Temporal Trends in the Incidence of Staphylococcus aureus Bacteremia in Olmsted County, Minnesota, 1998 to 2005: A Population-Based Study

Background. There is a paucity of population-based studies on Staphylococcus aureus bacteremia (SAB) in the United States. We determined the incidence of and trends in SAB in Olmsted County, Minnesota, over an 8-year period. Methods. A retrospective, population-based, cohort study was done to evaluate the initial episodes of SAB occurring in adult residents of Olmsted County, Minnesota, from 1 January 1998 through 31 December 2005, using the microbiology databases at Mayo Clinic and Olmsted Medical Center in Rochester, Minnesota. Results. Of 247 evaluable adult patients with SAB who were included in the incidence calculation, 143 (57.9%) were males, and the median age was 72.1 years (range, 19.5-98.5 years). Episodes of bacteremia were classified according to acquisition type: 58 (23.5%) were nosocomial (N-SAB), 145 (58.7%) were healthcare-associated (HCA-SAB), and 44 (17.8%) were community-acquired (CA-SAB). Methicillin-resistant S. aureus (MRSA) constituted 31.6% of the cases. No community-acquired MRSA bacteremia was noted. The age-adjusted incidence of SAB was 28.3 episodes/100,000 person-years for females and 53.5 episodes/100,000 person-years for males, with an age-and sex-adjusted rate of 38.2 episodes/100,000 person-years. The age-and sex-adjusted incidence of NSAB, HCA-SAB, and CA-SAB was 9.0, 22.6, and 6.6 episodes/100,000 person-years, respectively. The age-and sex-adjusted incidence of methicillin-susceptible S. aureus was 25.4 episodes/100,000 person-years, and that of MRSA was 12.4 episodes/100,000 person-years. Overall, the incidence rate increased with age but not over the calendar year. The exception was MRSA bacteremia, which increased at a rate of 19.8% (standard error, +/-5.5%) per year during the study. Conclusions. The incidence of SAB in adults remained stable in Olmsted County, Minnesota, from 1998 to 2005, but the proportion of episodes due to MRSA significantly increased over the 8-year period.