What is impetigo?
Impetigo is a common acute superficial bacterial skin infection (pyoderma). It is characterised by pustules and honey-coloured crusted erosions (“school sores”).
The term impetiginisation is used for superficial secondary infection of a wound or other skin condition. Ulcerated impetigo is called ecthyma.
What causes impetigo?
In nonbullous impetigo, staphylococci and/or streptococci invade a site of minor trauma where exposed proteins allow the bacteria to adhere.
Ecthyma is usually due to Streptococcus pyogenes, but co-infection with Staphylococcus aureus may occur.
Bullous impetigo is due to staphylococcal exfoliative toxins (exfoliatin A–D), which target desmoglein 1 (a desmosomal adhesion glycoprotein), and cleave off the superficial epidermis through the granular layer. No trauma is required, as the bacteria can infect intact skin.
Who gets impetigo?
Impetigo is most common in children (especially boys), but may also affect adults if they have low immunity to the bacteria. It is prevalent worldwide. Peak onset is during summer and it is more prevalent in developing countries.
The following factors predispose to impetigo.
- Atopic eczema
- Skin trauma: chickenpox, insect bite, abrasion, laceration, thermal burn, dermatitis, surgical wound
What are the clinical features of impetigo?
Primary impetigo mainly affects exposed areas such as the face and hands, but may also affect trunk, perineum and other body sites. It presents with single or multiple, irregular crops of irritable superficial plaques. These extend as they heal, forming annular or arcuate lesions.
Although many children are otherwise well, lymphadenopathy, mild fever and malaise may occur.
Nonbullous impetigo starts as a pink macule that evolves into a vesicle or pustule and then into crusted erosions. Untreated impetigo usually resolves within 2 to 4 weeks without scarring.
Ecthyma starts as nonbullous impetigo but develops into a punched-out necrotic ulcer. This heals slowly, leaving a scar.
Bullous impetigo presents with small vesicles that evolve into flaccid transparent bullae. It heals without scarring.
Complications from impetigo
Soft tissue infection
The bacteria causing impetigo can become invasive, leading to cellulitis and lymphangiitis; subsequent bacteraemia might result in osteomyelitis, septic arthritis or pneumonia.
Staphylococcal scalded skin syndrome
In infants under 6 years of age or adults with renal insufficiency, localised bullous impetigo due to certain staphylococcal serotypes can lead to a sick child with generalised staphylococcal scalded skin syndrome (SSSS). Superficial crusting then tender cutaneous denudation on face, in flexures, and elsewhere is due to circulating exfoliatin/epidermolysin, rather than direct skin infection. It does not scar.
Toxic shock syndrome
Toxic shock syndrome is rare and rarely preceded by impetigo. It causes fever, diffuse erythematous then desquamating rash, hypotension and involvement of other organs.
Group A streptococcal infection may rarely lead to acute post-streptococcal glomerulonephritis 3–6 weeks after the skin infection. It is associated with anti-DNase B and antistreptolysin O (ASO) antibodies.
Group A streptococcal skin infections have rarely been linked to cases of rheumatic fever and rheumatic heart disease. It is thought that this occurs because strains of group A streptococci usually found on the skin have moved to the throat (the more usual site for rheumatic fever-associated infection).
How is impetigo diagnosed?
Impetigo is usually diagnosed clinically but can be confirmed by bacterial swabs sent for microscopy (gram positive cocci are observed), culture and sensitivity.
Blood count may reveal neutrophil leucocytosis when impetigo is widespread.
Skin biopsy is rarely necessary. Histological features are characteristic.
- Gram-positive cocci
- Intraepidermal neutrophilic pustules,
- Dense inflammatory infiltrate in upper dermis
- Split through granular layer of epidermis without inflammation or bacteria
- Acantholytic cells
- Minimal inflammatory infiltrate in upper dermis
- Resembles pemphigus foliaceus
- Full thickness skin ulceration
- Gram stain shows cocci within dermis
What is the treatment for impetigo?
The following steps are used to treat impetigo.
- Cleanse the wound; use moist soaks to gently remove crusts
- Apply antiseptic (povidone iodine, hydrogen peroxide cream, chlorhexidine and others) or antibiotic ointment (fusidic acid, mupirocin or retapamulin) as prescribed
- Cover the affected areas.
- If impetigo is extensive, oral antibiotics are recommended, often flucloxacillin.
To prevent recurrence:
- Treat carrier sites: apply antibiotic ointment to nostrils
- Wash daily with antibacterial soap or soak in a bleach bath
- Identify and treat the source of re-infection, ie another infected person or carrier
To reduce the chance of passing the infection to another person:
- Avoid close contact with others
- Children must stay away from school until crusts have dried out
- Use separate towels and flannels
- Change and launder clothes and linen daily