Impetigo

Impetigo is a highly contagious superficial bacterial infection typically caused by Staphylococcus aureus (most common) and Streptococcus pyogenes. Impetigo most commonly presents in children aged 2 to 5 years with lesions that evolve from papules to vesicles to pustules, which eventually break down to form characteristic “honey-colored” crusts. Infection can either be primary (bacterial infection of healthy, intact skin) or secondary (infection of pre-existing abrasions). The diagnosis is clinical, and management includes topical or systemic antibiotic therapy. Complications of impetigo include post-streptococcal glomerulonephritis, cellulitis, and scarlet fever.

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Epidemiology and Etiology

Epidemiology

  • Any age can be affected
  • Most commonly affects children aged 25 years
  • Known to cause epidemics or outbreaks in preschools and schools (easily spread among individuals with close contact)

Etiology

  • Impetigo is a bacterial skin infection caused by:
    • Staphylococcus aureus: causes 80% of cases, both bullous and non-bullous forms
    • Streptococcus pyogenes (group A beta-hemolytic streptococcus): causes approximately 10% of cases, non-bullous forms only
    • S. aureus and S. pyogenes co-infection: occurs in approximately 10% of cases
  • 2 forms:
    1. Primary impetigo: bacterial infection of intact, healthy skin
    2. Secondary impetigo (impetiginization): more common, secondary infection of pre-existing skin lesions, such as scabies, insect bites, small cuts, eczema, etc.
  • Transmission: 
    • Highly contagious
    • Spread by direct contact with lesions or with nasal carrier
  • Risk factors
    • Children aged 25 years
    • Skin trauma (e.g., abrasion, lacerations, animal bite, or sting)
    • Poor hygiene
    • Crowded conditions
    • Warm, humid weather (increased incidence in the summer)
    • Participation in sports with skin-to-skin contact
    • Diabetes mellitus
    • Malnutrition

Clinical Presentation

There are 3 variants of impetigo:

Non-bullous impetigo

  • Most common: approximately 70% of cases
  • Previously known as “contagious impetigo”
  • Begins as a rash with papulesvesicles surrounded by erythema → pustules, which rupture and ooze exudate (pus and serous fluid) that dries → pruritic honey-colored crusts that heal with no scarring
  • Lesions are non-tender and pruritic
  • Lesions usually occur around the mouth and nose and/or on the hands

Bullous impetigo

  • 30% of cases
  • Begins as a rash with papules → vesicles → large, flaccid bullae, which are pruritic and rupture, oozing cloudy or yellow fluid (pus) → dries into brown crusts → may lead to scarring in severe infections 
  • Positive Nikolsky sign: sloughing off of the outermost layer of skin upon slight rubbing or lateral traction
  • Lesions usually occur on the trunk
  • May also present with systemic symptoms (fatigue, fever, weakness, general malaise)

Ecthyma

  • Rare; exact incidence unknown
  • Ulcerative form that affects deeper layers of the dermis (also known as “deep impetigo”)
  • Begins as a rash with papules → vesicles → sores that are painful, erythematous, and fluid- or pus-filled → coin-sized ulcers with a “punched-out” appearance covered with thick gray-yellow scabs → usually lead to scarring 
  • Lesions usually occur on the extremities
  • Ecthyma gangrenosum is most commonly associated with Pseudomonas bacteremia, seen in patients who are critically ill and immunocompromised; hemorrhagic pustules evolve into necrotic ulcers.

Diagnosis

Diagnosis is usually clinical, based on the natural sequence of the lesions and the presence of honey-colored crusts on pediatric patients aged 2–5 years.

  • Gram stain and culture of exudate are not usually required; only needed in the case of:
    • Resistance to therapy
    • Recurrence
    • Inconclusive clinical diagnosis
  • Imaging: X-ray, magnetic resonance imaging (MRI), and/or bone scintigraphy may be performed in the case of severe joint pain to confirm or rule out septic arthritis or osteomyelitis

Management and Complications

Management

Management depends on the type and severity of the infection.

  • Mild to moderate cases of non-bullous impetigo:
    • Topical antibiotic creams or ointments for 7 days (e.g., mupirocin, retapamulin)
  • Bullous impetigo, ecthyma, and severe cases of non-bullous impetigo:
    • First-line: oral antibiotics including cephalexin and dicloxacillin
    • If β-hemolytic streptococci are detected, oral penicillin is preferred.
  • Cases of methicillin-resistant S. aureus (MRSA) impetigo:
    • Clindamycin, trimethoprim-sulfamethoxazole, or doxycycline
    • Note: Retrospective data indicate that at standard doses, children < 8 years can receive up to 5 courses of doxycycline without detectable evidence of tooth staining. Drug-induced tooth discoloration in children had been reported with older classes of tetracyclines.

Prevention of spread

  • Regular handwashing
  • Covering of open skin lesions
  • Children may return to school 24 hours after beginning antibiotic treatment

Complications

  • Scarring or disfigurement
  • Poststreptococcal glomerulonephritis
  • Scarlet fever
  • Cellulitis
  • Septic arthritis 
  • Staphylococcal scalded skin syndrome
  • Septicemia or sepsis
  • Superimposed bacterial infection of primary impetigo lesions

Differential Diagnosis

The following conditions also present with bullae and serve as differential diagnoses for bullous impetigo:

  • Pemphigus vulgaris and bullous pemphigoid: autoimmune blistering disorders that present with tense blisters and bullae in the case of bullous pemphigoid and flaccid bullae and mucosal erosions in the case of pemphigus vulgaris
  • Second-degree burns: tissue injury caused by local heat exposure and heating of the respective tissue. Second-degree burns are superficial, limited to the epidermis, and present as hyperemia, wet wound bed, intact sensibility, and blistering. 
  • Varicella: primary infection with varicella zoster virus. Occurs most commonly in children and presents as a pruritic rash, with macules that rapidly develop into papules and then vesicles

The following conditions are other types of superficial skin infections that are included in the differential diagnoses of impetigo:

  • Erysipelas: acute superficial infection of the upper dermis and involving lymphatics. Usually caused by bacterial agents such as group A β-hemolytic streptococcus, it presents as a sharply demarcated skin lesion with erythema, edema, and warmth. 
  • Dermatitis: a general term for an edematous skin rash caused by an allergic reaction or a non-bacterial infection 
  • Cutaneous candidiasis: a superficial infection caused by Candida spp. Usually associated with immunodeficiency
  • Tinea: an infection of the skin caused by dermatophytes. There are 3 types of dermatophytes: Trichophyton, Epidermophyton, and Microsporum
  • Herpes simplex viral infection: a viral infection that presents as recurring, erythematous vesicles that turn into painful ulcerations, usually around in and around the mouth (HSV type 1 is more common at this site)
  • Folliculitis: a localized inflammation of the hair follicle or sebaceous glands primarily caused by S. aureus. It is characterized by erythema, tender papules, and/or pustules at the site of hair follicles.

References

  1. Baddour LM. Impetigo. (Literature review current through: July 2020). UpToDate Evidence-Based Medicine. https://www.uptodate.com/contents/impetigo?search=impetigo&source=search_result&selectedTitle=1~122&usage_type=default&display_rank=1
  2. Emanuel P. Impetigo. (2013). DermNet New Zealand (Supported by and contributed to by New Zealand Dermatologists on behalf of the New Zealand Dermatological Society Incorporated). https://dermnetnz.org/topics/impetigo-pathology/

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