A 42-year-old male nursing home resident presents with a 6-week history of severe, intractable pruritus and widespread hyperkeratotic crusted lesions over the hands, feet, elbows, and trunk. Examination shows thick, scaly plaques with minimal visible burrows. KOH mount of crusted material reveals numerous mites, eggs, and fecal pellets. Other residents and staff members are asymptomatic. What is the most likely diagnosis and the most appropriate initial management?

Explanation

## Diagnosis: Crusted (Norwegian) Scabies **Key Point:** Crusted scabies is a severe, highly contagious variant of scabies characterized by hyperkeratotic crusted plaques, minimal visible burrows, and massive mite burden (up to 1 million mites per patient vs. 10–15 in typical scabies). It occurs in immunocompromised, elderly, or institutionalized patients. **High-Yield:** The diagnostic triad is: 1. Hyperkeratotic crusted plaques (not typical burrows) 2. Massive mite burden on microscopy (KOH mount shows numerous mites, eggs, fecal pellets) 3. Immunocompromised or institutionalized host (elderly nursing home resident) ## Pathophysiology and Risk Factors ```mermaid flowchart TD A[Immunocompromise or advanced age]:::outcome --> B[Impaired Th1/Th2 response]:::outcome B --> C[Failure to mount adequate inflammatory response]:::outcome C --> D[Uncontrolled mite proliferation]:::outcome D --> E[Hyperkeratotic crusted plaques]:::outcome E --> F[Massive mite burden<br/>1 million+ mites]:::outcome F --> G[High transmission risk]:::urgent G --> H[Institutional outbreaks]:::urgent ``` **Risk Factors for Crusted Scabies:** - Advanced age - HIV/AIDS (CD4 <200 cells/µL) - Organ transplant recipients - Chronic corticosteroid use - Leukemia, lymphoma - Dementia or inability to scratch/treat ## Management: Combination Systemic + Topical Therapy | Aspect | Standard Scabies | Crusted Scabies | |--------|---|---| | **First-line oral agent** | Ivermectin (if indicated) | Ivermectin 200 µg/kg | | **Dosing schedule** | Single dose or day 0 + day 14 | Day 0, 14, 21, 28 (may need 4+ doses) | | **Topical agent** | Permethrin 5% alone | Permethrin 5% OR sulfur 10% | | **Topical frequency** | Twice, 7 days apart | Daily or every other day | | **Duration** | 2–3 weeks | 4–8 weeks or until clinical cure | | **Contact tracing** | Essential | **Mandatory** — institutional control measures | **Clinical Pearl:** Crusted scabies is a **medical emergency** in institutional settings. A single patient can infect dozens of residents and staff. Immediate isolation and treatment of all contacts is mandatory. ## Why Combination Therapy is Essential 1. **Ivermectin penetration:** Oral ivermectin achieves high systemic levels and penetrates thick crusts better than topical agents alone. 2. **Mite burden:** The massive number of mites requires repeated dosing (typically 4 doses at days 0, 14, 21, 28) to achieve cure. 3. **Topical synergy:** Concurrent topical permethrin or sulfur 10% (higher concentration than standard 5%) enhances mite kill and removes crusted material. 4. **Crusted material removal:** Gentle debridement of crusts with keratolytic agents (salicylic acid 5–10%) may improve penetration. **High-Yield:** Monotherapy (either ivermectin alone or topical permethrin alone) frequently fails in crusted scabies. Combination therapy is the standard of care. ## Why This Patient Does NOT Have Typical Scabies - **Minimal visible burrows:** Typical scabies presents with linear burrows; crusted scabies shows hyperkeratotic plaques. - **Massive mite burden:** KOH mount showing "numerous mites, eggs, and fecal pellets" is pathognomonic for crusted scabies; typical scabies shows 1–3 mites per specimen. - **Asymptomatic contacts:** In typical scabies, household contacts develop symptoms within 2–4 weeks. Asymptomatic contacts in a nursing home suggest the index patient has crusted scabies (high transmission risk but delayed symptom onset in contacts). - **6-week duration:** Crusted scabies often has a prolonged course before diagnosis due to atypical presentation. **Mnemonic — Crusted Scabies (CRUSTED):** **C**rusted hyperkeratotic plaques | **R**epeated ivermectin dosing (4+ doses) | **U**ncontrolled mite proliferation | **S**ystemic + topical combination therapy | **T**hick burden (1 million+ mites) | **E**lderly/immunocompromised host | **D**angerous institutional transmission.