Decreasing incidence of Staphylococcus aureus bacteremia over 9 years: greatest decline in community-associated methicillin-susceptible and hospital-acquired methicillin-resistant isolates

Background: The impact of community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) emergence on the epidemiology of S aureus bacteremia (SAB) is not well documented.

Methods: This was an observational study of adult (aged ≥18 years) inpatients with SAB in a single 808-bed teaching hospital during 2002-2003, 2005-2006, 2008-2009, and 2010 with period-stratified SAB rate, onset mode, patient characteristics, and outcome.

Results: We encountered a total of 1,098 cases over the entire study period. The rate decreased steadily over time (from 6.64/10(3) discharges in 2002-2003 to 6.49/10(3) in 2005-2006, 5.24/10(3) in 2008-2009, and 5.00/10(3) in 2010; P = .0001), with a greater decline in community-associated cases (0.99/10(3), 0.77/10(3), 0.58/10(3), and 0.40/10(3), respectively; P = .0005) compared with health care-associated cases (5.65/10(3), 5.72/10(3), 4.66/10(3), and 4.60/10(3), respectively; P = .005). The decline was principally in MSSA (3.11/10(3), 2.21/10(3), 2.24/10(3), and 1.75/10(3), respectively; P = .00006), including both community-associated (P = .0002) and health care-associated cases (P = .006). Although overall rate changes in MRSA were not significant (P = .09), hospital-onset MRSA decreased markedly (P < .00001), whereas CA-MRSA increased (P = .03). The all-cause 100-day mortality rate did not change significantly (25.6% for 2002-2003, 25.2% for 2005-2006, 28.1% for 2008-2009, and 32.2% for 2010; P = .10). Differences in MSSA/MRSA-associated mortality decreased (20.1% vs 30.6%, P = .03 for 2002-2003; 18.1% vs 28.9%, P = .05 for 2005-2006; 21.7% vs 32.9%, P = .05 for 2008-2009; and 29.3% vs 34.9, P = .5 for 2010).

Conclusions: SAB incidence is decreasing, with the greatest decline in community-associated MSSA and hospital-onset MRSA cases. Most health care-associated cases currently are community-onset. MRSA/MSSA-related mortality is comparable. These changes are likely related to the emergence of CA-MRSA and the inpatient-to-outpatient shift in health care.