What are the diagnostic criteria and treatment options for impetigo?

Impetigo Diagnosis and Treatment

Diagnostic Criteria

Impetigo is diagnosed clinically based on characteristic skin lesions, with cultures reserved for treatment failures, suspected MRSA, or recurrent infections. 1, 2

Clinical Presentation

Nonbullous impetigo (70% of cases):

• Begins as erythematous papules that rapidly evolve into vesicles, then pustules, finally forming characteristic thick honey-colored crusts 2, 3
• Most commonly affects the face and extremities 2
• Caused by Staphylococcus aureus, Streptococcus pyogenes, or both 1, 2, 4
• Pustules enlarge and break down over 4-6 days before crusting 2

Bullous impetigo (30% of cases):

• Caused exclusively by toxin-producing strains of S. aureus 2, 3
• Characterized by fragile, thin-roofed vesicopustules that form when toxins cleave the dermal-epidermal junction 2
• More likely to affect intertriginous areas 3
• Results in large, flaccid bullae 3

When to Obtain Cultures

Cultures of vesicle fluid, pus, or erosions should be obtained in the following situations:

• Treatment failure with appropriate antibiotics 5, 2
• Suspected MRSA infection 5, 2
• Recurrent infections 5, 2
• Outbreak settings requiring epidemiological tracking 1

Key Diagnostic Pitfall

If apparent impetigo does not respond to appropriate antibiotic therapy after 48-72 hours, consider alternative diagnoses including fungal infections (tinea faciei), which can mimic impetigo clinically. 6 This is particularly important as more than 50% of impetigo-like tinea cases have prolonged courses due to misdiagnosis. 6

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Treatment Algorithm

Step 1: Assess Disease Extent

For localized impetigo (few lesions, limited to one area):

• Use topical antibiotics as first-line therapy 1, 5, 2

For extensive impetigo (multiple lesions, multiple sites, or involving the scalp):

• Use oral antibiotics 1, 5, 2

Step 2: First-Line Topical Treatment (Localized Disease)

Topical mupirocin 2% ointment applied three times daily for 5-7 days is the gold standard first-line treatment. 1, 5, 4

• Mupirocin demonstrated 71% clinical efficacy versus 35% for placebo in FDA trials 4
• Mupirocin achieved 94% pathogen eradication versus 62% for placebo 4
• Topical antibiotics are superior to placebo (RR 2.24,95% CI 1.61-3.13) 2, 7

Alternative topical option:

• Retapamulin 1% ointment applied twice daily for 5 days 5, 3

Important note: Fusidic acid is equally effective to mupirocin but is not available in the United States. 7, 8

Step 3: Oral Antibiotic Treatment (Extensive Disease)

Indications for oral antibiotics:

• Extensive disease involving multiple sites 1, 5, 2
• Topical therapy is impractical (e.g., scalp involvement, numerous lesions) 1, 5
• Treatment failure with topical antibiotics 5, 2
• Presence of systemic symptoms 5, 2
• Large bullae present 3

For Methicillin-Susceptible S. aureus (MSSA):

• Dicloxacillin 250 mg four times daily for adults 5, 9
• Cephalexin 250-500 mg four times daily for adults 5, 9
• Adjust dosing by weight for children 5, 2
• Duration: 5-10 days 2

For Methicillin-Resistant S. aureus (MRSA):

• Clindamycin 300-450 mg three times daily for adults 5, 9
• Trimethoprim-sulfamethoxazole (TMP-SMX) - dose varies by formulation 5, 9
• Doxycycline (contraindicated in children under 8 years) 5, 9

For streptococcal infection alone (rare, culture-confirmed):

• Penicillin is the drug of choice 1
• For penicillin-allergic patients: macrolide or clindamycin 2

Step 4: Re-evaluation

Re-evaluate the patient if no improvement occurs after 48-72 hours of appropriate therapy. 5, 2

At this point:

• Obtain cultures if not already done 5, 2
• Consider MRSA coverage if not already provided 5
• Consider alternative diagnoses (fungal infection, eczema, other dermatoses) 6
• Adjust therapy based on culture and susceptibility results 1

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Critical Treatment Considerations

What NOT to Use

Penicillin alone is inadequate for empiric treatment of impetigo because it lacks coverage against S. aureus. 5, 9, 7 Penicillin was inferior to erythromycin (RR 1.29,95% CI 1.07-1.56) and cloxacillin (RR 1.59,95% CI 1.21-2.08) in clinical trials. 8

Topical disinfectants are inferior to antibiotics and should not be used. 3, 10, 8 When compared directly, topical antibiotics were significantly better than disinfecting treatments (RR 1.15,95% CI 1.01-1.32). 8

Comparative Efficacy

Topical mupirocin is slightly superior to oral erythromycin (pooled RR 1.07,95% CI 1.01-1.13) in multiple studies with 581 participants. 8 This supports using topical therapy as first-line for localized disease. 1, 2

Mupirocin and fusidic acid have equivalent efficacy (pooled OR 1.76,95% CI 0.69-2.16), with no significant difference between them. 1, 7

Special Populations

Pediatric considerations:

• Impetigo most commonly affects children aged 2-5 years 1, 3, 10
• Mupirocin demonstrated 78% efficacy in children versus 36% for placebo 4
• Tetracyclines (doxycycline, minocycline) are contraindicated in children under 8 years 5, 2
• All oral antibiotic dosing must be adjusted by weight 5, 2

Prevention of Spread

Infection control measures:

• Keep lesions covered with clean, dry bandages 5, 2
• Maintain good personal hygiene 5, 2
• The responsible microorganisms initially colonize unbroken skin before inoculation occurs through abrasions, minor trauma, or insect bites 1

Recurrent Impetigo

For patients with recurrent impetigo and nasal colonization:

• Mupirocin ointment applied twice daily to anterior nares for the first 5 days of each month reduces recurrences by approximately 50% 9
• Clindamycin 150 mg daily for 3 months may decrease subsequent infections by approximately 80% 9

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Complications and Special Circumstances

Poststreptococcal Glomerulonephritis

While rare in developed countries (<1 case per 1,000 population per year), poststreptococcal glomerulonephritis may complicate impetigo caused by certain strains of S. pyogenes. 1 However, no data demonstrate that treatment of impetigo prevents this sequela. 1

Exception: Systemic antimicrobials should be used during outbreaks of poststreptococcal glomerulonephritis to help eliminate nephritogenic strains. 2

Ecthyma

Ecthyma represents a deeper variant of impetigo, caused by S. aureus and/or streptococci, characterized by circular erythematous ulcers with adherent crusts rather than superficial vesicles. 2 This requires the same antibiotic coverage as impetigo but may require longer treatment duration. 2

Antibiotic Resistance Patterns

Growing resistance is a concern:

• Methicillin-resistant S. aureus (MRSA) prevalence is increasing 3
• Macrolide-resistant streptococcus is documented 3
• Mupirocin-resistant streptococcus has been reported 1, 3
• Empiric treatment must be based on local antibiograms 1

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Side Effect Profile

Topical antibiotics cause fewer side effects than oral antibiotics. 7, 8

• Oral antibiotics cause more gastrointestinal side effects (nausea, diarrhea, abdominal pain) 7, 8
• The reported number of side effects overall is low, and most are mild 8
• No side effects were reported in the mupirocin ointment group in FDA trials 4