Impetigo is a common, highly contagious bacterial infection of the superficial layers of the epidermis that is most often caused by Staphylococcus aureus and less often by Streptococcus pyogenes.
Traditionally, impetigo has been divided into two forms—bullous and nonbullous (impetigo contagiosa). Sometimes, these are clinically indistinguishable; therefore, it is probably less confusing to use the term impetigo to describe both of them.
Impetigo is transmitted from person to person, or from an infected fomite and can spread via autoinoculation.
Impetigo begins as a thin-roofed fragile vesicle or pustule that soon ruptures and leaves a peripheral collarette of scale or the flaccid remains of a bulla (Figs. 16.1-16.2).
Oozing serum dries and gives rise to the classic golden-yellow, “honey-colored,” crusted lesions that seem to be stuck on the skin (Fig. 16.3). After rupturing and desiccating they may appear “varnish-like” (Fig. 16.4).
Lesions often start at the site of a minor skin injury such as an abrasion, an arthropod bite reaction, or eczema that has been excoriated.
Whether pustular or bullous, impetigo is a self-limited process and heals without scarring in 2 to 6 weeks even without treatment. Postinflammatory hyperpigmentation may occur, especially in dark-skinned persons.
Ecthyma is a deeper, dermal presentation of impetigo, often caused by group A beta-hemolytic streptococci, and is characterized by thick, often blackened adherent crusts over ulcers. Unlike impetigo, which only affects the stratum corneum, ecthyma extends into the dermis. Ecthyma occurs most often on the lower extremities and can heal with scarring (Fig. 16.5).
Rarely progresses to systemic infection; poststreptococcal glomerulonephritis is a rare complication of impetigo caused by S. pyogenes.
Rarely, cellulitis, lymphangitis, scarlet fever, erysipelas, bacteremia with subsequent pneumonitis, septic arthritis, septicemia, and bacterial endocarditis may develop.
Staphylococcal scalded skin syndrome can develop in younger children or immunocompromised adults with bullous impetigo.
Bacterial culture and sensitivity testing are recommended prior to treatment especially if standard topical or oral treatment does not result in improvement.
Recurrent or persistent impetigo may indicate a carrier state in the patient or the patient's family. A bacterial culture of the nares may be obtained to determine whether a patient is a carrier of S. aureus.
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In adults, lesions may occur anywhere on the body; however, they arise most often on exposed areas such as on the hands and face (Fig. 16.6A,B) or in skin folds particularly the axillae (Fig. 16.7).
In children, the face (particularly in and around the nose and mouth) and the exposed parts of the body (e.g., arms, legs) are the typical sites of involvement.
Impetigo can, and often does, emerge as a secondary infection of a pre-existing skin disease (Fig. 16.8) or traumatized skin; it is then referred to as secondary impetiginization.
Conditions that may lead to secondary impetiginization include the following:
Differential Diagnosis 
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