Impetigo Krustosa

01Pathophysiology

Non-bullous impetigo is a superficial bacterial infection confined entirely to the epidermis, making it the most superficial of all skin infections. The bacteria, most commonly Staphylococcus aureus or Streptococcus pyogenes, do not penetrate intact skin on their own. They require a portal of entry through a break in the epidermal barrier. This is why vignettes so often mention a preceding insect bite, a scratch, an abrasion, or underlying atopic dermatitis. The breach allows the organisms to colonize the superficial layers of the skin.

Once inoculated, the bacteria proliferate within the stratum granulosum and stratum corneum. The host's innate immune response recruits neutrophils to the site, producing a collection of serous fluid and inflammatory cells beneath the stratum corneum. This initially forms a thin-walled vesicle or pustule on an erythematous base. Because the roof of this vesicle is so superficial (just a few cell layers of dead keratinocytes), it is extremely fragile and ruptures almost immediately. The dried serous exudate, combined with fibrin, cellular debris, and bacteria, creates the hallmark honey-colored (golden yellow) crust that is the visual signature of this disease.

The reason the infection spreads to adjacent areas, producing satellite lesions, is autoinoculation. The child scratches the pruritic lesion, picks up organisms under the fingernails, and transfers them to other sites. This also explains why the disease is highly contagious in close-contact environments like daycare centers and why it clusters around the face, where children frequently touch.

A critical concept to understand is that because the infection is so superficial, it heals without scarring. The dermis is not involved. This contrasts with deeper infections like ecthyma, where the same organisms invade into the dermis, producing ulceration that does scar. On an exam, if you see scarring or deep ulceration, you should be thinking of ecthyma (the deeper cousin of impetigo), not non-bullous impetigo.

The connection between S. pyogenes skin infection and post-streptococcal glomerulonephritis (PSGN) is a high-yield link. Nephritogenic strains of GAS that cause impetigo can trigger an immune complex-mediated glomerulonephritis 1 to 3 weeks after the skin infection. Importantly, rheumatic fever does not follow streptococcal skin infections; it follows pharyngeal streptococcal infections only. This distinction is tested repeatedly.

02Classification and Clinical Manifestation

Clinical Progression of Non-Bullous Impetigo:

The lesion goes through a predictable sequence that is often described step by step in a clinical vignette:

Regional lymphadenopathy is common and does not by itself indicate a deeper or more complicated infection. Systemic symptoms (fever, malaise) are typically absent in uncomplicated non-bullous impetigo. If the vignette includes high fever and systemic toxicity, you should question the diagnosis or consider a complication like cellulitis or bacteremia.

03Diagnostic Workup

Non-bullous impetigo is fundamentally a clinical diagnosis. In a straightforward vignette describing a child with honey-crusted perioral lesions, no further testing is needed and the correct next step is to initiate treatment.

Gram stain and culture become necessary in two scenarios that commonly appear on exams. The first is treatment failure, where the lesions do not respond to empiric therapy after 48 to 72 hours, raising concern for MRSA. The second is recurrent impetigo, where identifying the organism and its sensitivities guides targeted antibiotic selection and helps determine if the patient is a nasal carrier of S. aureus.

The distinction between anti-DNase B and ASO is a classic tested concept. The streptolysin O molecule that triggers the ASO response is inactivated by skin lipids, so ASO titers are unreliable after cutaneous streptococcal infections. Anti-DNase B is not affected by skin lipids and therefore is the preferred serologic marker for confirming prior streptococcal skin infection. The most common context where this appears is a child who presents with nephritic syndrome (hematuria, edema, hypertension) 2 to 3 weeks after impetigo, and you need to prove the streptococcal link.

Routine blood work (CBC, CRP, blood cultures) is not necessary for uncomplicated impetigo. If the question asks for the "next best step" in a healthy child with classic honey-crusted lesions, the answer is treatment, not labs.

04Management and Treatment

Acute Management:

For the majority of exam vignettes, the patient will have a few localized honey-crusted lesions on the face. The correct answer for initial management is topical mupirocin 2% ointment applied three times daily for 5 days. Gentle debridement of crusts with warm, soapy water before applying the ointment improves antibiotic penetration and is often mentioned as part of correct wound care.

Topical therapy alone is sufficient when the disease is limited in extent, the patient is immunocompetent, and there are no signs of deeper tissue involvement.

When to Escalate to Systemic Therapy:

Oral antibiotics are indicated when there are more than five lesions, when multiple body regions are involved, when the disease is spreading despite topical treatment, or when there is associated lymphadenopathy suggesting a higher bacterial burden. Cephalexin is the first-line oral agent because it covers both S. aureus (methicillin-sensitive strains) and S. pyogenes effectively.

A common trap is selecting amoxicillin or penicillin V as the oral agent. While these cover S. pyogenes, they are hydrolyzed by S. aureus beta-lactamase and therefore provide inadequate coverage for the most common causative organism. Always choose a beta-lactamase-resistant agent.

MRSA Considerations:

If the vignette mentions failure of first-line therapy, history of MRSA colonization, or community-acquired MRSA risk factors (incarceration, IV drug use, contact sports, household contact with MRSA), the correct pivot is to TMP-SMX or clindamycin. Note that TMP-SMX does not reliably cover S. pyogenes, so if mixed infection is suspected, combining TMP-SMX with a beta-lactam or using clindamycin alone (which covers both organisms) is preferred.

Important Contraindications:

Doxycycline must not be used in children younger than 8 years due to the risk of permanent tooth discoloration. TMP-SMX should be avoided in neonates and in patients with folate deficiency, sulfa allergy, or G6PD deficiency (risk of hemolytic anemia). Clindamycin carries a well-known risk of Clostridioides difficile colitis, which may appear as a complication in a multi-step question.

Prevention of Complications:

Antibiotic treatment of streptococcal impetigo does not prevent PSGN. This is the opposite of the pharyngitis paradigm, where treating strep pharyngitis with penicillin prevents rheumatic fever. The immune complex deposition that leads to PSGN has already been set in motion by the time of clinical presentation. Treatment of impetigo reduces contagion and accelerates healing but does not alter the risk of renal complications.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most commonly tested trap with non-bullous impetigo is the relationship between streptococcal skin infection and post-streptococcal complications. Students who remember that "strep causes rheumatic fever and PSGN" will incorrectly assume that treating impetigo with antibiotics prevents both. The tested principle is that PSGN can follow streptococcal impetigo, but rheumatic fever cannot. Rheumatic fever is exclusively a sequela of pharyngeal GAS infection. Additionally, antibiotics do not prevent PSGN even when given promptly for the skin infection, unlike the pharyngitis paradigm where penicillin prevents rheumatic fever.

The second common trap involves antibiotic selection. A vignette will describe classic impetigo and offer amoxicillin or penicillin V as an answer choice. These are tempting because students associate streptococcal infections with penicillin. However, because S. aureus is now the predominant cause of non-bullous impetigo and produces beta-lactamase, a penicillinase-resistant agent (cephalexin, dicloxacillin, or amoxicillin-clavulanate) is required. Selecting plain amoxicillin is the predictable wrong answer.

The third trap is the ASO titer. When a child develops nephritic syndrome weeks after impetigo and the question asks for the most appropriate serologic test, students often select ASO because it is the classic streptococcal marker. But ASO is unreliable after skin infections due to lipid-mediated inactivation of streptolysin O. The correct answer is anti-DNase B.

Finally, do not confuse non-bullous impetigo with ecthyma. If the vignette mentions deep ulceration, lower extremity involvement, or scarring, the answer is ecthyma, not impetigo. Both share the same causative organisms, but the depth of invasion is the distinguishing factor.

The core competency being tested with non-bullous impetigo questions is the ability to recognize a classic superficial skin infection by its visual description, select the appropriate first-line agent while avoiding common antibiotic errors, and correctly link streptococcal skin infection to its renal (but not cardiac) complication.