Key Points

  • Impetigo is a highly contagious superficial skin infection due to Staphylococcus aureus or less commonly Streptococcus pyogenes.
  • Secondary impetigo, also known as impetiginization, may occur when these bacteria enter the skin through a disrupted barrier such as in preexisting dermatoses like atopic dermatitis, scabies, superficial cuts and abrasions, and insect bites.
  • Honey-colored crusted erosions are the typical clinical morphology for lesions of impetigo contagiosa—also known as non-bullous impetigo. Superficial bullae or erosions with surrounding scale and minimal crust are the hallmarks of bullous impetigo. Finally, ulcerative and necrotic lesions with pus can occur with ecthyma, a deeper form of non-bullous impetigo primarily due to pyogenes.
  • Impetigo is mostly self-limited, though treatment should be considered when extensive involvement occurs, when deeper involvement occurs as in ecthyma, or during outbreaks.
  • An important consideration in the treatment of impetigo is the rising incidence of methicillin-resistant S. aureus (MRSA). Bacterial cultures for the determination of species and antibiotic sensitivity are recommended when treatment is being considered.
  • Bullous impetigo is due to the local production of exfoliative toxins (ETA and ETB) produced by S. aureus, phage group II.

Introduction

Impetigo is a contagious superficial skin infection most common in children, with a peak incidence between ages 2 and 5. A recent systematic review of the global epidemiology of impetigo found a median prevalence rate of 12.3% in children and 4.9% in adults. Risk factors include poor hygiene, lower socioeconomic status and warm climate.

The two major forms of impetigo, non-bullous impetigo (impetigo contagiosa) and bullous impetigo, can masquerade as noninfectious diseases, as both may lack characteristic signs of acute infection (i.e., dolor, calor, tumor, and in rare cases may not be erythematous) and because of their tendency to form dry crusts. The clustering of lesions around facial orifices and exposed areas of the body is an important diagnostic clue. A rare deeper, ulcerative form of impetigo called ecthyma most commonly occurs on the lower extremities. Impetigo, in all its forms, can complicate preexisting skin conditions such as atopic dermatitis, scabies, insect bites, and cuts and abrasions. The presence of honey-colored crusts or erosions distinguishes impetigo contagiosa from bullous impetigo, which is typified by bullous lesions or erosions with peripheral scale and minimal or no crust. Ecthyma is notable for its distinctive purplish, ulcerated appearance that may be associated with necrosis and pus.

While in the past S. pyogenes was the most commonly implicated pathogen in non-bullous impetigo, it has been replaced more recently by S. aureus. Often, the infection can be due to a combination of these two common pathogens. Bullous impetigo is due to the local production of exfoliative toxins (ETA and ETB) produced by S. aureus, phage group II species. Systemic production of these toxins is the cause of staphylococcal scalded skin syndrome. Finally, ecthyma is due almost exclusively to S. pyogenes.

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