In the laboratory, isolates of MRSA were identified on mannitol salt agar screen plates supplemented with oxacillin (6 mg/L), confirmed by detection of the mecA
genes via a polymerase chain reaction (PCR) assay.7
Susceptibilty testing was done with VITEK (bioMerieux, Inc. Haselwood, Mo.) or Kirby Bauer methods, plus a D-test for clindamycin susceptibility.8
Typing was done with pulsed-field gel electrophoresis (PFGE) after DNA extraction and digestion with Sma1
in a standard protocol.9
All isolates were further tested to confirm methicillin resistance and for the presence of PVL with a triplex PCR assay, which simultaneously detects lukS-PV
The MRSA isolates were typed for SCCmec
with a multiplex PCR SCCmec
typing assay into subtypes I, II, III, IVa, IVb, IVc, IVd and V.11
Isolates were typed for spa
(a polymorphism of the S. aureus
and a representative selection of isolates was further tested with multilocus sequence typing (MLST).14
The identification of MRSA isolates matching the USA300 strain was based on the similarity of PFGE patterns to control strains provided by the National Microbiology Laboratory of the Public Health Agency of Canada.
For case findings and definitions, we used laboratory reports provided by the regional laboratory of USA300 MRSA isolates to identify potential cases; in those with more than one positive isolate, the first report was used. This laboratory processes all specimens submitted from community physicians, clinics, hospitals and long-term care facilities in the CHR. A study case was defined as a patient with a positive isolate for the USA300 strain of MRSA between Jan. 1 and Sept. 30, 2004; a clinical infection; and a history of residence in or travel to the CHR during the 2 years before symptom onset. We used a history of one of the following during the 2 years before symptom onset to assign patients a high-risk status: illicit drug use (i.e., heroin, cocaine, crack cocaine, or abuse of morphine or other prescription opiates); incarceration in Alberta anytime since Jan. 1, 2002; or homelessness (having no fixed address, or staying in homeless shelters). Patients without any such history were identified as low-risk.
For demographic, clinical and risk information, we used chart reviews of medical records at hospitals, primary care clinics and a local corrections facility; an electronic search of the provincial Corrections Services admission database, HIV and hepatitis C registries, and hospital admission databases; and interviews with patients and their primary care providers. The information collected on potential risk factors included the presence of comorbid disease and histories of incarceration, use of group-living community facilities (detoxification centres or homeless shelters), travel, drug-use behaviours and types of drugs used, wound-care behaviours and previous hospital admissions.
We used EpiData version 3.0 (The EpiData Association, Denmark) to collect data, which was then analyzed with EpiINFO6 version 6.04 (US Centers for Disease Control and Prevention) and SPSS version 12.0.1. To compare medians, we used the Wilcoxon rank–sum test; to compare proportions, the χ2 test or Fisher's exact test, as appropriate.
We calculated the cumulative incidence of USA300 MRSA infection using CHR population estimates from Alberta Registry population data (from the Calgary Health Region internal Web site; accessed 2005 Aug. 10): 15 540 for the high-risk population (6000 homeless persons, 3540 injection drug users and 6000 individuals admitted to the local corrections facility at least once per year) and 1 127 825 for the low-risk population.15
Relative risks of infection were calculated with 95% confidence intervals (CIs).
To identify people with USA300 infections that were potentially health care–acquired, we used criteria from the Canadian Nosocomial Infections Surveillance Program (i.e., patients with infections that developed ≥ 3 days after admission to hospital, or who stayed in a hospital or resided in a long-term care facility at any time during the 12 months before symptom onset).16