What is impetigo?

What is Impetigo?

Impetigo is a highly contagious bacterial infection of the superficial layers of the epidermis, predominantly affecting children aged 2-5 years, caused by Staphylococcus aureus and/or Streptococcus pyogenes. 1, 2, 3

Clinical Characteristics

Epidemiology and Transmission

• Impetigo is the most common bacterial skin infection in children worldwide, with a global disease burden exceeding 140 million cases 3, 4
• The infection spreads readily through direct skin contact or sharing of towels, clothing, and bedding 2
• Incidence decreases with age, though adults can also be affected 4

Pathogenesis

• S. aureus colonizes the nasal epithelium first, then spreads to skin sites 5
• S. pyogenes (group A beta-hemolytic streptococci) colonizes the skin directly by binding to fibronectin exposed by minor trauma, insect bites, or scratches 2, 5
• The bacteria enter through breaks in the skin barrier such as cuts, insect bites, eczema, or herpetic lesions 3

Clinical Presentation: Two Distinct Forms

Nonbullous Impetigo (70% of cases)

• Caused by S. aureus and/or S. pyogenes 1, 3
• Characterized by honey-colored crusts on the face and extremities 1, 2, 4
• Presents as discrete purulent lesions with characteristic golden-yellow crusting 1
• More commonly affects exposed areas of skin 3

Bullous Impetigo (30% of cases)

• Caused exclusively by S. aureus 3, 4
• Results in large, flaccid, fluid-filled blisters that rupture, leaving a thin brown crust 2, 3
• More likely to affect intertriginous areas (skin folds) 1

Natural History and Complications

Disease Course

• Both types typically resolve within 2-3 weeks without scarring when untreated 3
• The disease is generally mild and considered self-limited 4
• However, treatment is recommended to reduce spread and shorten clinical course 4

Complications (Rare)

• The most serious complication is post-streptococcal glomerulonephritis following infection with certain strains of S. pyogenes 1, 3
• No evidence demonstrates that antibiotic treatment prevents this sequela 1
• Other complications are uncommon 3

Microbiological Considerations

Rising Antibiotic Resistance

• Community-acquired methicillin-resistant S. aureus (CA-MRSA) is an emerging etiological agent of increasing concern 1, 6
• Approximately 10% of S. aureus isolates in impetigo are MRSA 7
• Macrolide resistance in S. pyogenes has gradually increased from 4-5% (1996-1998) to 8-9% (1999-2001) in the United States 1
• Resistance to mupirocin, retapamulin, fusidic acid, and erythromycin has been widely reported 6
• Nearly 100% of S. pyogenes strains remain susceptible to penicillin and 99.5% to clindamycin 1

Diagnostic Approach

Clinical Diagnosis

• Diagnosis is most often based on clinical presentation alone 4
• The distinctive honey-colored crusted lesions aid in diagnosis 4
• Physical examination reveals erythema, tenderness, and induration 1

When to Consider Cultures

• Aspiration of skin is not helpful in 75-80% of cellulitis cases 1
• Blood cultures are rarely positive (<5% of cases) 1
• Culture should be obtained in patients who do not respond to initial empirical therapy or in immunocompromised hosts 1