Impetigo

What is impetigo?

Impetigo is a common acute superficial bacterial skin infection characterised by pustules and honey-coloured crusted erosions ('school sores').

The word 'impetiginisation' is used for superficial secondary infection of a wound or other skin condition. Ulcerated impetigo is called ecthyma.

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 dermatitis
• Scabies
• Skin trauma: chickenpox, insect bite, abrasion, laceration, thermal burn, dermatitis, surgical wound

What causes impetigo?

Impetigo is most often caused by Staphylococcus aureus. Non-bullous impetigo can also be caused by group A beta-haemolytic streptococcus (Streptococcus pyogenes).

Nonbullous impetigo

In nonbullous impetigo, staphylococci and streptococci invade a site of minor trauma where exposed proteins allow the bacteria to adhere.

Ecthyma

Ecthyma is usually due to Strep pyogenes, but co-infection with Staph aureus may occur.

Bullous impetigo

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.

What are the clinical features of impetigo?

Primary impetigo mainly affects exposed areas such as the face and hands, but may also affect 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

Nonbullous impetigo starts as a pink macule that evolves into a vesicle or pustule and then into erosions with a honey-coloured crust. Untreated impetigo usually resolves within 2 to 4 weeks without scarring.

Ecthyma

Ecthyma starts as nonbullous impetigo but develops into a punched-out necrotic ulcer that heals slowly, leaving a scar.

Bullous impetigo

Bullous impetigo presents with small vesicles that evolve into flaccid transparent bullae. It heals without scarring.

Bullous impetigo in a neonate

See more images of impetigo.

Complications from impetigo

Soft tissue infection

The bacteria causing impetigo can become invasive, leading to cellulitis and lymphangitis; subsequent bacteraemia might result in osteomyelitis, septic arthritis or pneumonia.

Staphylococcal scalded skin syndrome

In infants under six years of age or adults with renal insufficiency, localised bullous impetigo due to specific staphylococcal serotypes can lead to a sick patient with generalised staphylococcal scalded skin syndrome (SSSS). Superficial crusting then tender cutaneous denudation on the face, in flexures, and elsewhere is due to circulating exfoliatin/epidermolysin, rather than a direct skin infection. It does not scar.

Toxic shock syndrome

Toxic shock syndrome is rarely preceded by impetigo. It causes fever, diffuse erythematous then desquamating rash, hypotension and involvement of other organs.

Post-streptococcal glomerulonephritis

Group A streptococcal infection may 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.

Rheumatic fever

Group A streptococcal skin infections have 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 move 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.

A blood count may reveal an elevated white cell count due ot increased neutrophils when impetigo is widespread.

Skin biopsy is rarely necessary. The histological features of impetigo are characteristic.

Non-bullous impetigo

• Gram-positive cocci
• Intraepidermal neutrophilic pustules,
• Dense inflammatory infiltrate in the upper dermis

Bullous impetigo

• Split through the granular layer of the epidermis without inflammation or bacteria
• Acantholytic cells
• Minimal inflammatory infiltrate in the upper dermis
• Resembles pemphigus foliaceus

Ecthyma

• Full-thickness skin ulceration
• Gram stain shows cocci within the dermis

What is the treatment for impetigo?

General measures

• Cleanse the wound; use moist soaks to remove crusts gently.
• Apply antiseptic 2–3 times daily for five days (povidone-iodine, hydrogen peroxide 1% cream, chlorhexidine, superoxidised solution and others).
• Cover the affected areas.

Specific measures

• Oral antibiotics are recommended if:

• • Symptoms are significant or severe (fever, malaise)
  • There are more than three lesions
  • There is a high risk of complications
  • The infection is not resolving or is unlikely to resolve.

Suitable oral antibiotics in New Zealand are flucloxacillin 500 mg four times daily for 5 days (adult dose), and in case of allergy or bacterial resistance, trimethoprim + sulphamethoxazole 960 mg, twice daily, for five days (adult dose), erythromycin 800 mg twice daily for 5 days, or cephalexin 1 g twice daily for 5 days [2].

In New Zealand, antibiotic ointment (fusidic acid, mupirocin or retapamulin) is discouraged, due to the potential to develop bacterial resistance and contact allergic dermatitis.

Preventative measures

• Treat carrier sites: apply antiseptic ointment to nostrils
• Wash daily with antibacterial soap or soak in a bleach bath
• Cut nails and keep hands clean
• Identify and treat the source of re-infection, usually another infected person or carrier in the household.

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 or for 24 hours after starting oral antibiotics
• Use separate towels and flannels
• Change and launder clothes and linen daily.