Prevalence of methicillin-resistant Staphylococcus aureus in skin and soft tissue infections in patients presenting to Canadian emergency departments

Background: Community-associated methicillin-resistant Staphylococcus aureus (MRSA) is an increasingly common cause of skin and soft tissue infection (SSTI) worldwide. The prevalence of MRSA in SSTIs across Canada has not been well described. Studies in the United States have shown significant geographic variability in the prevalence of MRSA. This study characterizes the geographic prevalence and microbiology of MRSA in patients presenting to Canadian emergency departments with SSTIs. Methods: Using a prospective, observational design, we enrolled patients with acute purulent SSTIs presenting to 17 hospital emergency departments and 2 community health centres (spanning 6 Canadian provinces) between July 1, 2008, and April 30, 2009. Eligible patients were those whose wound cultures grew S. aureus. MRSA isolates were characterized by antimicrobial susceptibility testing and pulsed-field gel electrophoresis. All patients were subjected to a structured chart audit, and patients whose wound swabs grew MRSA were contacted by telephone to gather detailed information regarding risk factors for MRSA infection, history of illness, and outcomes. Results: Of the 1,353 S. aureus-positive encounters recorded, 431 (32%) grew MRSA and 922 (68%) wounds grew methicillin-susceptible S. aureus. We observed significant variation in both the prevalence of MRSA (11-100%) and the proportion of community-associated strains of MRSA (0-100%) across our study sites, with a significantly higher prevalence of MRSA in western Canada. Interpretation: MRSA continues to emerge across Canada, and the prevalence of MRSA in SSTIs across Canada is variable and higher than previously expected.