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Updated May 16, 2014

Impetigo is a common bacterial infection of the upper layers of the skin caused by Streptococcus pyogenes and Staphylococcus aureus. It is highly contagious and usually treated with a topical antibiotic.

Cause of Impetigo

In industrialized countries most cases of impetigo are caused by Staphylococcus aureus, but in developing countries Streptococcus pyogenes is the main causitive agent. Mixed infections caused by both bacteria are common and result when a primary infection is invaded by the other bacteria.

Factors that Make Impetigo Worse

Impetigo tends to occur in areas of minor breaks in the skin such as insect bites, cuts, or abrasions. Impetigo can also occur in breaks in the skin caused by skin conditions such as eczema, scabies, herpes, chickenpox, or contact dermatitis. Often people harbor the Staphylococcus bacteria inside their nose and don't even know it because it does not cause an infection inside the nose. However, direct contact with infected nasal fluid can infect other parts of the body or other people. The following are common contributing factors:
  • Warm, humid climate or environment
  • Traumatized skin
  • Poor hygiene
  • Overcrowding
  • Nasal colonization with bacteria

Appearance of Impetigo Rash

Impetigo is classified as bullous or non-bullous. Both types have a distinct appearance and cause.

Non-Bullous Impetigo

  • More common - 70% of impetigo infections
  • Caused by either Streptococcus or Staphylococcus species
  • Hallmark of appearance is a thick "honey-colored" crust
  • Occurs mainly on face or limbs
  • Only large lesions are painful
  • Generally does not cause a fever
  • Heals without scarring

Bullous Impetigo

  • Less common form of impetigo
  • Occurs mainly in newborns and younger children
  • Caused by a specific sub-group of Staphylococcus aureus
  • Blisters form in response to a toxin produced by the bacteria
  • Occurs on face, trunk, hands, and buttocks
  • Generally does not cause a fever
  • Heals without scarring

How Impetigo is Diagnosed

Impetigo is often diagnosed clinically, based on the characteristic appearance. However, because impetigo can be confused with several other skin conditions, certain tests may be needed to make the diagnosis. The most common diagnostic tests performed are a gram stain or bacterial culture obtained by swabbing the base of a lesion with a cotton swab.


Non-bullous impetigo is treated with the topical antibiotic mupirocin (Bactroban) applied to the lesion four times a day until the lesion is gone for three days. Bullous impetigo and resistant non-bullous infections are treated with oral antibiotics. Penicillin and amoxicillin are not good antibiotics to use because of increasing resistance rates. Antibiotic classes that are effective include the macrolides (erythromycin, azithromycin, or clarithromycin) and the cephalosporins (cephalexin, cefprozil, or cefdinir).

How to Prevent Impetigo

The following measures may help prevent impetigo from developing:
  • Frequently wash areas of minor skin trauma with soap and water
  • Avoid overcrowded living conditions
  • People with recurrent impetigo should have the inside of the nose cultured to determine if they are a carrier of Staphylococcus aureus
  • Staph carriers should be treated with topical mupirocin (Bactroban) inside the nose
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