In non-bullous impetigo, which organism is the most common causative agent in developed countries, and what is the characteristic clinical appearance?

## Non-Bullous Impetigo: Epidemiology and Clinical Features **Key Point:** In developed countries, *Staphylococcus aureus* is now the most common cause of non-bullous impetigo, accounting for >50% of cases. However, *Streptococcus pyogenes* (Group A Streptococcus) remains a significant co-pathogen and is still prevalent in developing countries. ### Epidemiologic Shift Historically, *S. pyogenes* was the primary cause of impetigo worldwide. Over the past 2–3 decades, *S. aureus* prevalence has increased significantly in developed nations due to: - Emergence of community-acquired MRSA (CA-MRSA) - Increased antibiotic resistance - Greater colonization rates in the community ### Clinical Presentation of Non-Bullous Impetigo 1. **Initial lesion:** Erythematous macule or papule 2. **Progression:** Rapid evolution to vesicles and pustules 3. **Characteristic crusting:** Honey-colored (golden-yellow) or "mellitin" crusts 4. **Distribution:** Face (especially around nose and mouth), extremities, areas of trauma 5. **Regional lymphadenopathy:** Often present (satellite lymphadenitis) **High-Yield:** The honey-colored crust is pathognomonic for impetigo and is the key diagnostic feature that distinguishes it from other pyogenic infections. ### Comparison: Bullous vs. Non-Bullous Impetigo | Feature | Non-Bullous | Bullous | |---------|-------------|----------| | **Frequency** | ~70% of cases | ~30% of cases | | **Causative organism** | *S. aureus* (now most common) + *S. pyogenes* | *S. aureus* (exfoliative toxin-producing strains) | | **Appearance** | Honey-colored crusts on erythematous base | Flaccid, clear bullae that rupture easily | | **Depth** | Subcorneal pustules | Intraepidermal bullae | | **Pathology** | Superficial pustulation | Acantholysis (desmoglein-1 cleavage) | **Clinical Pearl:** Post-streptococcal glomerulonephritis (PSGN) is a potential complication of impetigo caused by *S. pyogenes*, particularly in tropical and subtropical regions. This risk is independent of treatment, emphasizing the importance of early antimicrobial therapy. [cite:Park 26e Ch 5]