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 N-SAB, 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.