What is the pathophysiology, signs, symptoms, diagnosis, management, and differential diagnoses of impetigo?

Impetigo: Comprehensive Clinical Overview

Pathophysiology

Impetigo is a highly contagious superficial bacterial infection of the epidermis caused by Staphylococcus aureus and/or Streptococcus pyogenes. 1, 2

• Nonbullous impetigo (70% of cases) results from direct bacterial invasion of the epidermis by S. aureus or S. pyogenes, causing localized inflammation and characteristic honey-colored crusting 3, 4
• Bullous impetigo (30% of cases) is caused exclusively by S. aureus strains that produce exfoliative toxins, which remain localized to the infection site and cause intraepidermal cleavage, resulting in fragile fluid-filled vesicles and flaccid bullae 5, 6
• The infection predominantly affects children aged 2-5 years, with a global disease burden exceeding 140 million cases 2, 3
• Community-acquired MRSA (CA-MRSA) is an emerging pathogen of increasing concern in impetigo 1, 7

Signs and Symptoms

Nonbullous Impetigo

• Honey-colored, crusted lesions are the pathognomonic finding 3, 4
• Lesions typically appear on the face and extremities 4
• May develop on previously intact skin or secondarily infect insect bites, eczema, or herpetic lesions 4
• Discrete purulent lesions with surrounding erythema 1

Bullous Impetigo

• Large, flaccid bullae containing clear to turbid fluid 5, 6
• More likely to affect intertriginous areas (skin folds) 4
• Bullae rupture easily, leaving a collarette of scale 6

General Features

• Both types are highly contagious 2, 3
• Systemic symptoms (fever, malaise) are typically absent in uncomplicated cases 4
• Lesions usually resolve within 2-3 weeks without scarring 4

Diagnosis

Diagnosis is primarily clinical, based on the characteristic appearance of honey-colored crusted lesions or flaccid bullae. 3, 4

• Bacterial culture is indicated when treatment fails, MRSA is suspected, or in cases of recurrent infections 8
• Culture specimens should be obtained from vesicle fluid, pus, or erosions 8
• Routine cultures are not necessary for typical presentations in otherwise healthy children 4

Management

First-Line Treatment: Limited Disease

Topical mupirocin 2% ointment applied three times daily for 5-7 days is the first-line treatment for localized impetigo. 7, 2, 9

• Mupirocin demonstrates 71% clinical efficacy versus 35% for placebo, with 94% pathogen eradication rates 9
• Retapamulin 1% ointment applied twice daily for 5 days is an effective alternative 7, 2
• Topical therapy is appropriate when lesions are limited to a few sites and topical application is practical 7, 2

Second-Line Treatment: Extensive Disease

Oral antibiotics for 7 days are recommended when impetigo involves multiple sites, topical therapy is impractical, topical treatment has failed, or systemic symptoms are present. 7, 2

For Methicillin-Susceptible S. aureus (MSSA):

• Dicloxacillin 250 mg four times daily (adults) 7, 8
• Cephalexin 250-500 mg four times daily (adults) 7, 8
• Pediatric dosing must be weight-adjusted 7, 8

For Suspected Community-Acquired MRSA:

• Clindamycin 300-450 mg three times daily (adults) 7, 2, 8
• Trimethoprim-sulfamethoxazole (dose varies by formulation) 2, 8
• Doxycycline (contraindicated in children under 8 years) 7, 8

Empiric MRSA Coverage Indications

Consider empiric CA-MRSA therapy in patients with residence in long-term care facilities, failure to respond to first-line therapy, or high local MRSA prevalence. 7, 2

Treatment Duration and Monitoring

• Topical antibiotics: 5-7 days 7, 8
• Oral antibiotics: 7 days 7
• Re-evaluate if no improvement occurs after 48-72 hours of therapy 7, 8
• Complete the full antibiotic course even if symptoms improve quickly to prevent complications such as post-streptococcal glomerulonephritis 2

Adjunctive Measures

• Keep lesions covered with clean, dry bandages 7, 8
• Maintain good personal hygiene to prevent spread 7, 8
• Identify and treat asymptomatic nasal carriers of S. aureus in the patient and close contacts 6

Critical Pitfalls to Avoid

• Penicillin alone is NOT effective for impetigo as it lacks adequate coverage against S. aureus 7, 8, 4
• Topical disinfectants are inferior to antibiotics and should not be used 7, 4
• Tetracyclines (doxycycline, minocycline) must not be used in children under 8 years due to risk of dental staining 7, 8
• Trimethoprim-sulfamethoxazole provides inadequate streptococcal coverage and should not be used as monotherapy when streptococcal infection is suspected 4

Differential Diagnoses

Primary Considerations

• Bullous pemphigoid: Autoimmune blistering disorder with tense bullae, typically in elderly patients, distinguished by immunofluorescence studies 6
• Herpes simplex virus infection: Grouped vesicles on erythematous base, often with prodromal tingling; viral culture or PCR confirms diagnosis 4
• Varicella (chickenpox): Generalized distribution of vesicles in various stages, systemic symptoms, characteristic "dewdrop on rose petal" appearance 4
• Contact dermatitis: History of exposure to irritant/allergen, pruritic rather than painful, lacks honey-colored crusting 4

Secondary Considerations

• Ecthyma: Deeper ulcerative form of impetigo extending into the dermis, results in scarring 4
• Staphylococcal scalded skin syndrome: Widespread painful blistering with superficial denudation, positive Nikolsky sign, requires systemic antibiotics 5, 6
• Insect bite reactions with secondary infection: History of bites, distribution pattern consistent with exposure 4
• Atopic dermatitis with secondary impetiginization: Underlying chronic eczematous changes with acute crusting 4

Patient Counseling

• Impetigo is highly contagious; avoid close contact with others, especially children, until 24-48 hours after starting antibiotics 2, 3
• Do not share towels, clothing, or personal items during the infection period 7
• Wash hands frequently with soap and water, especially after touching lesions 7
• Keep fingernails short to minimize scratching and spread 7
• Complete the full course of antibiotics even if lesions improve quickly to prevent recurrence and complications 2
• Lesions typically heal without scarring if treated appropriately 4
• Return for re-evaluation if no improvement after 2-3 days of treatment 7, 8
• Children may return to school/daycare 24 hours after starting antibiotic therapy if lesions can be covered 7