Epidemiology of Staphylococcus aureus blood and skin and soft tissue infections in the US Military Health System, 2005-2010

Context: Rates of hospital-onset methicillin-resistant Staphylococcus aureus (MRSA) infections are reported as decreasing, but recent rates of community-onset S aureus infections are less known. Objectives: To characterize the overall and annual incidence rates of community-onset and hospital-onset S aureus bacteremia and skin and soft tissue infections (SSTIs) in a national health care system and to evaluate trends in the incidence rates of S aureus bacteremia and SSTIs and the proportion due to MRSA. Design, Setting, and Participants: Observational study of all Department of Defense TRICARE beneficiaries from January 2005 through December 2010. Medical record databases were used to identify and classify all annual first-positive S aureus blood and wound or abscess cultures as methicillin-susceptible S aureus or MRSA, and as community-onset or hospital-onset infections (isolates collected >3 days after hospital admission). Main Outcome Measures: Unadjusted incidence rates per 100 000 person-years of observation, the proportion of infections that was due to MRSA, and annual trends for 2005 through 2010 (examined using the Spearman rank correlation test or the Mantel-Haenszel χ² test for linear trend). Results: During 56 million person-years (nonactive duty: 47 million person-years; active duty: 9 million person-years), there were 2643 blood and 80 281 wound or abscess annual first-positive S aureus cultures. Annual incidence rates varied from 3.6 to 6.0 per 100 000 person-years for S aureus bacteremia and 122.7 to 168.9 per 100 000 person-years for S aureus SSTIs.Theannual incidence rates for community-onset MRSA bacteremia decreased from 1.7 per 100 000 person-years (95% CI, 1.5-2.0 per 100 000 person-years) in 2005 to1.2 per 100 000 person-years (95% CI, 0.9-1.4 per 100 000 person-years) in 2010 (P=.005 for trend). The annual incidence rates for hospital-onset MRSA bacteremia also decreased from 0.7 per 100 000 person-years (95% CI, 0.6-0.9 per 100 000 person-years) in 2005 to 0.4 per 100 000 person-years(95% CI, 0.3-0.5 per 100 000 person-years) in 2010 (P=.005 for trend). Concurrently, the proportion of community-onset SSTI due to MRSA peaked at 62% in 2006 before decreasing annually to 52% in 2010 (P<.001 for trend). Conclusion: In the Department of Defense population consisting of men and women of all ages from across the United States, the rates of both community-onset and hospital-onset MRSA bacteremia decreased in parallel, while the proportion of community-onset SSTIs due to MRSA has more recently declined.