectin due to neurotoxicity risk 6
- Apply to entire body including scalp 6
Crusted (Norwegian) Scabies
Combination therapy is mandatory for this highly contagious variant with massive mite burden. 4, 5
- Topical: 5% permethrin cream applied daily for 7 days, then twice weekly until cure 4, 5
- Oral: Ivermectin 200 μg/kg on days 1,2,8,9, and 15 4, 5
- Single-application permethrin will fail in crusted scabies 4
- Topical therapy alone is insufficient given the mite burden and thick crusts 4
Alternative Treatments (When First-Line Options Unavailable)
Lindane 1%
- Apply thinly from neck down, wash off after 8 hours 2, 5
- CONTRAINDICATIONS: children <10 years, pregnant/lactating women, extensive dermatitis, post-bath application 2, 4, 5, 6
- Risk of seizures and neurotoxicity with improper use 2, 4
Crotamiton 10%
- Apply nightly for 2 consecutive nights, wash off 24 hours after second application 2, 9
- Less effective than permethrin 2
Sulfur 6% Ointment
- Apply nightly for 3 nights 4, 5
- Safe alternative for infants and pregnant women when permethrin unavailable 4
Contact and Environmental Management
Simultaneous treatment of all contacts is critical to prevent reinfection. 4, 5, 6
Contact Tracing
- Treat all persons with sexual, close personal, or household contact within the preceding month, even if asymptomatic 4, 5, 6
- For institutional outbreaks: treat entire at-risk population 5
Environmental Decontamination
- Machine wash and dry bedding, clothing, and towels using hot cycle 2, 4, 5, 6
- Alternative: dry-clean or remove from body contact for at least 72 hours 2, 4, 5
- Fumigation of living areas is unnecessary 2, 4, 5
- Vacuum furniture and carpets; isolate non-launderables for minimum 2 days (or 3 weeks for rigorous approach) 1
Follow-Up and Treatment Failure
Pruritus may persist for up to 2 weeks after successful treatment and does not indicate treatment failure. 4, 5, 6
When to Reassess
- Evaluate at 1-2 weeks if symptoms persist 4, 5, 6
- Consider retreatment after 2 weeks only if live mites are observed or symptoms persist beyond expected timeframe 4, 5, 6
- Persistent pruritus alone is NOT an indication for retreatment as it may represent sensitization reaction 6
Common Causes of Treatment Failure
- Failure to treat all close contacts simultaneously 4, 5
- Inadequate application (missing scalp/face in high-risk patients, insufficient coverage) 4, 5, 1
- Not repeating ivermectin dose after 2 weeks 4, 5
- Reinfection from untreated contacts or inadequately decontaminated fomites 4, 5, 1
- Using lindane after bathing or in contraindicated populations 4, 5
Critical Pitfalls to Avoid
- Never use lindane in children <10 years, pregnant/lactating women, or after bathing due to neurotoxicity risk 2, 4, 5, 6
- Never skip the second dose of ivermectin at 2 weeks 4, 5, 6
- Never treat crusted scabies with single-application permethrin alone 4
- Never expect immediate symptom resolution—allow up to 2 weeks for post-treatment pruritus to resolve 4, 5, 6
- Never apply permethrin to eyes; flush immediately if contact occurs 8
- Never use ivermectin in children <15 kg 6