Impetigo: Taking on a common skin infection

Figure 1. Honey-colored crusts are the characteristic lesions of impetigo.
Figure 1. Honey-colored crusts are the characteristic lesions of impetigo.

Impetigo is a common skin infection most often caused by Staphylococcus aureus or Streptococcus pyogenes. The infection can be primary, occurring in undetected minor superficial breaks in the skin, or secondary to a pre-existing skin disease, a process known as “impetiginization.” For example, impetiginization can occur in lesions of eczema, molluscum contagiosum, scabies, or herpes infections. While classic impetigo is most commonly seen in children 2-6 years old, patients of all ages are susceptible.

Types of impetigo

Remember the classic “honey-colored crusts” described in textbooks and test questions? How about the picture of the adorable 2-year-old with golden-yellow crusts from the bottom of her nose to the top of her lip? Those lesions are characteristic of classic nonbullous impetigo (Figure 1). The disease begins with transient superficial vesicles or pustules so small that their rupture may go unnoticed, even by the patient. This is followed by the formation of golden yellow (honey-colored) lesions. Children often have impetigo infections on their face, neck, hands, and groin/diaper region. Impetigo lesions can be painless or sometimes sore and may be itchy.

A commonly missed variant is bullous impetigo (Figure 2). Bullous impetigo presents as vesicles and bullae (a vesicle larger than 5 mm) containing clear yellow or cloudy fluid. If the roofs of the vesicles or bullae are removed, shallow, moist erosions are seen. Most often caused by S. aureus, bullous impetigo occurs most commonly on the trunk, arms, and legs and consists predominantly of bullae.

The least common but most serious form of impetigo is ecthyma (Figure 3), which occurs when the infection penetrates deeper into the dermal layer of the skin. The presenting purulent and painful sores develop into ulcers on the hands and feet. A thick, gray-yellow crust can form. Most cases of ecthyma will leave scars, even if treated promptly.

Diagnosis

Impetigo is most often diagnosed by clinical appearance. Gram's stain depicting gram-positive cocci in chains or clusters or cultures of S. aureus or S. pyogenes are found with this infection. Cultures are confirmatory but, in consideration of cost, should be performed only when the infection does not respond to antibiotic therapy, involvement is severe, or the patient is at high risk for methicillin-resistant S. aureus (MRSA) infections, e.g., a health-care worker or teacher. Individuals with recurrent impetigo should have a nasal culture, and those who are found to be carriers of staphylococci should be started on mupirocin (Bactroban) applied three times a day to the nares, umbilicus, and anal region for approximately one week. Opinions on the duration and frequency of therapy vary from 5-10 days in a onetime treatment to one week per month for six months. Persistent infections or recurrent cases should be cultured to rule out MRSA. 

Patients who present with crusted lesions should be evaluated for perioral dermatitis, seborrheic dermatitis, herpes simplex, allergic contact dermatitis, and scabies. If bullae are present, consider allergic contact dermatitis, herpes simplex or zoster, folliculitis, bullous pemphigoid, and porphyria cutanea tarda. History, along with lesional distribution, clinical appearance, and quality (e.g., whether they are itchy or painful), will help distinguish the diagnosis.

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