What is an infected rash?

What is an Infected Rash?

An infected rash is a skin eruption that has become secondarily colonized or invaded by pathogenic bacteria (most commonly Staphylococcus aureus), resulting in signs of active infection such as purulent drainage, crusting, increased warmth, expanding erythema, or systemic symptoms like fever. 1

Primary vs. Secondary Infection

The term "infected rash" typically refers to secondary bacterial superinfection of a pre-existing dermatologic condition rather than a primary infectious exanthem:

• Secondary infection occurs when disrupted skin barrier from conditions like atopic dermatitis, contact dermatitis, or traumatic lesions becomes colonized with bacteria, leading to impetigo-like changes with honey-colored crusting, weeping, or pustule formation 1
• Primary infectious rashes (viral exanthems, rickettsial diseases, bacterial toxin-mediated eruptions) are caused directly by the infectious agent itself and represent the disease manifestation rather than a complication 2, 3

Key Distinguishing Features of Secondary Bacterial Infection

When evaluating whether a rash has become infected, look for these specific clinical signs:

• Purulent exudate or honey-colored crusting overlying the primary lesions 1
• Expanding erythema beyond the original rash borders with increased warmth and tenderness 1
• Pustule formation or vesicles containing cloudy fluid 1
• Systemic symptoms including fever, malaise, or regional lymphadenopathy developing after the initial rash appeared 1
• Pain or tenderness that is disproportionate to the original rash 1

High-Risk Conditions for Secondary Infection

Certain underlying conditions dramatically increase infection risk:

• Atopic dermatitis has the highest risk due to skin barrier defects, type 2 inflammation, S. aureus colonization (present in 70-90% of lesional skin), and cutaneous dysbiosis 1
• Impetigo represents superficial bacterial infection most commonly affecting the face and extremities of children, often arising from minor trauma or insect bites 4
• Eczema herpeticum is a life-threatening viral superinfection (HSV) of atopic dermatitis that can mimic bacterial infection but requires antiviral rather than antibiotic therapy 1

Critical Pitfall: Distinguishing Secondary Infection from Primary Infectious Exanthems

Do not confuse secondary bacterial superinfection with primary infectious diseases that present with fever and rash, as management differs fundamentally:

• Rocky Mountain Spotted Fever presents with fever followed 2-4 days later by blanching pink macules on ankles/wrists that progress to maculopapular then petechial lesions—this is the disease itself, not a secondary infection, and requires immediate doxycycline 2, 3
• Meningococcemia causes rapidly progressive petechial/purpuric rash with fever—this is primary invasive bacterial disease requiring emergent antibiotics, not a secondarily infected rash 3
• Scarlet fever produces diffuse erythematous sandpaper-textured rash with fever from streptococcal toxin—this is primary toxin-mediated disease 4

Management Approach

For true secondary bacterial infection of pre-existing rash:

• Topical antibiotics (mupirocin) for localized superficial infection without systemic symptoms 1
• Oral antibiotics (cephalexin, dicloxacillin, or doxycycline) for extensive involvement or systemic symptoms, targeting S. aureus and Streptococcus pyogenes 1
• MRSA coverage (trimethoprim-sulfamethoxazole, doxycycline, or clindamycin) if community MRSA prevalence is high or patient has risk factors 1
• Preventive therapy emphasizes skin barrier improvement with emollients and anti-inflammatory treatment of the underlying condition rather than prophylactic antibiotics 1

The use of antibiotics for atopic dermatitis exacerbations without overt signs of infection remains controversial and requires further study 1

When to Suspect Life-Threatening Primary Infection Instead

If a patient presents with fever AND rash together, immediately consider these life-threatening conditions that require urgent intervention rather than treating as a secondarily infected rash:

• RMSF: Fever with rash appearing 2-4 days later, progressing from blanching macules to petechiae, involving palms/soles by days 5-6; mortality 5-10% if untreated 2, 3
• Meningococcemia: Rapidly progressive petechial/purpuric rash with high fever and altered mental status 3, 5
• Toxic shock syndrome: Diffuse erythroderma with fever, hypotension, and multiorgan involvement 3
• Kawasaki disease (children): ≥5 days fever with polymorphous exanthem, requiring diagnosis within 10 days to prevent coronary aneurysms 3

Start empiric doxycycline immediately without waiting for laboratory confirmation if RMSF is suspected based on fever, rash, headache, and tick exposure or endemic area exposure 6