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BACKGROUND: In 2004, an outbreak of the USA300 strain of methicillin-resistantStaphylococcus aureus(MRSA) was identified in persons with histories of homelessness, illicit drug use or incarceration in the Calgary Health Region (Calgary, Alberta). A prevalence study was conducted to test the hypotheses for factors associated with USA300 colonization or infection.METHODS: Participants were recruited at sites accessed by this marginalized population. Health care staff administered a questionnaire and collected crack pipes and nasal, axillary and skin infection swabs. Pipes and swabs were cultured according to standard techniques. MRSA isolates were further characterized by polymerase chain reaction (mecA, Panton-Valentine leukocidin and Staphylococcal cassette chromosomemec) and typing methods (pulsed-field gel electrophoresis, staphylococcal protein A typing and multilocus sequence typing). Colonization or infection was determined by having any one of nasal, axillary, skin infection or pipe swabs positive for USA300. Colonized participants had one or more nasal, axillary or pipe swab positive for USA300; infected participants had one or more skin infection swab positive for USA300.RESULTS: The prevalence of USA300 colonization or infection among 271 participants was 5.5% (range 3.1% to 9.0%). USA300 cases were more likely to report manipulation of skin infections (OR 9.55; 95% CI 2.74 to 33.26); use of crack pipes was not significant despite identification of the USA300 strain on two of four crack pipes tested. USA300 cases were more likely to report drug use between sex trade workers and clients (OR 5.86; 95% CI 1.63 to 21.00), and with casual sex partners (OR 5.40; 95% CI 1.64 to 17.78).CONCLUSION: Ongoing efforts to promote the appropriate treatment of skin infections in this population are warranted. The association of USA300 colonization or infection and drug use with sexual partners suggest a role for sexual transmission of the USA300 strain of MRSA.

Background: The USA300 strain of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) can cause severe infection and is increasingly recognized as a cause of community outbreaks. In 2004, an outbreak was identified in the Calgary Health Region (CHR). Methods: MRSA isolates were identified with standard methods at a central regional laboratory and typed via pulsed-field gel electrophoresis (PFGE). Isolates were tested by PCR for mecA , Panton–Valentine leukocidin (PVL), SCC mec , and spa genes. Cases were defined as such if a clinical isolate of the USA300 strain was noted between January 1 and September 30, 2004, and the patient had lived or traveled in CHR within 2 years before symptom onset. Demographic, clinical and risk data on all such cases were collected from several sources for statistical analysis. A case was defined as high-risk if the patient had a history of drug use, homelessness or incarceration. Results: Of 40 isolates with the USA300 PFGE pattern, all tested positive for PVL, SCC mec type IVa and spa type 008. Almost all infections (39/40, 98%) involved skin and soft tissues, except for 1 death from necrotizing hemorrhagic pneumonia; a notable proportion (38%) required hospital admission or intravenous antimicrobial therapy. The outbreak centred on the high-risk population in CHR (70%; risk ratio 169.4, 95% confidence interval 86.1–333.0). Interpretation: People with histories of illicit drug use, homelessness or recent incarceration were at highest risk for infection with CA-MRSA. The emergence and spread of this virulent strain has important implications for treatment and public health in Canada.

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