What is impetigo and how is it treated?

Impetigo: Diagnosis and Management

Impetigo is a highly contagious bacterial skin infection affecting the superficial layers of the epidermis that should be treated with topical mupirocin or retapamulin for limited disease, or oral antibiotics active against Staphylococcus aureus for extensive disease. 1

Clinical Characteristics

Impetigo presents in two main forms:

1. Nonbullous impetigo (70% of cases):

   • Caused by Staphylococcus aureus or Streptococcus pyogenes
   • Characterized by honey-colored crusts on the face and extremities
   • Begins as erythematous papules that rapidly evolve into vesicles and pustules
   • These rupture, forming the characteristic honey-colored crusts on an erythematous base 1, 2

2. Bullous impetigo (30% of cases):

   • Caused exclusively by toxin-producing strains of S. aureus
   • Results in large, flaccid bullae that may rupture
   • More likely to affect intertriginous areas 1, 2

Impetigo primarily affects children 2-5 years of age, though it can occur at any age. The condition typically resolves within 2-3 weeks without scarring but may leave temporary depigmented areas 1, 3.

Diagnosis

Diagnosis is primarily clinical based on:

• Characteristic appearance of lesions (honey-colored crusts or bullae)
• Lesions commonly affecting exposed areas of the body (face and extremities)
• Well-localized but frequently multiple lesions
• Regional lymphadenitis may occur, but systemic symptoms are usually absent 1

Bacterial cultures should be obtained before initiating therapy to guide subsequent treatment, especially in cases that don't respond to empiric therapy 1.

Treatment

For Limited Disease:

Topical therapy is first-line for limited impetigo 1, 4:

• Mupirocin 2% ointment applied three times daily for 5-7 days (FDA-approved with 71% clinical efficacy rate versus 35% for placebo) 5
• Retapamulin twice daily for 5 days 1

Clinical trials have demonstrated that topical mupirocin is slightly superior to oral erythromycin (93% vs 78.5% efficacy) with fewer side effects 5.

For Extensive Disease:

Oral antibiotics are recommended for extensive impetigo (multiple lesions or large affected areas) 1, 6:

• 7-day regimen with an agent active against S. aureus:
  • For MSSA: Dicloxacillin or cephalexin
  • For suspected MRSA: Doxycycline, clindamycin, or trimethoprim-sulfamethoxazole 1

Important note: Penicillin alone is not effective for impetigo treatment as it has poor activity against S. aureus 1, 2.

Special Populations:

• Neonates: IDSA recommends intravenous vancomycin as first-line treatment. For full-term neonates with very limited disease, topical mupirocin may be adequate 1

Treatment Algorithm:

1. Assess extent of disease:

   • Limited (few lesions): Use topical antibiotics
   • Extensive (multiple lesions): Use oral antibiotics

2. For limited disease:

   • First-line: Mupirocin 2% ointment three times daily for 5-7 days
   • Alternative: Retapamulin twice daily for 5 days

3. For extensive disease (7-day course):

   • No MRSA risk factors: Dicloxacillin or cephalexin
   • MRSA risk factors present: Clindamycin, doxycycline, or trimethoprim-sulfamethoxazole

4. Follow-up:

   • Ensure resolution within 7-14 days
   • If no improvement after 3-4 days, reassess diagnosis and consider bacterial culture

Prevention and Control

Impetigo remains contagious until 24 hours after starting appropriate antimicrobial therapy. Prevention measures include 1:

• Good hygiene practices
• Avoiding sharing personal items
• Covering lesions with bandages
• Washing hands frequently
• Cleaning and disinfecting surfaces
• Avoiding close contact activities until 24 hours after starting treatment

Complications

While rare, the most serious potential complication is poststreptococcal glomerulonephritis, which can occur following infection with certain strains of S. pyogenes. However, treatment of impetigo has not been proven to prevent this sequela 6, 2.

Emerging Concerns

Increasing prevalence of antibiotic-resistant bacteria is a growing concern, with methicillin-resistant S. aureus (MRSA), macrolide-resistant streptococcus, and mupirocin-resistant streptococcus all documented 2. This highlights the importance of obtaining cultures in non-responsive cases.