Diagnosis and Treatment of Scabies
Diagnosis
Scabies diagnosis is primarily clinical, based on characteristic findings of burrows, pruritic rash in typical locations, and nocturnal itching, with confirmation by microscopic or dermatoscopic identification of mites, eggs, or fecal pellets. 1
Clinical Presentation
- Classic scabies presents with burrows (thread-like papules), inflammatory changes in intertriginous areas, and intense nocturnal pruritus 1, 2
- Typical locations include finger webs, wrists, axillary folds, abdomen, buttocks, inframammary folds, and male genitalia 2
- In debilitated or elderly patients, presentation may be atypical—burrows and pruritus may be absent, with only hyperkeratosis, papules, or vesicles present 1
- In infants and geriatric patients, the scalp, hairline, neck, temple, and forehead may be infested 3
Diagnostic Confirmation
- Definitive diagnosis requires microscopic identification of mites, eggs, or fecal pellets (scybala) from skin scrapings 2, 4
- Dermatoscopy (epiluminescence microscopy) can detect mites directly and is increasingly used as a diagnostic tool 2, 5, 4
- Clinical diagnosis alone may be necessary in outbreak settings, though incorrect diagnosis can lead to pseudo-outbreaks with widespread "psychogenic" scabies (itching without actual infestation) 1
Important caveat: Diagnosis is often made retrospectively when healthcare workers or visitors develop the characteristic rash after exposure to an index patient 1
Treatment
First-Line Treatment for Uncomplicated Scabies
Permethrin 5% cream is the first-line treatment, applied from the neck down (including scalp in infants) for 8-14 hours, then washed off—one application is generally curative. 6, 7, 3
- Application technique: Thoroughly massage into skin from head to soles of feet, paying particular attention to all folds and creases, under fingernails, and up to the edge of all body orifices 6, 3
- Dosage: Usually 30 grams is sufficient for an average adult 3
- Preferred populations: Infants, young children, pregnant women, and lactating women 6, 7
Alternative First-Line Treatment
Oral ivermectin 200 μg/kg is an effective alternative, repeated in 2 weeks, and should be taken with food to increase bioavailability. 6, 7, 2
- Contraindications: Children weighing less than 15 kg due to neurotoxicity risk 6, 7
- Advantages: Particularly useful for patients with poor adherence, immunosuppression, or in outbreak settings 2, 4
- No renal adjustment needed 6
Second-Line Treatments
- Crotamiton: Massage into skin from chin down, repeat after 24 hours, cleansing bath 48 hours after last application 8
- Benzyl benzoate: Good second-line option 4
- Lindane 1%: Should be avoided in children <10 years, pregnant/lactating women, and persons with extensive dermatitis due to neurotoxicity risk 6, 7
Special Situations
Crusted (Norwegian) Scabies
Combination therapy is required: topical 5% permethrin cream applied daily for 7 days then twice weekly, PLUS oral ivermectin 200 μg/kg on days 1,2,8,9, and 15. 6, 7
- Isolation required for affected patients 4
- High infectivity due to heavy mite burden—debilitated older adults are often heavily infested 1
Contact and Environmental Management
All persons with close personal, sexual, or household contact within the preceding month must be examined and treated simultaneously, even if asymptomatic. 6, 7
- Failure to treat contacts simultaneously is the most common cause of treatment failure 6
- Decontaminate bedding and clothing by machine washing/drying on hot cycle, dry cleaning, or removing from body contact for at least 72 hours 6, 7
- Fumigation of living areas is unnecessary 6
- Scabies mites survive on clothing for up to 4 days without skin contact but only 1-2 days at room temperature (21°C) 1, 4
Follow-Up and Persistent Symptoms
Rash and pruritus may persist for up to 2 weeks after successful treatment—this is NOT treatment failure and does NOT indicate need for retreatment. 6, 7, 3
- Retreatment indicated only if: Symptoms persist beyond 2 weeks AND demonstrable living mites are observed after 14 days 6, 7, 3
- Reasons for true treatment failure include inadequate application, reinfection from untreated contacts/fomites, not repeating ivermectin dose, and possible emerging permethrin resistance 6, 9
Critical Pitfalls to Avoid
- Not treating all close contacts simultaneously—the leading cause of treatment failure 6
- Inadequate topical application—must include under nails, all body folds, and up to edge of all orifices 6
- Using lindane after bathing—increases absorption and neurotoxicity risk 6
- Not repeating ivermectin at 2 weeks—essential for complete eradication 6
- Expecting immediate symptom resolution—pruritus persists up to 2 weeks normally 6, 7, 3
- Misdiagnosing outbreaks—can lead to unnecessary mass treatment for "psychogenic" scabies 1