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Community Acquired Methicillin-Resistant Staphylococcus Aureus (CA-MRSA) Skin Infections

Basics

  • CA-MRSA affects healthy hosts, unlike hospital-acquired MRSA.
  • Causes mainly skin and soft tissue infections (abscesses, furuncles, carbuncles).
  • Severe infections: osteomyelitis, sepsis, necrotizing fasciitis, necrotizing pneumonia.
  • System(s) affected: skin, soft tissue.

Epidemiology

  • Affects all ages, generally younger; female > male.
  • SSTI incidence peaked ~2010; pediatric rates declined after 2011.
  • 25-30% of U.S. colonized with S. aureus; ~7% with MRSA.
  • CA-MRSA isolated in ~60% of ED SSTIs; accounts for up to 75% of staphylococcal infections in children.

Etiology and Pathophysiology

  • Epidemic began ~1999; USA300 clone predominant.
  • Distinguished from HA-MRSA by:
  • Lack of multidrug resistance.
  • Presence of exotoxin virulence factors.
  • Type IV staphylococcal cassette chromosome carrying mecA gene.

Risk Factors

  • 50% have no obvious risk.
  • Antibiotic use in past month (cephalosporins, fluoroquinolones).
  • Abscess or "spider bite" history.
  • IV/intradermal drug use, HIV, dialysis catheter.
  • Close contact, daycare attendance, long-term care, athletes, incarceration.

Prevention

  • CA-MRSA colonizes nares, oropharynx, inguinal areas.
  • Transmitted via environmental/household contact.
  • CDC prevention guidance for athletes.

Diagnosis

  • History: risk factors, prior CA-MRSA, “spider bite” confusion.
  • Exam: abscess with cellulitis; erythema, warmth, tenderness, fluctuance.
  • Culture purulent lesions if systemic signs or immunocompromised.
  • Ultrasound to differentiate abscess vs cellulitis; abscess <0.4 cm may not need I&D.
  • Imaging (CT/MRI) only if necrotizing fasciitis suspected; do not delay surgery.

Differential Diagnosis

  • SSTIs caused by other organisms.

Treatment

General

  • Incision and drainage (I&D) for abscesses; loop drainage alternative.
  • Antibiotics guided by culture and local susceptibility.
  • Extended antibiotics not needed for nonsuppurative cellulitis.
  • No routine decolonization recommended.
  • Restrict contact if wounds uncovered.
  • Elevate affected area.

Medications

First Line

  • TMP/SMX DS 1-2 tabs PO q12h (children: 8-12 mg/kg/day TMP).
  • Doxycycline 100 mg PO q12h (children >8 years dosing adjusted).

  • Clindamycin 300-450 mg PO q6h (check D-zone test for inducible resistance).

  • CA-MRSA resistant to β-lactams, often macrolides and quinolones.

  • Rifampin not used as monotherapy.

Second Line (Severe/HA-MRSA)

  • Vancomycin IV 1 g q12h (children 40 mg/kg/day).
  • Linezolid 600 mg IV/PO q12h (children dosing varies).
  • Clindamycin IV 600 mg q8h.
  • Daptomycin 4 mg/kg/day IV (avoid if pulmonary involvement).
  • Ceftaroline 600 mg IV q12h (pediatric dosing variable).

Special Populations

  • Tetracyclines contraindicated <8 years.
  • TMP/SMX not recommended <2 months.
  • Daptomycin not for <1 year.
  • Tetracyclines contraindicated in pregnancy; TMP/SMX avoided in 1st/3rd trimester.

Referral

  • Infectious disease consult if refractory or decolonization planned.
  • Surgical consult for serious SSTIs (necrotizing fasciitis).

Admission Criteria

  • Systemic illness, extensive SSTI, comorbidities, immunocompromise.
  • SSTI complications (sepsis, necrotizing fasciitis).
  • Alternatives: observation units, outpatient parenteral therapy.

Ongoing Care

  • Follow up in 48 hours for outpatients.
  • Monitor for systemic symptoms or worsening.

Patient Education

  • Cover draining wounds.
  • Hand hygiene with soap and water (alcohol sanitizers ineffective).
  • Avoid sharing contaminated personal items.
  • Clean contaminated surfaces with diluted bleach solution.
  • CDC MRSA education resources.

Prognosis

  • Improvement expected within 48 hours in outpatients.

Complications

  • Necrotizing pneumonia, empyema.
  • Necrotizing fasciitis.
  • Sepsis syndrome.
  • Pyomyositis, osteomyelitis.
  • Purpura fulminans.
  • Disseminated septic emboli, endocarditis.

Clinical Pearls:

  • Incise and drain abscesses; send for culture.
  • Local susceptibility guides empiric antibiotic therapy.
  • TMP/SMX, doxycycline, and clindamycin are first-line oral agents for uncomplicated infections.
  • Expect improvement within 48 hours.