Impetigo and Ecthyma
On this page
Direct answer
Impetigo and ecthyma are superficial bacterial skin infections, usually caused by Staphylococcus aureus or Group A Streptococcus, and they differ in how deep the infection extends. Impetigo stays in the epidermis and produces honey-coloured crusts or flaccid bullae, while ecthyma penetrates the dermis and forms an ulcer under a thick crust. Streptococcal skin infection carries a risk of post-streptococcal glomerulonephritis, and mupirocin ointment treats limited disease.
What you must remember
- Non-bullous impetigo: the common form, with honey-coloured crusts on a red base, often around the nose and mouth.
- Bullous impetigo: flaccid bullae with a thin roof, seen mainly in infants and young children, usually caused by Staphylococcus aureus producing exfoliative toxin.
- Ecthyma: a deeper ulcer covered by a thick, adherent crust, typically on the legs, which heals with scarring.
- Organisms: Staphylococcus aureus and Group A Streptococcus, alone or together.
- Glomerulonephritis risk: post-streptococcal glomerulonephritis follows nephritogenic Group A streptococcal infection, typically three to six weeks after a skin infection.
- Topical therapy: mupirocin ointment for limited, localised disease.
- Systemic therapy: oral antibiotics such as flucloxacillin, cloxacillin or cephalexin for extensive, bullous or deeper disease, guided by local sensitivity.
A stepwise approach from examination to follow-up
Impetigo is a common and easily treated infection, but the depth and extent of disease decide the treatment. Examine the lesions for depth and extent before choosing between topical and systemic therapy.
- Assess the depth: decide whether the lesion is superficial with a honey crust or deeper with an ulcer under a thick crust.
- Assess the extent: count the lesions and note whether the infection is localised or widespread.
- Check for systemic features: look for fever, toxicity or widespread blistering, which need urgent review.
- Treat localised disease: apply mupirocin ointment to the lesions for a limited number of days.
- Treat extensive disease: use an oral antibiotic chosen by local sensitivity, with cover for ecthyma and bullous lesions.
- Monitor for complications: check the urine for haematuria, and examine for oedema and hypertension in the weeks after a streptococcal infection.
The table below compares the three clinical forms by depth, appearance and organism.
| Form | Depth | Key appearance | Common organism |
|---|---|---|---|
| Non-bullous impetigo | epidermis | honey-coloured crusts | S. aureus or S. pyogenes |
| Bullous impetigo | epidermis | flaccid bullae with thin roof | S. aureus |
| Ecthyma | dermis | ulcer under thick crust | mainly S. pyogenes |
Depth explains the treatment. Epidermal disease responds to topical therapy, whereas ecthyma and widespread disease need systemic cover, and the clinician should not treat them as the same condition.
Where impetigo questions catch candidates
Impetigo looks simple, but examiners test the depth distinction and the streptococcal complication.
- Trap: treating ecthyma with topical mupirocin alone Correct: ecthyma extends into the dermis, so systemic antibiotics are usually needed alongside topical care.
- Trap: assuming post-streptococcal glomerulonephritis follows only pharyngitis Correct: it also follows streptococcal skin infection, especially in tropical settings, so urine should be checked after impetigo too.
- Trap: confusing bullous impetigo with bullous pemphigoid Correct: bullous impetigo is an infection of children with flaccid bullae and a bacterial cause, whereas pemphigoid is autoimmune and usually affects older adults.
Frequently asked questions
Which organisms cause impetigo?
Impetigo is caused mainly by Staphylococcus aureus and Group A Streptococcus, either alone or together. Bullous impetigo is most often due to Staphylococcus aureus producing exfoliative toxin, while non-bullous impetigo can be caused by either organism.
How does ecthyma differ from non-bullous impetigo?
Ecthyma extends into the dermis and forms an ulcer under a thick, adherent crust, whereas non-bullous impetigo stays in the epidermis with honey-coloured crusts. Ecthyma heals with scarring, and it often needs systemic antibiotics.
Why is post-streptococcal glomerulonephritis a concern after impetigo?
Post-streptococcal glomerulonephritis can follow impetigo caused by nephritogenic Group A streptococcal strains, typically three to six weeks after the skin infection. Patients should be monitored for haematuria, oedema and raised blood pressure during this period.
When is topical mupirocin enough for impetigo?
Topical mupirocin ointment is usually enough for limited, localised impetigo with few lesions and no systemic features. Extensive, bullous or deeper disease such as ecthyma needs oral antibiotics, with the choice guided by local sensitivity patterns.
Which systemic antibiotics are used for extensive impetigo?
Oral flucloxacillin, cloxacillin or cephalexin are commonly used for extensive impetigo, with the choice guided by local sensitivity. Erythromycin or clindamycin are options for patients who cannot take penicillin-type drugs.