Incidence

• SSTI incidence for adult ambulatory care peaked in 2010 at 35 per 1,000 population and has since plateaued.
• SSTI incidence for pediatric ambulatory care visits peaked in 2011 at 26 per 1,000 population, decreasing to 13 per 1,000 in 2015.
• The incidence of MRSA-related hospitalizations decreased from 2010 to 2014.
• Among people who inject drugs, the incidence of MRSA-related skin abscesses is increasing. Patients should receive substance misuse disorder care and be linked with syringe exchange programs.

Prevalence

• Local epidemiology patterns vary.
• 25–30% of U.S. population colonized with S. aureus; up to 7% are colonized with MRSA.
• CA-MRSA isolated in ~60% of SSTIs presenting to emergency departments (range 15–74%).
• CA-MRSA accounts for up to 75% of all community staphylococcal infections in children.

Initial Tests (lab, imaging)

• Wound cultures establish definitive diagnosis. Culture a purulent lesion if there are systemic signs of illness or if the patient is immunocompromised (1)[B].
• Susceptibility testing; many labs use oxacillin instead of methicillin.
• “D-zone disk-diffusion test” evaluates for inducible clindamycin resistance if CA-MRSA is resistant to erythromycin.
• Ultrasound may help identify abscesses versus a non-drainable phlegmon; evidence suggests abscesses shallower than 0.4 cm may not need incision and drainage (I&D) (2).
• Look for fascial plane edema on CT or MRI if necrotizing fasciitis is suspected. DO NOT DELAY surgical intervention to obtain imaging in such cases.

Diagnostic Procedures/Other

I&D for purulent lesions; needle aspiration not recommended (1). An effective alternative to I&D is the loop drainage technique.

First Line

Antibiotics for CA-MRSA SSTIs: 7- to 14-day course (depends on severity and clinical response):

• TMP/SMX: DS (160 mg TMP and 800 mg of SMX) 1 to 2 tablet(s) PO q12h; children, 8 to 12 mg/kg/day PO of trimethoprim component in 2 divided doses
• Doxycycline or minocycline: 100 mg PO q12h; children, >8 years and <45 kg, 2 to 5 mg/kg/day PO in 1 to 2 divided doses, not to exceed 200 mg/day; >8 years and >45 kg, use adult dosing; taken with a full glass of water
• Clindamycin: 300 to 450 mg PO q6h; children, 30 to 40 mg/kg/day PO in 3 divided doses; taken with full glass of water; check D-zone test in erythromycin-resistant, clindamycin-susceptible S. aureus isolates (a positive test indicates induced resistance—choose a different antibiotic).
• CA-MRSA is resistant to β-lactams (including oral cephalosporins and antistaphylococcal penicillins) and often macrolides, azalides, and quinolones.
• Although most CA-MRSA isolates are susceptible to rifampin, this drug should never be used as a single agent because of concerns for resistance. The role of combination therapy with rifampin in CA-MRSA SSTIs is not clearly defined.
• There has been increasing resistance to clindamycin, both initial (~33%) and induced.
• Although CA-MRSA isolates are susceptible to vancomycin, oral vancomycin cannot be used for CA-MRSA SSTIs due to limited absorption.

Second Line

Treat severe CA-MRSA SSTIs requiring hospitalization and HA-MRSA SSTIs using:

• Vancomycin: generally, 1 g IV q12h (30 mg/kg/day IV in 2 divided doses); children: 40 mg/kg/day IV in 4 divided doses; vancomycin-like antibiotics that require only 1 or 2 doses are also available.
• Linezolid: 600 mg IV/PO q12h; children, uncomplicated: <5 years of age, 30 mg/kg/day IV/PO in 3 divided doses; 5 to 11 years of age, 20 mg/kg/day IV/PO in 2 divided doses; >11 years, use adult dosing; children, complicated: birth to 11 years, 30 mg/kg/day IV/PO in 3 divided doses; for older children, use adult dosing.
  • Linezolid seems to be more effective than vancomycin for treating people with SSTIs, but current studies have high risk of bias.
• Clindamycin: 600 mg IV q8h; children, 10 to 13 mg/kg/dose IV q6–8h up to 40 mg/kg/day
• Daptomycin: 4 mg/kg/day IV; children, 1 to <2 years, 10 mg/kg IV once daily; 2 to 6 years, 9 mg/kg IV once daily; 7 to 11 years, 7 mg/kg IV once daily; 12 to 17 years, 5 mg/kg IV once daily; ≥18 years, adult dosing
  • Do not use if pulmonary involvement.
• Ceftaroline: 600 mg IV q12h; children, 0* to <2 months, 6 mg/kg IV q8h; 2 months to <2 years, 8 mg/kg IV q8h; ≥2 years to <18 years and ≤33 kg, 12 mg/kg IV q8h; ≥2 years to <18 years and >33 kg, 400 mg IV q8h OR 600 mg IV q12h; ≥18 years, adult dosing.
  • *Gestational age ≥34 weeks and postnatal age ≥12 days.

Pediatric Considerations

• Tetracyclines not recommended for patients aged <8 years for CA-MRSA (in contrast with their position as treatment of choice in tickborne rickettsial diseases).
• TMP/SMX is not recommended for patients aged <2 months.
• Daptomycin is not recommended in pediatric patients <1 year of age (risk of neuromuscular, and/or nervous system side effects).
• Daptomycin dosage adjustment for pediatric patients with renal impairment has not been established.
• Ceftaroline dosage adjustment for pediatric patients with CrCl ≤50 mL/min/1.73 m² has not been established.

Pregnancy Considerations

• Tetracyclines are contraindicated.
• TMP/SMX not recommended in 1st or 3rd trimester

Geriatric Considerations
A recent review notes no prospective trials in this age group and recommends use of general adult guidelines.

Patient Monitoring

For outpatients: promptly return for care with systemic symptoms, worsening local symptoms, or failure to improve within 48 hours. Consider a follow-up within 48 hours of initial visit to assess response and review culture.