Impetigo

What is Impetigo?
Pathophysiology and Bacterial Causes
Non-Bullous Impetigo: Honey-Crusted Lesions
Bullous Impetigo: Staphylococcal Exfoliative Toxins
Ecthyma: Deep Ulcerative Form
Risk Factors and Epidemiology
Complications: Post-Streptococcal Glomerulonephritis
Diagnosis
Treatment: Topical and Oral Antibiotics
Prevention and Decolonization
Research Papers
Connections
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What is Impetigo?

Impetigo is the most common bacterial skin infection in children worldwide. It is a highly contagious, superficial infection of the epidermis — the outermost layer of the skin — that spreads easily through direct contact with infected skin or contaminated objects. Despite its alarming appearance, impetigo carries an excellent prognosis and resolves completely with appropriate treatment, leaving no scars in its most common form.

There are two principal clinical forms:

Non-bullous impetigo — accounts for approximately 70% of cases; characterized by the hallmark golden-yellow honey-crusted lesions.
Bullous impetigo — accounts for approximately 30% of cases; characterized by thin-walled fluid-filled blisters (bullae) on normal-appearing surrounding skin.

Impetigo affects people of all ages but predominates in children aged 2–5 years, who lack the protective immunity developed through repeated skin colonization with common bacteria. According to World Health Organization data, approximately 162 million children worldwide have impetigo at any given time, making it one of the most prevalent infectious diseases of childhood. It is a leading cause of school absenteeism globally.

Warm, humid climates dramatically increase prevalence — impetigo is endemic in tropical and subtropical regions, particularly in Indigenous and low-income communities where overcrowding and skin trauma from insect bites are common. In temperate climates, cases peak in summer and early autumn. Uncomplicated impetigo carries no mortality, but rare complications — notably post-streptococcal glomerulonephritis — require awareness and appropriate follow-up.

Pathophysiology and Bacterial Causes

Impetigo is caused by two principal gram-positive bacterial pathogens: Staphylococcus aureus and Streptococcus pyogenes (Group A Streptococcus, GAS). The relative contribution of each organism has shifted over recent decades — S. aureus is now the dominant pathogen in most regions, capable of causing both non-bullous and bullous forms, while GAS causes non-bullous impetigo only.

Methicillin-resistant S. aureus (MRSA) has emerged as an increasing concern in community-acquired impetigo, particularly in the United States, where community-associated MRSA (CA-MRSA) strains (most commonly USA300) have been identified in impetigo outbreaks among children, athletes, and prison populations.

Mechanism of infection — S. aureus bullous form: Certain phage group II strains of S. aureus produce exfoliative toxins ET-A and ET-B, which are serine proteases. These toxins cleave desmoglein 1 — the desmosomal cadherin protein that anchors keratinocytes together in the superficial epidermis (stratum granulosum). Cleavage of desmoglein 1 causes an intraepidermal split at this level, producing the characteristic thin-roofed bulla on normal-appearing skin. Crucially, because this process is enzymatic (proteolytic) rather than inflammatory, there is no surrounding erythema or induration — the bulla forms on skin that looks completely normal until rupture.

Mechanism of infection — S. aureus/GAS non-bullous form: GAS produces several virulence factors that facilitate superficial skin spread: streptokinase (dissolves fibrin clots), DNases, hyaluronidase (degrades connective tissue matrix), and M proteins (resist phagocytosis). These organisms exploit breaks in the skin barrier to establish infection in the epidermis.

Barrier disruption is the critical prerequisite: Intact skin is highly resistant to impetigo. Colonization and infection require a breach — from eczema/atopic dermatitis, varicella, herpes simplex, insect bites, abrasions, or lacerations. This is why impetigo so frequently complicates eczema flares in children.

It is important to distinguish impetigo from deeper bacterial skin infections: cellulitis involves the dermis and subcutaneous fat (presents with spreading erythema, warmth, and tenderness without primary bullae or honey crust), while erysipelas involves the upper dermis and superficial lymphatics (raised, well-demarcated, intensely red plaques). Impetigo is confined to the epidermis.

Non-Bullous Impetigo: Honey-Crusted Lesions

Non-bullous impetigo is the most common form, accounting for approximately 70% of all cases. Its hallmark is the golden-yellow "honey-crusted" lesion — a pathognomonic finding that allows confident clinical diagnosis at the bedside without laboratory confirmation in typical cases.

Clinical evolution:

Small erythematous macule or papule appears at the site of skin trauma or pre-existing lesion.
Rapidly evolves to a thin-walled vesicle or pustule (usually within hours to 1–2 days).
The fragile vesicle/pustule ruptures spontaneously, releasing a serous or seropurulent exudate.
The exudate dries to form the characteristic golden-yellow to amber honey-colored crust, which adheres firmly to the underlying erythematous base.
Satellite lesions may develop at adjacent sites through autoinoculation (the child scratches the lesion and transfers organisms to other skin areas).

Favored locations: The face (perioral and perinasal regions are most common), followed by the extremities. Periorbital impetigo requires ophthalmological attention due to risk of orbital cellulitis.

Associated findings: Mild surrounding erythema is typical. Regional lymphadenopathy is common, particularly submandibular nodes with facial lesions. Systemic symptoms (fever, malaise) are typically absent — their presence should prompt evaluation for deeper infection or bacteremia.

Contagion: Highly contagious through direct contact with the serous/purulent exudate of active lesions. Fomite transmission (towels, bedding) is possible but less significant than direct contact. Children with active lesions should be excluded from school and daycare until 24 hours after initiating antibiotic therapy.

Healing: When treated, lesions resolve without scarring — the infection is confined to the epidermis, which regenerates completely. Untreated lesions may persist for weeks but also eventually self-resolve in most immunocompetent individuals; however, complications risk and contagion duration argue strongly for treatment.

Causative organisms: S. aureus alone (most common in developed countries), mixed S. aureus + GAS, or GAS alone (more common in tropical/developing settings).

Bullous Impetigo: Staphylococcal Exfoliative Toxins

Bullous impetigo accounts for approximately 30% of impetigo cases and is caused exclusively by toxigenic strains of S. aureus (phage group II, particularly phage types 3A, 3B, 3C, 55, and 71). The defining pathological mechanism is the local action of exfoliative toxins ET-A and ET-B, which cleave desmoglein 1 in the stratum granulosum.

Distinguishing clinical features:

Thin-walled, flaccid bullae arising on skin that appears completely normal (no surrounding erythema, induration, or inflammation at onset — this distinguishes bullous impetigo from other blistering conditions like herpes simplex or contact dermatitis, which show surrounding inflammation).
Bullae begin clear and progress to cloudy yellow fluid as neutrophils accumulate.
Bullae rupture easily, leaving a moist, red, glistening erosion with a peripheral "collarette" of scale — the remnant of the blister roof at the edge.
Unlike non-bullous impetigo, there is no honey crust — the roof is too thin and the exudate too scant to form a substantial crust.

Favored locations: Trunk, buttocks, axillae, and intertriginous areas — moist skin folds facilitate blister formation. Less commonly the face and extremities.

Neonatal susceptibility: Neonates are particularly vulnerable to bullous impetigo because they lack antibodies against exfoliative toxins (maternal IgG wanes, and neonates have not yet developed their own immunity). Neonatal impetigo can cause nursery outbreaks.

Scalded Skin Syndrome (SSSS): When exfoliative toxins are produced in sufficient quantity to enter the bloodstream systemically — particularly in neonates, young infants, and immunocompromised adults — the toxins can cleave desmoglein 1 throughout the entire skin surface remotely from the primary infection site. This causes staphylococcal scalded skin syndrome: widespread superficial skin peeling resembling thermal burns, with a positive Nikolsky sign (gentle lateral pressure on normal skin causes the epidermis to slide off). SSSS requires hospitalization and IV antibiotics; mortality in children is low (~3%) but higher in immunocompromised adults (~60%).

Lower contagion than non-bullous: Paradoxically, bullous impetigo is somewhat less contagious than non-bullous — the intact bulla contains fewer organisms than the freely weeping exudate of ruptured non-bullous vesicles, and toxin-mediated blistering does not depend on high bacterial density at the transmission site.

Ecthyma: Deep Ulcerative Form

Ecthyma is the deepest form of impetigo, distinguished by its penetration through the epidermis into the dermis. While superficial impetigo heals without a scar, ecthyma invariably causes scarring because the dermis is involved in the destructive process.

Causative organism: S. pyogenes (GAS) most commonly; sometimes S. aureus. The same organisms as non-bullous impetigo, but the infection goes deeper — usually because of delayed or absent treatment, host immunocompromise, impaired lymphatic drainage (lymphedema), or particularly virulent strains.

Clinical appearance: The pathognomonic finding is a "punched-out" ulcer with a violaceous (purple-gray) border, covered by a thick, adherent "oyster shell" crust. When the crust is lifted, a shallow ulcer with an erythematous base is revealed. The surrounding skin shows induration and erythema. Individual lesions are 0.5–3 cm in diameter.

Favored locations: Lower extremities, predominantly the legs and feet — a distribution explained by the prevalence of insect bites and trauma in these areas, plus dependent edema impeding local immune response. Also buttocks, perineum (particularly in incontinent patients).

Clinical course: Chronic and indolent — lesions persist for weeks to months if untreated, unlike superficial impetigo which may self-resolve. Healing is slow even with antibiotics (weeks), and scarring is the rule.

Risk factors:

Insect bites and neglected traumatic wounds on the lower extremities
Lymphedema (impairs local immune surveillance)
Malnutrition and protein deficiency
Diabetes mellitus (impaired wound healing + neutrophil dysfunction)
Immunosuppression (HIV, chemotherapy)
Homelessness and skin-barrier compromise from environmental exposure

Complications: Ecthyma can progress to cellulitis, lymphangitis, and — in severely immunocompromised patients — bacteremia. The thick overlying crust must be soaked and debrided before antibiotic therapy can be effective. Ecthyma gangrenosum (caused by Pseudomonas aeruginosa, not streptococcus) must be excluded in neutropenic patients — it appears similar but is associated with systemic infection and carries high mortality.

Risk Factors and Epidemiology

Impetigo's global prevalence of ~162 million affected children at any one time reflects the convergence of biological susceptibility, environmental factors, and social determinants of health.

Age: Children aged 2–5 years are most susceptible. The immature adaptive immune system has not yet developed memory responses against common skin pathogens, and behavioral factors (contact play, poor hand hygiene, nail biting, scratching insect bites) facilitate transmission and autoinoculation. Adolescents and adults can develop impetigo, particularly through contact sports (wrestling — "impetigo gladiatorum") or when skin barrier is disrupted.

Climate: Warm, humid climates dramatically increase prevalence. Heat and humidity promote bacterial growth on skin surfaces and increase insect biting activity. Tropical and subtropical regions — sub-Saharan Africa, South and Southeast Asia, the Pacific Islands, and northern Australia (particularly Indigenous communities) — bear a disproportionate burden. Temperate-climate seasonal peaks in late summer reflect similar thermodynamic factors.

Skin barrier disruption — the most critical risk factor:

Atopic dermatitis (eczema): Most important risk factor in developed countries. The defective filaggrin-based skin barrier in eczema dramatically facilitates S. aureus colonization and invasion. Children with eczema have S. aureus colonization rates of 70–90% (vs. ~20% in healthy children).
Varicella (chickenpox): A leading cause of secondary bacterial superinfection in unvaccinated children.
Herpes simplex lesions, insect bites, lacerations, abrasions, scabies burrows.

Socioeconomic and environmental factors:

Overcrowding in households, daycare centers, and schools — facilitates person-to-person transmission.
Poor access to clean water — limits hand hygiene (the single most effective preventive measure).
Contact sports (wrestling, rugby, football) — skin abrasions plus close body contact.
Shared towels, razors, sports equipment.

Microbiological risk factors:

Nasal carriage of S. aureus (present in ~30% of the general population) is a reservoir for skin inoculation — nares-to-skin autoinoculation explains many recurrent cases.
MRSA prevalence in the community (particularly USA300 in North America) is an increasing concern for treatment-refractory impetigo.
Nephritogenic GAS strains (M-types 49, 57, 59, 61) circulate in tropical/subtropical endemic settings and carry specific risk of post-streptococcal glomerulonephritis.

Complications: Post-Streptococcal Glomerulonephritis

Impetigo is generally a self-limited infection with excellent outcomes. However, one complication — post-streptococcal glomerulonephritis (PSGN) — deserves specific attention because it can cause permanent renal damage and is unique to GAS-associated skin infection.

Post-Streptococcal Glomerulonephritis (PSGN):

PSGN is an immune complex-mediated glomerulonephritis triggered by specific nephritogenic strains of GAS following skin infection. Key concepts:

GAS skin infection, not S. aureus: Only GAS (not S. aureus) causes PSGN. This makes accurate microbiological diagnosis important in endemic settings.
Critical distinction from pharyngeal GAS: Pharyngeal GAS infection causes both rheumatic fever and PSGN. Skin GAS infection causes PSGN only — it does not cause rheumatic fever (rheumatic fever requires mucosal pharyngeal infection). This distinction has major public health implications.
Nephritogenic M-types: Not all GAS strains cause PSGN — only those bearing specific nephritogenic M-proteins, particularly M-types 49, 57, 59, and 61, which are prevalent in tropical/subtropical regions (different strains from the pharyngeal nephritogenic type 12).
Incubation period: PSGN develops 3–6 weeks after the skin infection (longer than the 1–3 weeks after pharyngeal infection) — this latency reflects the time for immune complex deposition and glomerular inflammation to develop.
Clinical presentation: Hematuria (gross or microscopic, "coca-cola" colored urine), edema (periorbital, peripheral), hypertension, oliguria, proteinuria. The nephritic syndrome in its full form occurs in a minority; many cases present with asymptomatic urinary abnormalities only.
Prognosis: In children, PSGN is usually self-limiting — the vast majority recover full renal function over weeks to months. Adults have worse outcomes, with a higher rate of progression to chronic kidney disease.
Antibiotic treatment does NOT prevent PSGN: Unlike rheumatic fever (where early penicillin treatment prevents the complication), once GAS skin infection is established, antibiotic treatment does not reliably prevent PSGN. Antibiotics are still indicated to treat the infection and reduce transmission.

Other complications:

Cellulitis: Spread of infection from superficial impetigo to dermis/subcutaneous tissue; requires oral or IV antibiotics.
Lymphadenitis: Infection of regional lymph nodes; may require incision and drainage if abscess forms.
Bacteremia/septicemia: Rare in healthy immunocompetent children; more common in neonates and immunocompromised patients.
Staphylococcal scalded skin syndrome (SSSS): Systemic dissemination of exfoliative toxins from bullous impetigo; primarily a risk in neonates (see Bullous section).
Orbital cellulitis: Serious complication of periorbital impetigo; requires urgent ophthalmological evaluation and IV antibiotics.

Diagnosis

Impetigo is primarily a clinical diagnosis in typical presentations. The combination of golden honey-crusted lesions on the face of a child, or thin-walled bullae on normal-appearing skin, is sufficiently distinctive that laboratory confirmation is rarely required for initial management.

When to culture: Bacterial culture (swab from beneath a lifted crust — NOT from the surface, which yields contaminants) is indicated in:

Recurrent or treatment-refractory impetigo (MRSA suspicion)
Outbreak investigation in institutional settings (daycare, schools, military)
Neonatal impetigo (to guide antibiotic selection and identify nursery reservoirs)
Atypical presentations or diagnostic uncertainty
Immunocompromised patients

Laboratory investigations: No routine bloodwork is required for uncomplicated impetigo. In outbreak settings with confirmed GAS, urinalysis at 3–6 weeks after infection is reasonable to screen for asymptomatic PSGN (hematuria, proteinuria).

Differential diagnosis — key conditions to exclude:

Tinea (ringworm): Annular scaly plaques without honey crust; KOH preparation reveals fungal hyphae; bacterial culture negative. Tinea capitis can mimic perinasal impetigo.
Herpes simplex (HSV): Grouped vesicles on erythematous base (unlike bullous impetigo's bullae on normal skin); recurrent pattern; positive Tzanck smear (multinucleated giant cells) or PCR. Eczema herpeticum (Kaposi's varicelliform eruption) can be catastrophic — extensive grouped vesicles on eczematous skin require emergency treatment with acyclovir.
Varicella (chickenpox): Viral prodrome (fever, malaise); simultaneous lesions at all stages (macule, papule, vesicle, crust); centripetal distribution; whole-body involvement.
Contact dermatitis: History of allergen/irritant exposure; pruritic vesicular or papular rash; no honey crust; no regional lymphadenopathy; patch testing may clarify.
Pemphigus vulgaris/foliaceus: Autoimmune blistering disease; positive Nikolsky sign; skin biopsy + direct immunofluorescence shows IgG deposits at keratinocyte surfaces. Affects adults; rare in children.
Bullous pemphigoid: Tense (not flaccid) bullae; elderly patients; positive indirect immunofluorescence; IgG against hemidesmosomal proteins.

Treatment: Topical and Oral Antibiotics

Treatment selection depends on the extent of infection, clinical form (non-bullous vs. bullous), likelihood of MRSA, and patient age. The overarching principle: topical antibiotics for limited disease; oral antibiotics for widespread, bullous, ecthymatous, or recurrent infection.

Topical Antibiotics (Limited Non-Bullous Impetigo)

For patients with few lesions confined to a small area, topical antibiotics are the preferred approach — they achieve high local concentrations, have fewer systemic side effects, and cause less disturbance to the gut microbiome than oral antibiotics.

Mupirocin 2% ointment (Bactroban): First-line topical agent. Applied to affected areas three times daily for 5 days. Highly effective against MSSA (methicillin-susceptible S. aureus) and GAS. Limited activity against MRSA (approximately 15–20% of community MRSA strains are mupirocin-resistant where it has been used extensively). Crusts should be gently soaked and removed before application to maximize penetration.
Retapamulin 1% ointment (Altabax): A pleuromutilin antibiotic; effective alternative to mupirocin; BID × 5 days. Also poor MRSA coverage. Not available in all countries.
Fusidic acid 2% cream/ointment: Widely used in Europe and Australia (not available in the US). Effective against MSSA and GAS. Resistance can emerge with monotherapy; used TID × 5–7 days.
Advantage of topical therapy: Randomized trials demonstrate equivalent efficacy to oral antibiotics for limited non-bullous impetigo, with fewer adverse effects and no contribution to systemic antibiotic resistance.

Oral Antibiotics (Widespread, Bullous, Ecthymatous, or MRSA Impetigo)

Oral antibiotics are required when lesions are too numerous or widespread for topical treatment, when bullous impetigo is present, for ecthyma, for recurrent impetigo, when topical therapy has failed, or when MRSA is confirmed or suspected.

For MSSA (non-MRSA) impetigo:

Cephalexin (first-generation cephalosporin): 25–50 mg/kg/day in 3–4 divided doses × 7 days. First-line oral agent for MSSA impetigo in children. Well-tolerated, inexpensive, widely available.
Dicloxacillin (penicillinase-resistant penicillin): 12.5–25 mg/kg/day divided QID × 7 days. Excellent MSSA coverage; poor palatability in children.
Amoxicillin-clavulanate: Alternative for MSSA + GAS mixed infection; broader spectrum than needed for typical impetigo but useful when infection is recurrent or mixed flora suspected.
Note: Plain amoxicillin or amoxicillin alone should NOT be used — most S. aureus strains (>90%) produce beta-lactamase and are resistant.

For MRSA impetigo:

Trimethoprim-sulfamethoxazole (TMP-SMX): Highly effective against CA-MRSA (community-associated MRSA); 1–2 DS tablets or pediatric weight-based dosing BID × 7 days. Drug of choice when MRSA is confirmed or community MRSA prevalence is high. Note: poor GAS coverage, so if GAS is also suspected, add a beta-lactam.
Doxycycline: Effective against MRSA; 100 mg BID × 7 days. Not recommended under age 8 (permanent tooth staining). Reasonable choice for older children and adults.
Clindamycin: Effective against MRSA if susceptibility confirmed (inducible clindamycin resistance [iMLSB] must be excluded by D-zone test). 10–20 mg/kg/day divided TID × 7 days.

Ecthyma: Same oral antibiotic regimens as above, extended to 7–10 days. Crusts must be soaked and debrided at each dressing change to allow antibiotic penetration and removal of necrotic material.

Prevention and Decolonization

Primary prevention:

Hand hygiene: The single most effective preventive measure. Hand washing with soap and water after touching lesions, before meals, and after diaper changes disrupts the contact-transmission chain.
Keep skin wounds, insect bites, and abrasions clean and covered with sterile dressings.
Prompt and effective treatment of eczema/atopic dermatitis to restore the skin barrier — this is particularly important for children with recurrent impetigo complicating eczema.
Avoid sharing towels, face cloths, razors, and sports equipment.
Trim fingernails short in young children to reduce scratching-related autoinoculation.

School/daycare exclusion policy: Children with active impetigo lesions should be excluded from school, daycare, and contact sports until they have completed at least 24 hours of antibiotic therapy AND all lesions are crusted over. This 24-hour antibiotic window substantially reduces transmissibility; exclusion beyond this point is generally unnecessary.

Decolonization for recurrent impetigo: Patients with recurrent impetigo (3 or more episodes per year) or household clusters should be evaluated for nasal carriage of S. aureus — the primary reservoir that seeds repeated skin infections.

Nasal mupirocin ointment: Applied to both nares twice daily for 5 days, then repeated monthly for 3–6 months in recurrent carriers. Eliminates nasal S. aureus carriage in ~80% of treated individuals. Most effective intervention for preventing recurrent S. aureus skin infections.
Chlorhexidine gluconate body washes (2–4%): Daily wash × 5–14 days reduces skin surface S. aureus load. Part of the standard "decolonization bundle" for recurrent MRSA infections. Allow to sit on skin 1 minute before rinsing; avoid eyes and mucous membranes.
Household contacts: Simultaneous treatment of all household members is essential when recurrence pattern suggests family transmission. Otherwise, successfully treated index patients are rapidly recolonized from untreated family contacts.
Environmental decolonization: Launder bed linens, towels, and clothing in hot water (>60°C/140°F); clean shared surfaces with antiseptic solutions during active outbreaks.

No vaccine available: Despite decades of research, there is no licensed vaccine against S. aureus or GAS impetigo. Vaccine development against GAS is challenging because M-protein-based vaccines risk cross-reactive antibodies against cardiac tissue (molecular mimicry — the same mechanism underlying rheumatic fever). Research into non-M-protein GAS vaccine candidates continues.

Research Papers

Hay RJ et al. The global burden of skin disease in 2010: an analysis of the prevalence and impact of skin conditions. J Invest Dermatol. 2014. PMID 23886120
Moran GJ et al. Methicillin-resistant S. aureus infections among patients in the emergency department. N Engl J Med. 2006. PMID 16481636
Iwatsuki K et al. Staphylococcal exfoliative toxins: how they cause diseases. J Invest Dermatol. 2006. PMID 17108096
Carapetis JR et al. Skin infections and infestations in Aboriginal communities in northern Australia. Australas J Dermatol. 2005. PMID 15985040
Stevens DL et al. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the IDSA. Clin Infect Dis. 2014. PMID 24973422
Bowen AC et al. The global epidemiology of impetigo: a systematic review of the population prevalence of impetigo and pyoderma. PLoS One. 2015. PMID 26317533
Cole C et al. Topical antibiotics for skin infections: a systematic review. Br J Gen Pract. 2018. PMID 29255099
Bangert S et al. Bacterial skin and soft tissue infections. Aust Prescr. 2011. PMID 22174450
Mölstad S et al. Penicillin treatment of impetigo contagiosa. Scand J Infect Dis. 1992. PMID 1509239
Martin JM et al. Impetigo caused by community-acquired methicillin-resistant Staphylococcus aureus in children. Pediatrics. 2010. PMID 20231194
Nardi NM, Schaefer TJ. Impetigo. StatPearls. 2023. PMID 30725860
Sladden MJ, Johnston GA. Common skin infections in children. BMJ. 2004. PMID 15242917

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