Evolving Epidemiology of Staphylococcus aureus Bacteremia
Background: National estimates suggest that the epidemiology of invasive methicillin-resistant S. aureus (MRSA) infections is changing. USA300—the most common community-associated MRSA strain in the US by pulsed-field gel electrophoresis (PFGE)—has led to community-onset (CO) and hospital-onset (HO) bloodstream infections (BSIs). The objective was to examine the epidemiology of S. aureus—MRSA and methicillin-susceptible S. aureus (MSSA)—BSIs and to determine the proportion of MRSA BSIs due to USA300.
Methods: From 2007-2013, we used electronic surveillance data to examine the incidence of CO (≤3 days into hospitalization, irrespective of prior healthcare exposure) and HO (>3 days) S. aureus BSIs at Stroger Hospital, the major public hospital in Chicago. Available MRSA isolates from 2007-2012 underwent PFGE analysis. We modeled change in BSI incidence using a Poisson regression model.
Results: African-Americans were significantly more likely than others to have a MRSA BSI rather than MSSA BSI (OR=2.1; 95%CI, 1.6, 2.8, p<0.001); Hispanic ethnicity was negatively associated with MRSA BSI (OR=0.5; 95%CI, 0.4, 0.8, p<0.001). Younger age was associated with having a USA300 infection (p=0.002).
The incidence of HO-MRSA BSIs significantly declined during the study period (p=0.04). The incidence of HO-MSSA BSIs was stable during the early study period (p=0.22) but declined following 2010 (Fig 1). While the incidence of CO-MSSA BSIs significantly declined over time (p=0.04), the incidence of CO-MRSA BSIs remained unchanged over the 7 year study period (p=0.92) (Fig 2).
88% of MRSA BSI isolates were genotyped. The proportion of genotyped MRSA BSIs due to USA300 was 69%. From Time 1 (2007-09) to Time 2 (2010-12), the proportion of CO-MRSA BSIs due to USA300 MRSA remained stable, 75% vs 77%. For HO-MRSA BSIs, over half (57%) of genotyped strains were USA300 MRSA with a proportion of 60% in Time 1 and 53% in Time 2.
Conclusion: The incidence of HO-MRSA and HO-MSSA BSIs declined during the study period while the rate of CO-MRSA BSIs was unchanged. Over half of MRSA BSIs were due to USA300 and the proportion of CO and HO MRSA BSIs due to USA300 was stable over time. Our findings suggest that USA300 MRSA is endemic not only in the community but also in certain healthcare settings.
B. Hota, None
R. A. Weinstein, None
K. Popovich, None