A 28-year-old man with HIV/AIDS (CD4 count 45 cells/μL) presents with widespread crusted plaques over his entire body, including palms, soles, and face. Skin scrapings show hundreds of mites per field. He reports only mild itching despite the extensive infestation. What is the most appropriate initial treatment for this patient?

Explanation

## Crusted (Norwegian) Scabies in Immunocompromised Host ### Clinical Recognition **Key Point:** The combination of **severe immunosuppression** (CD4 <50), **extensive crusted plaques** (palms, soles, face), **high mite burden** (hundreds per field), and **minimal pruritus** is pathognomonic for **crusted scabies**. ### Why Crusted Scabies Is Different **High-Yield:** Crusted scabies occurs almost exclusively in severely immunocompromised patients (HIV CD4 <50, hematologic malignancy, organ transplant). The host cannot mount an adequate immune response, allowing mites to proliferate unchecked (up to 1 million per patient vs. 10–15 in classical scabies). #### Pathophysiology - Defective Th1/Th2 response → impaired itch sensation - Massive mite replication → hyperkeratotic crusted lesions - High transmissibility → infection control priority ### Treatment Algorithm for Crusted Scabies ```mermaid flowchart TD A[Crusted Scabies Diagnosis]:::outcome --> B{Severity & CD4 Count?}:::decision B -->|CD4 <50, extensive| C[Ivermectin 200 mcg/kg]:::action C --> D[Repeat at Day 7 & Day 14]:::action D --> E[Add topical sulfur 10% daily]:::action E --> F[Immune reconstitution with ART]:::action F --> G[Cure achieved]:::outcome B -->|Mild/localized| H[Permethrin monotherapy]:::action H --> I[May fail; escalate to ivermectin]:::urgent ``` ### Ivermectin for Crusted Scabies **Clinical Pearl:** Ivermectin is the treatment of choice for crusted scabies because: 1. **Systemic penetration** → reaches mites in thick crusts where topical agents fail 2. **High mite burden** → requires multiple doses (day 1, 7, 14) to kill successive generations 3. **Immunocompromised host** → cannot rely on host immune response; need aggressive pharmacotherapy #### Ivermectin Dosing for Crusted Scabies - **Dose:** 200 mcg/kg orally - **Schedule:** Day 1, then repeat at day 7 and day 14 (minimum 3 doses) - **Adjunct:** Topical sulfur 10% ointment daily during treatment - **Monitoring:** Clinical response by week 4; repeat if necessary ### Comparison: Topical vs. Systemic in Crusted Scabies | Agent | Route | Efficacy in Crusted | Rationale | Limitations | |-------|-------|-------------------|-----------|-------------| | **Ivermectin** | Oral | **Excellent** | Penetrates crusts; systemic; multiple doses | Teratogenic; requires repeat dosing | | Permethrin 5% | Topical | **Poor** | Cannot penetrate thick crusts | High failure rate; inadequate for crusted disease | | Sulfur 10% | Topical | **Moderate** | Adjunctive; slow-acting | Never monotherapy for crusted; requires weeks | | Crotamiton 10% | Topical | **Poor** | Insufficient penetration | Not recommended for crusted scabies | **Mnemonic:** **CRISP** — **C**rusted scabies, **R**equires **I**vermectin **S**ystemic, **P**lus topical sulfur. ### Why Permethrin Fails Here **Warning:** Permethrin is ineffective in crusted scabies because the thick hyperkeratotic crusts prevent adequate drug penetration. Topical agents alone have failure rates >50% in this population. Permethrin should NOT be used as monotherapy for crusted disease. ### Immune Reconstitution **Clinical Pearl:** Concurrent initiation or optimization of antiretroviral therapy (ART) is essential. As CD4 count recovers (>200), the host immune response improves and scabies may spontaneously resolve or become easier to treat. [cite:Irvine 3e Ch 8; Harrison 21e Ch 197]