## First-Line Treatment of Bullous Impetigo **Key Point:** Bullous impetigo is caused exclusively by *Staphylococcus aureus* (producing exfoliative toxins ETA/ETB), and systemic antibiotics are required for widespread disease. Oral **cephalexin** (a first-generation cephalosporin) is the drug of choice. ### Bullous vs Non-Bullous Impetigo: Treatment Differences | Feature | Non-Bullous | Bullous | |---------|-------------|---------| | **Causative organism** | *S. pyogenes* + *S. aureus* | *S. aureus* only | | **Lesion type** | Pustules, honey crusts | Flaccid blisters, erosions | | **Localized disease** | Topical mupirocin effective | Topical therapy alone INSUFFICIENT for widespread disease | | **First-line systemic** | Cephalexin | **Cephalexin** | | **Alternative systemic** | Amoxicillin-clavulanate | Amoxicillin-clavulanate | | **Duration** | 7 days | 7–10 days | ### Why Cephalexin for Bullous Impetigo? 1. **Targeted coverage:** *S. aureus* is the sole pathogen in bullous impetigo. Cephalexin (a first-generation cephalosporin) provides excellent anti-staphylococcal activity and is the standard first-line oral agent per AAD guidelines, IADVL guidelines, and major dermatology textbooks (Rook's, Andrews'). 2. **Beta-lactamase stability:** Cephalexin is inherently stable against staphylococcal beta-lactamases, making the addition of clavulanate unnecessary. 3. **Narrow-spectrum advantage:** Cephalexin's narrower spectrum is preferred over amoxicillin-clavulanate to minimize disruption of normal flora and reduce adverse effects (e.g., diarrhea) — particularly important in a 4-year-old child. 4. **Why NOT amoxicillin-clavulanate?** Amoxicillin-clavulanate is broader-spectrum and is reserved as an alternative when mixed flora (e.g., *S. pyogenes* + *S. aureus*) is suspected, or when cephalexin is not tolerated. It is NOT the preferred first-line agent for bullous impetigo caused solely by *S. aureus*. 5. **Why NOT topical agents?** Topical mupirocin and bacitracin are effective for localized non-bullous impetigo but are insufficient for bullous impetigo, which involves deeper dermal infection and systemic toxin production. **High-Yield:** For MRSA-suspected bullous impetigo, trimethoprim-sulfamethoxazole or clindamycin is used. For community-acquired MSSA (the usual case), cephalexin remains first-line. ### Dosing - **Cephalexin:** 25–50 mg/kg/day divided every 6–8 hours for 7–10 days. - For a 20 kg child: 250 mg every 6–8 hours. **Clinical Pearl:** Bullous impetigo in a child with lesions on the abdomen and thighs (widespread) mandates systemic therapy. Cephalexin is the drug of choice; amoxicillin-clavulanate is an alternative, not the first-line agent. **Mnemonic:** **CBS** — **C**ephalexin for **B**ullous impetigo caused by **S**taphylococcus aureus. [cite: Andrews' Diseases of the Skin 13e Ch 14; Rook's Textbook of Dermatology 9e Ch 25; AAD Impetigo Guidelines 2014; KD Tripathi Essentials of Medical Pharmacology 8e]
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