Dermatology & Pediatrics & Infectious Diseases & Family Medicine
Impetigo
Impetigo is the most common superficial bacterial skin infection in clinical practice, particularly in children aged 2 to 10. It is caused mainly by Staphylococcus aureus - the sole agent in 70-80 % of cases in North America - or by Streptococcus pyogenes (group A beta-hemolytic streptococcus - SBHA), or by a co-infection of the two germs. Impetigo is a contagious infection, transmitted by direct contact with lesions or contaminated objects, and spreads easily in pre-school, school and home environments. There are two main clinical forms: crusty (non-bullous) impetigo - the most common form (70 % of cases), characterized by vesicopustules that rapidly develop into yellowish, melicoid crusts - and bullous impetigo - a rarer form (30 % of cases), exclusively due to exfoliatin-producing S. aureus phage group II (ET-A and ET-B), characterized by flaccid bullae with clear or cloudy contents. Impetigo is superficial - limited to the epidermis and stratum corneum - which distinguishes it from ecthyma (a deep ulcerated form reaching the dermis) and cellulitis (a deep cutaneous infection). Treatment of localized impetigo is based on topical (mupirocin or fusidic acid) or systemic antibiotic therapy, depending on the extent and severity of the infection. The rare but serious complication is post-streptococcal glomerulonephritis (PSG), which can occur 1 to 3 weeks after SBHA impetigo - but antibiotic therapy does not prevent this complication.
Microbiology, pathophysiology, and clinical presentation
- Pathogen agents, virulence mechanisms, and predisposing factors: causative agents of impetigo in North America: Staphylococcus aureus: main agent → 70-80 % of impetigos → crusty form + bullous form (exfoliatin-producing strains) → almost universal penicillin resistance (plasmid beta-lactamase) → community MRSA (methicillin-resistant Staphylococcus aureus): <3-5 % of outpatient impétigos in Canada → increasing in some communities → Streptococcus pyogenes (SBHA): solely responsible in 15-25 % of cases → co-infection with S. aureus in 10-15 % → M neph serotypes (nephritogenic M-types: M2 + M49 + M55 + M57 + M60) → risk of post-streptococcal glomerulonephritis → Trask 2003 - Postgraduate Medical Journal: epidemiology of impetigo + evolution of causative agents → S. aureus now predominant vs SBHA dominant in 1960s-80s; virulence mechanisms: S. aureus - virulence pathways in bullous impetigo : exfoliatins A and B (ET-A = chromosomal gene-encoded serine protease + ET-B = plasmid-encoded) → cleave desmoglein 1 (intercellular adhesion protein of granular layer desmosomes) → intraepidermal detachment → flaccid bubbles beneath granular layer → analogy with SSSS (Staphylococcal Scalded Skin Syndrome) but localized in bullous impetigo + S. aureus - crusty impetigo: cutaneous colonization → invasion of epidermal microlesions → activation of inflammatory response → pustulation → rupture → meliceric crusts → SBHA - virulence mechanisms: anti-phagocytic M protein (resistance to opsonization) + superantigens (SPE-A + SPE-B + SPE-C) → erythrogenic toxins → fibronectin-binding proteins → glomerular lesions (GNS) by deposition of anti-M protein immune complexes + impetigo-promoting factors: skin abrasions (insect bites + scratches + eczema + wounds + chickenpox - secondary impetigo ++) + heat and humidity + lack of hygiene + close contact (day-care centers + nurseries + camps + contact sports) + immunodepression (HIV + corticoids) + diabetes + age (infants + children especially) + nasal carriage of S. aureus (main reservoir - 20-30 % of the general population) + malnutrition (developing countries); epidemiology: 111 million cases of impetigo in children worldwide (WHO) → prevalence in developed countries: 2-10 % of children + distribution: summer + spring (heat + humidity + insects + outdoor activities) + geographical location: tropical zones + hot climate + seasonal in temperate zones
- Clinical Presentation — Crusted Impetigo and Bullous Impetigo: crusty (non-bullous) impetigo - most frequent form (70 %): location: face (especially perioral + peri-nasal) + limbs + hands + cervical region → begins as superficial, thin-walled vesicles or pustules → wall ruptures very rapidly (sometimes clinically imperceptible) → oozing of a yellowish serous + purulent exudate → rapid drying → formation of melicotic crusts (honey-brown-yellowish color + crisp - very characteristic) → crusts may be thick + grayish or greenish in case of coinfection → slightly raised erythematous base → lesions not very painful (except when crusts are mobilized) → sometimes pruritic → reactive satellite adenopathies → centrifugal extension + satellite lesions by auto-inoculation + no fever in uncomplicated forms + healing without scarring in 7-14 days under treatment → natural duration without treatment: 2-4 weeks → bullous impetigo - minority form (30 %) - exclusively S. aureus phage group II: location: trunk + face + extremities → thin-walled, transparent flaccid bubbles + sometimes cloudy (discrete erythematous background) → serous content initially → evolves to yellowish or cloudy purulent content + 1-3 cm in diameter → rupture easily → leave a moist superficial erosion surrounded by a scaly border (epidermal collar) → no melicoid crusts (unlike crusty impetigo) → less contagious but more rapidly extensive + can affect infants and newbornsborn (omphalitis + neonatal impetigo) + if very extensive → SSSS syndrome (staphylococcal scalded skin syndrome) → dermatological emergency → ecthyma : deep + ulcerative form (necrosis of epidermis + superficial dermis) → SBHA predominant → thick crusts + grayish or blackish → underlying ulcer + slow to heal + frequent scarring → more frequent in tropical areas + in immunocompromised patients + treatment identical to impetigo but more prolonged; mainly clinical diagnosis: impetigo is generally a clinical diagnosis → bacterial culture: indicated if : frequent recurrence + resistance to treatment + extensive or severe lesions + immunodepression + community epidemic context (MRSA) → sampling: swab under the scab (not on the scab) + contents of an intact bulla → antibiogram will guide 2nd-line treatment if necessary → ASLO serology (antistreptolysin O) + DNASE B: not routinely indicated in uncomplicated impetigo → useful if GNS suspected (edema + hematuria + HTA in the weeks following infection)
Treatment, Complications, and Public Health Measures
| Treatment / appearance | Data, methods and results | Key studies and recommendations |
|---|---|---|
| Topical treatment — mupirocin and fusidic acid Mupirocin — fusidic acid — retapamulin — topical treatment — crust softening — duration — resistance |
Topical treatment - indicated in localized forms (512 mg/L): often plasmidic + transferable → emergence in some MRSA - remain vigilant → epidemiological surveillance + fusidic acid (Fucidin) 2 % cream or ointment: mechanism: inhibition of protein elongation (elongation factor EF-G) → bacteriostatic to bactericidal depending on concentration → spectrum: S. aureus + coagulase-negative staphylococci + SBHA → dosage: application × 3/d × 5-7 days → efficacy similar to mupirocin + available over-the-counter in Canada in certain formulations → resistance to fusidic acid: more frequent than mupirocin in certain regions (Northern Europe) → emerging in S. aureus → avoid if local resistance documented + retapamulin (Altabax) 1 % ointment: topical pleuromutilin → mechanism different from other agents → spectrum: S. aureus MSSA + SBHA → no efficacy on MRSA → FDA approval 2007 → limited availability in Canada → alternative if resistant to mupirocin and fusidic acid + ozenoxacin (Xepi) cream 1 %: topical fluoroquinolone → FDA approval 2017 → effective against MRSA and SBHA → limited availability in Canada → Moran 2012 - Journal of the American Medical Association: topical mupirocin vs placebo + bacteriological cultures → superior for localized impetigo | Koning 2012 - Cochrane Review (meta-analysis of 68 clinical trials): topical mupirocin ≥ systemic antibiotic therapy for localized impetigo → cure rate 87-94 % → strong recommendation + Moran 2012 - JAMA: topical antibiotics in bacterial skin infections → mupirocin effective + Williamson 2014 - Cochrane: fusidic acid vs mupirocin → comparable efficacy + resistance to fusidic acid more of a concern in some areas + IDSA 2014 Clinical Practice Guidelines (Stevens - Clinical Infectious Diseases): topical mupirocin or fusidic acid = 1st-line treatment for localized non-bullous impetigo + mupirocin or systemic antibiotic therapy for more extensive forms + Stevens 2014 - Clinical Infectious Diseases (IDSA Guidelines): topical antibiotics recommended for localized impetigo → systemic if extensive or bullous + Health Canada + CADDAC (Canadian Anti-Microbial Resistance Alliance): recommendations on the prudent use of topical antibiotics |
| Systemic antibiotic therapy — indications, choice, and duration Cephalexin — cloxacillin — amoxicillin-clavulanate — clarithromycin — TMP-SMX — MRSA — resistance — indications — duration — treatment failure |
Indications for systemic antibiotic therapy in impetigo : extensive forms (>5 lesions or surface area >9 cm²) + extensive bullous impetigo + rapidly progressive lesions + extensive facial involvement (periorbital area + eyelids) + suppurated adenopathies + fever + systemic signs + immunosuppression + treatment failure topical treatment × 7 days + suspected or documented MRSA + recurrent impetigo despite correct local treatment + high risk of GNS (documented nephritogenic SBHA serotype or local epidemic) → systemic antibiotics - 1st line (forms without MRSA suspicion) : cefalexin (Keflex): 1st-generation cephalosporin → antistaphylococcal (SAMS) + antistreptococcal activity → dosage: adult 500 mg × 4/d × 7 days → child 25-50 mg/kg/d in 4 doses × 7 days → very good coverage of S. aureus MSSA + SBHA → reference treatment in North America → cloxacillin (Orbenin): antistaphylococcal penicillin M → narrow spectrum (S. aureus MSSA) → adult 250-500 mg × 4/d × 7 days → fasting for optimal absorption → less convenient (4 intakes/d fasting) + amoxicillin-clavulanate (Clavulin): broad spectrum + MSSA + SBHA + resistant to beta-lactamases → alternative if several mixed etiologies suspected → less preferable in 1st line (wider selection pressure) → macrolides (clarithromycin - Biaxin + azithromycin - Zithromax): reserve for documented beta-lactam allergy → resistance rate of S. aureus resistance to macrolides: 10-25 % in North America → less effective → clarithromycin 500 mg × 2/d × 7 days + azithromycin 500 mg D1 then 250 mg D2-J5 → community MRSA: TMP-SMX (Bactrim - Septra): double-strength 160/800 mg × 2/d × 5-7 days → effective against community MRSA (CA-SARM) + resistant SBHA but TMP-SMX SBHA : variable resistance - risk of ineffectiveness on SBHA alone → combine with a beta-lactam if SBHA suspected → doxycycline 100 mg × 2/d × 7 days: effective on community MRSA + not in children <8 years (tooth staining) → clindamycin 300-450 mg × 3/d × 7 days: S. aureus MSSA + MRSA (if susceptible) + SBHA → check for inducible resistance to clindamycin (D-test) before use → if MRSA documented resistant to clindamycin → TMP-SMX or doxycycline → hospitalizations (severe forms or SSSS): oxacillin or cloxacillin IV + or vancomycin IV if MRSA + flucloxacillin IV if available → infant (7 days for simple forms + in case of documented culture → adapt according to antibiogram. | Stevens 2014 - Clinical Infectious Diseases (IDSA Practice Guidelines for Skin and Soft Tissue Infections): cefalexin or dicloxacillin = 1st-line treatment of extensive impetigos + MRSA → TMP-SMX or doxycycline → Moran 2006 - NEJM (NEJM US study 11 emergency centers): S. aureus → 59 % MRSA → TMP-SMX + clindamycin = most effective treatments against CA MRSA + Kiani 1997 - Journal of International Medical Research: 5-day treatment not less than 10 days for mild forms → reduction in duration without loss of efficacy + Goldstein 2020 - UpToDate: review of available agents + Jeng 2020 - Journal of the American Academy of Dermatology: community-acquired MRSA and skin infections + Bangert 2012 - American Family Physician: impetigo - treatment and complications → Canadian Paediatric Society (CPS) 2022: recommendations on the treatment of bacterial skin infections in children → cefalexin as 1st-line treatment + TMP-SMX if MRSA suspected + AMMI Canada 2020: recommendations on empirical antibiotic therapy for skin infections → local sensitivities to be monitored |
| Complications — post-streptococcal glomerulonephritis, SSSS, and preventive measures GNS — ASLO — DNASE B — hematuria — edema — hypertension — SSSS — school exclusion — transmission — nasal decolonization — prevention — recurrence |
Post-streptococcal glomerulonephritis (GNS) - immune complication of SBHA impetigo: mechanism: deposition of anti-protein M immune complexes → complement activation → glomerular inflammation + Couser 1993 - Kidney International: mechanisms of post-streptococcal GNS + delay after impetigo: 1-6 weeks (3 weeks on average - vs 1-3 weeks after streptococcal pharyngitis) → nephritogenic SBHA serotypes: M2 + M49 + M55 + M57 + M60 (cutaneous serotypes - different from pharyngeal serotypes M1 + M3 + M12) → incidence of GNS after nephritogenic SBHA impetigo: 5-10 % in epidemics → often underdiagnosed (frequent asymptomatic forms) + clinical picture of GNS: edema of lower limbs + periorbital + oliguria + macroscopic hematuria (coca-cola or tea-colored urine) + arterial hypertension + proteinuria + hypo-complementemia (lowered C3 + normal C4 - alternate pathway activation) → ASLO (antistreptolysin O): often normal or low in impetigo (unlike pharyngeal infections) + DNASE B + anti-hyaluronidase (AHase): more sensitive for impetigo → dosage recommended if GNS suspected → biology: hematuria + proteinuria + haematic cylinders (urine + urine cytology) + creatinine + C3/C4 complement → complete renal workup + IMPORTANT: antibiotic treatment of impetigo does not prevent GNS (unlike SBHA pharyngitis, where early treatment reduces the risk of RAA) → it eradicates SBHA + reduces contagiousness → SAPL regresses spontaneously in 90-95 % of cases in 6-8 weeks → symptomatic treatment (antihypertensives + sodium restriction + diuretics if necessary) ; staphylococcal epidermal necrolysis syndrome (SSSS - Staphylococcal Scalded Skin Syndrome): severe complication of extensive bullous impetigo → circulating exfoliatins A and B → generalized cleavage of desmoglein 1 → extensive epidermal detachment → «scalded skin» appearance → positive Nikolsky sign (skin slips under pressure) → mainly affects infants + children <5 years (adults eliminate exfoliatins + have immunity) + immunocompromised adults → adult mortality: 50-60 % → in children: <5 % if treated early → urgent hospitalization + IV vancomycin + wound care (such as burns) + rehydration + corticosteroids CONTRAINDICATED (aggravate epidermolysis) → differential diagnosis: toxic epidermal necrolysis (NET - Lyell): drug-induced + no Nikolsky's sign in the same distribution + ; preventive measures and school avoidance: contagiousness: impetigo is contagious by direct contact → soiled lesions or hands → up to 24-48h after start of effective antibiotic treatment → children can return to nursery or school: 24-48h after the start of effective antibiotic treatment + or if all lesions are covered (adhesive bandage or clothing if not accessible) + hygiene rules: wash hands frequently + do not share towels + clothing + cut nails short + avoid scratching + nasal decolonization with mupirocin if nasal carriage of S. aureus in case of frequent recurrences (nasal carriage = main reservoir) → Creech 2015 - New England Journal of Medicine (NEJM - MRSA decolonization): decolonization protocol (nasal mupirocin + chlorhexidine bath) → reduction of recurrent S. aureus infections → applicable to recurrent impetigo + treatment of secondary cases in the entourage if family epidemic | Couser 1993 - Kidney International: post-streptococcal GNS → immune complex mechanisms + complement alternate pathway + Carapetis 2016 - Nature Reviews Disease Primers: global epidemiology of SBHA infections → GNS + RAA → global impact + Stevens 2014 - Clinical Infectious Diseases (IDSA Guidelines): SSSS → hospitalization + IV vancomycin + burn care + corticosteroids contraindicated + Creech 2015 - NEJM: decolonization S. aureus + MRSA → nasal mupirocin + chlorhexidine → reduction of recurrent infections + Cruickshank 2000 - BMJ: school eviction + impetigo → return after 24-48h of effective treatment → Canadian Paediatric Society (CPS) 2022: recommendations on school avoidance + hygiene measures + Groupe Hospitalo-Universitaire Necker (Pediatrics France): GNS → C3 lowered + DNASE B + hematuria + favorable prognosis 90-95 % + Bangert 2012 - American Family Physician: review of complications of impetigo + Moran 2006 - NEJM: community MRSA → 59 % of S. aureus in cutaneous infections in emergencies → reorientation of therapeutic choices + AMMI Canada 2020 + CPS 2022: Canadian recommendations on cutaneous infections in children |
| Recurrent impetigo, Staphylococcus aureus carriage, and special situations Recurrence — nasal carriage — decolonization — chlorhexidine — nasal mupirocin — eczema + impetigo — immunocompromised — infant — diabetes — community MRSA — daycare outbreak |
Recurrent impetigo - investigations and decolonization strategies: definition: ≥3 episodes per year or frequent episodes in the same family/community → workup: culture of active lesions + nasal culture (patient + family contacts) → if nasal carriage of S. aureus documented → decolonization protocol: nasal mupirocin 2 % ointment: apply in each nostril × 2/d × 5 days → eradicate nasal carriage + chlorhexidine soap 4 % or 2 %: decolonization baths × 2/d × 5-14 days → reduce skin bacterial load + persistence of lesions : use mupirocin solutions on recurrent wounds → can be repeated × 1/month if recurrence → Creech 2015 - NEJM (NEJM IDSA RCT): nasal + cutaneous decolonization → 40 % reduction in recurrent S. aureus → impetigo secondary to eczema (atopic dermatitis): eczema = major risk factor for impetigo → impaired barrier skin + colonization with S. aureus colonization (95 % of eczemas) → frequent superimposed impetigo → treatment: topical or systemic antibiotic therapy depending on extent + optimization of eczema treatment (emollients + topical corticoids) → avoid treating asymptomatic eczema colonization with antibiotics (no benefit + resistances) + infant impetigo (<6 months): potentially more severe forms → easier dissemination (immature immunity) → hospitalization if bullous + feverish → IV vancomycin if SSSS suspected → close monitoring → impetigo in the immunocompromised (HIV + chemotherapy + corticoids): extensive + recurrent + resistant forms → systemic antibiotic therapy from the outset + cultures + antibiogram → MRSA to be evoked + impetigo in diabetics: more torpid evolution + risks of diffusion + slow healing → culture + antibiogram + systemic antibiotic therapy + glycemic control → impetigo in the community (daycare + school + camp): report to public health if epidemic identified → treatment of all symptomatic cases + collective hygiene measures (hand washing + surface cleaning + eviction) → contact infection surveillance network (CIUSSS or CISSS depending on region) → community MRSA (CA-MRSA) in collective impetigos: molecular typing recommended → clonal diffusion → combat sport + wrestling contacts (tinea gladiatorum + impétigo gladiatorum) → friction + micro-trauma + sweats + skin-to-skin contact → recurrent field impétigo + prophylactic treatment of sports impétigo: preventive topical mupirocin if wounds + micro-trauma during sports season → discuss temporary exclusion if uncovered active lesions + wrestling areas / contact sports | Creech 2015 - NEJM (NEJM decolonization trial): nasal mupirocin + cutaneous chlorhexidine → 40 % reduction in recurrent S. aureus infections → reference protocol for carriers + Eells 2012 - Emerging Infectious Diseases: MRSA decolonization in communities → efficacy → recommendations for extension + Moran 2006 - NEJM: CA-SARM → 59 % of S. aureus in emergencies → TMP-SMX + clindamycin = treatments of choice + Leitch 1998 - British Medical Journal: impetigo in school communities → eviction measures + hygiene + effectiveness of interventions + Bangert 2012 - American Family Physician: comprehensive review of forms of impetigo + recurrences + Carapetis 2016 - Nature Reviews Disease Primers: impetigo in developed and developing countries + impact + prevention + CPS 2022: Canadian Pediatric Society + recommendations for impetigo in children + infants + the community + INESSS 2022 (Institut national d'excellence en santé et services sociaux Québec): antibiotics in pediatric dermatology → review of Quebec practices and recommendations. |
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For localized impetigo without signs of severity, topical treatment (mupirocin or fusidic acid × 5–7 days) is as effective as oral antibiotics and should be preferred to limit selection pressure: systemic antibiotic therapy is reserved for extensive, bullous forms with general signs, or in immunocompromised patients. Children can return to daycare or school 24 to 48 hours after the start of effective antibiotic treatment. Antibiotic therapy does not prevent post-streptococcal glomerulonephritis - so it's important to watch for the appearance of edema, hematuria or hypertension 1 to 6 weeks after streptococcal impetigo.
Situations requiring urgent medical attention
Infant (<6 months) or child with extensive bullous impetigo + generalized skin peeling + «scalded skin» appearance + positive Nikolsky sign (skin slippage on pressure) + fever → SSSS (staphylococcal scalded skin syndrome) → pediatric emergency → hospitalization → IV vancomycin → burn unit care → rehydration → DO NOT administer corticosteroids → pediatric mortality <5 % if treated early.
Child with a history of recent streptococcal impetigo (1–6 weeks) presenting with periorbital edema or lower extremity edema + dark urine (cola or tea colored) + hypertension + oliguria → probable post-streptococcal glomerulonephritis → medical emergencies → renal workup (creatinine + urea + ionogram) + ECBU + fundus + additional workup → hospitalization if severe involvement.
Extensive facial impetigo in an immunocompromised adult or child with rapid peri-orbital extension + pain + fever + suppurative adenopathy + peri-orbital edema → risk of periorbital cellulitis or cavernous sinus thrombosis → medical emergencies → IV antibiotic therapy + infectious workup + imaging if orbital involvement suspected.
Recurrent impetigo within the same family or community (daycare + school) with multiple affected children + suspicion of community-acquired MRSA → probable MRSA epidemic → cultures + antibiogram + decolonization protocol for all carriers → contact regional public health department (CISSS or CIUSSS) + INESSS for Quebec recommendations.
Consult at Clinique Omicron
The doctors at Clinique Omicron diagnose and treat impetigo in children and adults, prescribe topical and systemic antibiotics according to Canadian recommendations, perform cultures if indicated (relapse + resistance + MRSA suspected), inform about school exclusion and hygiene measures, and refer to a specialist in case of complications (GAS + SSSS). Consultations are available at several service points in Quebec and via telemedicine. To make an appointment, visit cliniqueomicron.ca.
The content of this page is for informational purposes only and does not substitute professional medical advice. Any child with rapidly evolving skin lesions, fever, or general signs should be medically evaluated without delay.
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