Staphylococcus aureus bloodstream infections:: Risk factors, outcomes, and the influence of methicillin resistance in Calgary, Canada, 2000-2006

Background. Reports have suggested that the epidemiological profile of invasive Staphylococcus aureus infections is changing. We sought to describe the epidemiological profile of S. aureus bacteremia and to assess whether the incidence and severity of and the antimicrobial resistance rates associated with this bacteremia are increasing. Methods. Population-based surveillance for S. aureus bacteremias was conducted in the Calgary Health Region (population, 1.2 million) during 2000-2006. Results. The annual incidence of S. aureus bacteremia was 19.7 cases/100,000 population. Although rates of health care-associated and nosocomial methicillin-susceptible S. aureus (MSSA) bacteremia were similar throughout the study, rates of community-acquired MSSA bacteremia gradually decreased, and rates of methicillin-resistant S. aureus (MRSA) bacteremia dramatically increased. The clonal type predominantly isolated was CMRSA-2 (i.e., Canadian [C] MRSA-2), but CMRSA-10 (USA300) strains have been increasingly isolated, especially from community-onset infections, since 2004. Dialysis dependence, organ transplantation, HIV infection, cancer, and diabetes were the most important risk factors and were comparable for MSSA and MRSA bacteremias. The overall case-fatality rate was higher among individuals with MRSA (39%) than among those with MSSA (24%; P < .0001). The annual overall population mortality rate associated with S. aureus bacteremia did not significantly change during the study. Conclusions. Although the overall influence of S. aureus bacteremia has not significantly changed, MRSA has emerged as an important etiology in our region.