What is the diagnosis and treatment for impetigo?

Diagnosis and Treatment of Impetigo

Diagnosis

Impetigo is primarily a clinical diagnosis based on characteristic skin findings, though Gram stain and culture of pus or exudates are recommended to identify whether Staphylococcus aureus and/or β-hemolytic Streptococcus is the cause, particularly when treatment fails, MRSA is suspected, or in recurrent infections. 1

Clinical Presentation

• Nonbullous impetigo (70% of cases): Begins as erythematous papules that rapidly evolve into vesicles and pustules, then rupture to form the characteristic honey-colored crusts, typically on the face and extremities 2, 3

• Bullous impetigo (30% of cases): Caused exclusively by toxin-producing S. aureus, characterized by fragile, thin-roofed vesicopustules that form large, flaccid bullae, more commonly affecting intertriginous areas 2, 3

• Ecthyma: A deeper infection than impetigo with circular, erythematous ulcers and adherent crusts, caused by S. aureus and/or streptococci 2

Diagnostic Testing

• Culture is reasonable but not mandatory in typical cases, though treatment without cultures is acceptable for straightforward presentations 1

• Cultures should be obtained when: treatment fails, MRSA is suspected, recurrent infections occur, or during outbreaks 4, 2

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Treatment Approach

Localized Disease (Few Lesions)

For limited impetigo, topical mupirocin 2% ointment applied three times daily for 5-7 days is the first-line treatment. 4, 2

• Topical mupirocin is FDA-approved for impetigo due to S. aureus and S. pyogenes, with clinical efficacy rates of 71-93% versus 35% for placebo 5

• Alternative topical option: Retapamulin 1% ointment applied twice daily for 5 days 1, 4

• Topical antibiotics are superior to placebo (RR 2.24,95% CI 1.61-3.13) and equally effective as oral antibiotics for localized disease 6

Extensive Disease or When Topical Therapy is Impractical

Oral antibiotics should be used when impetigo is extensive, involves multiple sites, topical therapy is impractical, treatment with topical antibiotics has failed, or systemic symptoms are present. 1, 2

For Methicillin-Susceptible S. aureus (MSSA):

• Dicloxacillin 250 mg four times daily (adults) or weight-adjusted dosing for children 4

• Cephalexin 250-500 mg four times daily (adults) or weight-adjusted dosing for children 4

• Duration: 7 days for impetigo; 5-10 days for ecthyma 1, 2

For Methicillin-Resistant S. aureus (MRSA):

• Clindamycin 300-450 mg three times daily (adults) 4

• Trimethoprim-sulfamethoxazole (TMP-SMX) at standard dosing 1, 4

• Doxycycline at standard dosing (NOT for children under 8 years) 1, 4

For Streptococci Alone:

• Oral penicillin is the recommended agent when cultures yield streptococci alone 1, 2

Comparative Efficacy

• Topical mupirocin is slightly superior to oral erythromycin (pooled RR 1.07,95% CI 1.01-1.13) 6

• Penicillin is inferior to other oral antibiotics: less effective than erythromycin (RR 1.29,95% CI 1.07-1.56) and cloxacillin (RR 1.59,95% CI 1.21-2.08) 6

• Mupirocin and fusidic acid have similar efficacy (RR 1.03,95% CI 0.95-1.11) 6

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Special Considerations

Pediatric Dosing

• All oral antibiotic dosing must be adjusted by weight for children 4, 2

• Tetracyclines (doxycycline, minocycline) are contraindicated in children under 8 years due to tooth discoloration risk 4, 2

Outbreak Management

• Systemic antimicrobials should be used during outbreaks of poststreptococcal glomerulonephritis to eliminate nephritogenic strains of S. pyogenes from the community 1, 2

Adjunctive Measures

• Keep lesions covered with clean, dry bandages and maintain good personal hygiene to prevent spread 4, 2

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Treatment Duration and Follow-Up

• Topical treatment: 5-7 days 4, 2

• Oral antibiotics: 5-10 days (7 days for standard impetigo) 1, 2

• Re-evaluate if no improvement after 48-72 hours of appropriate therapy 4, 2

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Common Pitfalls and Caveats

Penicillin alone is not effective for impetigo as it lacks adequate coverage against S. aureus, which is the most common causative organism. 4

• Topical disinfectants are inferior to antibiotics and should not be used as primary treatment 3, 6

• If impetigo is not responding to appropriate therapy, consider alternative diagnoses such as herpes simplex, contact dermatitis, or fungal infections 4

• Growing antibiotic resistance is a concern: MRSA, macrolide-resistant streptococcus, and mupirocin-resistant streptococcus are all documented 3

• Side effects are more common with oral antibiotics (primarily gastrointestinal) compared to topical treatment 6