Section 20. Cutaneous Bacterial Infections

Impetigo is a common superficial infection of the skin characterized by honey-colored crusts or bullae, typically caused by S. aureus, sometimes caused by Streptococcus pyogenes, or both.

Insight

Bacterial resistance such as community-acquired methicillin-resistant Staphylococcus aureus (MRSA) is becoming more prevalent; thus, treatment of infections must take evolving resistance patterns into account.

Synonyms Bullous impetigo, blistering distal dactylitis, impetigo contagiosa.

Epidemiology

Age Preschool children, young adults.

Gender M = F.

Incidence Common; 10% of dermatology visits.

Season Peak summer and fall.

Etiology Bullous impetigo is caused by phage group II Streptococcus. Vesiculopustular impetigo is caused by β-hemolytic Streptococcus.

Predisposing Factors Colonization of the skin and/or nares of the patient or patient’s family members, warm temperatures, high humidity, poor hygiene, atopic diathesis, skin trauma.

Pathophysiology

Crusted impetigo is caused by S. aureus or occasionally Streptococcus pyogenes at sites of skin trauma. Bullous impetigo is caused by a S. aureus exfoliative toxin, which binds to desmoglein I, cleaving its extracellular domain, resulting in an intraepidermal blister.

History

The skin lesions begin as erythematous areas, which may progress to superficial vesicles and bullae that rupture and form honey-colored crusts. The skin lesions are contagious and spread by person-to-person contact or fomites. Systemic symptoms are rare but can include fever and lymphadenopathy.

Physical Examination

Skin Findings

Type Macules, vesicles, bullae, crusts, and erosions (Fig. 20-1).

Color Pink, yellow “stuck-on” crusts.

Size 1 to 3 cm.

Shape Round or oval.

Arrangement Discrete, confluent, or satellite lesions from autoinoculation.

Distribution Face, arms, legs, buttocks, distal fingers (Fig. 20-2), toes.

Differential Diagnosis

In the early vesicular stage, impetigo may simulate varicella, herpes simplex, or candidiasis. The bullous stage may be confused with bullous insect bites, autoimmune bullous dermatoses, or burns. The crusted stage may resemble eczematous dermatoses or tinea infections.

Laboratory Examinations

Dermatopathology Acantholytic cleft in the stratum granulosum with leukocytes and may show scattered gram-positive cocci. Bacteria are not present within the blister cavity of bullous lesions.

Bacterial ...