Methicillin-Resistant Staphylococcus aureus Colonization or Infection in Canada: National Surveillance and Changing Epidemiology, 1995-2007

OBJECTIVE. To determine the incidence and describe the changing epidemiology of methicillin-resistant Staphylococcus aureus (MRSA) colonization or infection in Canadian hospitals from 1995-2007. SETTING. Forty-eight hospitals participating in the Canadian Nosocomial Infection Surveillance Program. DESIGN. Prospective, laboratory-based surveillance for incident cases of MRSA colonization or infection among hospitalized patients. METHODS. Clinical and epidemiologic data were obtained by review of hospital records. Standard criteria were used to determine whether MRSA colonization or infection was present and whether the MRSA strain was healthcare associated or community associated. A representative subset of isolates was characterized by use of pulsed-field gel electrophoresis and staphylococcal cassette chromosome (SCC) mec typing. RESULTS. From 1995 to 2007, a total of 37,169 hospitalized patients were newly identified as either infected or colonized with MRSA, and the overall incidence of both MRSA colonization and MRSA infection increased from 0.65 to 11.04 cases per 10,000 patient-days (P < .001). Of these 37,169 patients, 11,828 (32%) had an MRSA infection, and infection rate increased from 0.36 to 3.43 cases per 10,000 patient-days. The proportion of community-associated MRSA strains increased from 6% to 23% (P < .001). The most common genotype (47% of isolates) was CMRSA-2 (USA100/800); in 2007, CMRSA-10 (USA300) was the second most common strain (27% of isolates), associated with SCCmec type IV. Patients with CMRSA-10 were predominantly from western Canada and were more likely to be children (odds ratio [OR], 10.0 [95% confidence interval {CI}, 7.4-13.4]) and to have infection (OR, 2.3 [95% CI, 1.9-2.7]), especially skin and/or soft tissue infection (OR, 5.9 [95% CI, 5.0-6.9]). CONCLUSIONS. The overall incidence of both MRSA colonization and MRSA infection increased 17-fold in Canadian hospitals from 1995 to 2007. There has also been a dramatic increase in cases of community-associated MRSA infection due to the CMRSA-10 (USA300) clone. Continued surveillance is needed to monitor the ongoing evolution of MRSA colonization or infection in Canada and globally.