For FY 1999–FY 2008, a mean (± SD) of 48,940 (± 3,926) unique veterans accessed care in the VAMHCS each year. The mean annual numbers of acute care and nursing home or intermediate care admissions was 5,854 (± 199) and 919 (± 161) corresponding to 23,183 (± 1,743) and 98,902 (± 10,893) inpatient days, respectively.

We have described the incidence of all S. aureus infections during a 10-year period in a large US-based population (49,000 persons) using person-level data including clinical culture and administrative data to identify and classify infections. Our results suggest significant increases in overall S. aureus infections from FY 1999 through FY 2008; the largest increases were associated with community-onset MRSA infections of skin and soft tissue and an overall decrease in incidence of invasive S. aureus infections.

During the study period, we implemented many new hospital infection–control practices—including the use of alcohol-based hand gels for hand hygiene (2003), central line bundles (2006), MRSA surveillance cultures (intensive care units in 2003, expanded to acute care in 2007), and chlorhexidine bathing of all surgical patients (2009)—which may have contributed to the decreased incidence of invasive infections. Previous studies suggest that improved infection control practices have contributed to fewer catheter-related and central line–associated bloodstream infections (20,21). However, attributing the decrease to a single practice is difficult, if not impossible, and few published studies exist with which we can compare our invasive S. aureus results.

Laupland et al. estimated an annual incidence of invasive S. aureus of 28 cases per 100,000 population, but their results may not be comparable because they were based primarily on MSSA infections (12). Klevens et al. estimated an incidence of invasive MRSA infection of 32 cases per 100,000 population, 75% were bacteremias, and 27% were hospital-acquired on the basis of 2005 data from 9 US cities (13). Their study reported a noticeably higher incidence of invasive MRSA in Baltimore, Maryland, USA, of 117 per 100,000 compared with estimates of 20–50 per 100,000 population at other sites. Our estimated rate of invasive MRSA infection (160 cases/100,000 veterans) is commensurate during the same year (2005), and our calculated proportion of invasive hospital-onset MRSA in 2005 (20%) also was similar. Klevens et al. reported estimates per 100,000 persons; therefore a direct comparison of rates of invasive hospital-onset MRSA infection is not feasible. However, our rates are potentially higher given the inpatient days adjustment accounting for the first 48 hours in the definition for hospital-onset infection. Another study by Laupland et al. reported an annual incidence of S. aureus bloodstream infections during 2000–2006 of 20 per 100,000 population (14). However, few MRSA cases were identified in this study, thereby making it difficult to compare with our results for which MRSA caused almost half of infections.

For several reasons, our incidence rates of invasive infections are higher than those previously reported. Black race has previously been reported as a marker for increased risk for invasive MRSA infections (13,22). Therefore, we would expect a higher incidence of all S. aureus infections, given that in our study 49% of patients were black. Additionally, given that our study was performed on data from a primarily urban population located in or around Baltimore, we could attribute the higher incidence of infections to suspected risk factors that are more prevalent in this location, including intravenous drug use.

Our study identified increases in noninvasive S. aureus infections, particularly around 2003, which most likely are associated with the emergence of the USA300 MRSA clone that has led to increases in community-associated MRSA, specifically in SSTIs (1,22). Also, despite dramatic increases in noninvasive community-onset MRSA infections, we did not observe a proportionate increase in invasive community-onset MRSA as might be expected if USA300 MRSA had the same propensity as non-USA300 MRSA to invade the bloodstream. No population-based studies have been published with which to compare overall noninvasive S. aureus infections, and few exist for comparison of noninvasive MRSA infections. For instance, Liu et al. recently reported annual (2004–2005) incidence rates of community-acquired and hospital-acquired MRSA among residents in San Francisco, California, USA, community-associated of 316 and 31 cases per 100,000 population, respectively, for which most cultures were from skin and soft tissue (8). The results of our study for community-onset MRSA are slightly lower but similar. Crum et al. reported a dramatic increase during 2002–2004 in community-associated MRSA infections, of which most were classified as SSTIs, and an incidence rate of 155 cases per 100,000 persons from 2004 data (7). The FY 2004 incidence of noninvasive community-associated MRSA in our study was 188 cases per 100,000 veterans, which is similar to that reported by Crum et al.

Our study adds new information to the existing literature and has several strengths. A major strength is its calculation of annual incidence of all S. aureus infections for a 10-year period by using actual numbers of patients at risk, admissions, and inpatient days. Our estimates of incidence are more accurate than those in previous studies, which were based on census-level data (11–13,23). Noninvasive infections were identified by using an automated approach that required both positive clinical culture and confirmed ICD-9-CM code for infection. This definition is more rigorous, thereby producing higher positive predictive values than clinical cultures alone, particularly for infections of bone and skin or soft tissue (16). In addition, this automated approach enabled us to identify and classify types of S. aureus infection, which is useful for understanding the overall population distribution of infection. Access to comprehensive, patient-level information, including prior hospitalizations, prior MRSA infections, and surgeries, allowed us determine the epidemiologic class for all MRSA infections. This study was performed in a population receiving standardized health care; therefore, findings should be free of bias associated with access to care or duration and type of treatment received.

Our study also has several limitations. First, the VAMHCS population of adult, mostly male patients living in the mid-Atlantic region does not fully represent the overall US population. We are unable to extrapolate these findings to children, a population for which an increase of community-onset MRSA SSTIs has been reported (24). Previous reports suggest that men are at higher risk than women for S. aureus infections; therefore, our estimates may overestimate true rates for women (8,12,13). Second, although we did not perform molecular typing on the S. aureus–positive isolates, we expect that a significant proportion (>80%) of noninvasive MRSA infections were caused by the USA300 MRSA strain (1). Given that USA300 MRSA reportedly varies across the United States, our findings may not be generalizable to populations in which MRSA strains differ. Third, we may have underestimated incidence because our definition of S. aureus infection required a positive clinical or blood culture. However, given the standardized access to care in the VAMHCS, we expect that cultures were uniformly collected in patients who had clinical signs or symptoms of infection. Fourth, the clinical culturing rate may have increased during our study period, which would contribute to overestimates in the incidence of S. aureus infection, particularly noninvasive infections. Johnston et al. observed an increase in the absolute number of SSTI cultures obtained in the VAMHCS Emergency Care Service (1). However, they determined that the proportion of MRSA infections increased, even though the proportion of MSSA remained the same, which suggests a true increase in MRSA. We observed similar patterns: whereas MRSA infections increased, MSSA infections remained relatively stable, and the annual number of S. aureus cultures did not significantly change.

In conclusion, this large, population-based study demonstrated an increase in the overall incidence of S. aureus infections during FY 1999–FY 2008, which was driven by a rapid increase in noninvasive, community-onset, MRSA skin and soft tissue infections. This increase was most striking during and after 2003, which is coincident with the time during which the USA300 clone became a major contributor to noninvasive S. aureus infections. Despite this increase, incidence of invasive community-onset MRSA infections did not significantly increase, and the overall MSSA infections and noninvasive MSSA infections remained generally stable. These results suggest a shift in the distribution of S. aureus infections to more noninvasive community-onset MRSA infections. This information is useful for interpreting changes in the epidemiology of S. aureus infections, which may help guide additional prevention strategies focused on reducing community-onset S. aureus infections. To further understand these trends, additional studies are warranted to identify risk factors for S. aureus infection and to describe the epidemiology of S. aureus infections across the entire population.

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1. On the basis of the current retrospective cohort study by Dr. Tracy and colleagues, which of the following statements about the overall incidence of Staphylococcus aureus infections in the Veterans Affairs Maryland Healthcare System between fiscal years 1999 and 2008 is most likely correct?A. Incidence of all S. aureus infections significantly decreasedB. Changes in incidence of all S. aureus infections were most striking during and following 2005C. 10-year average overall incidence per 100,000 veterans was 749 ± 132D. The findings are applicable to pediatric units2. You are an infection-control officer in a Maryland veterans hospital. On the basis of the study by Dr. Tracy and colleagues, which of the following statements about trends in invasive vs noninvasive S. aureus infections, methicillin susceptibility, and location of onset and infection site is most likely to apply to infection control at your hospital?A. About half of S. aureus infections are likely to be invasiveB. A projected increase in overall S. aureus infections is most likely to be driven by invasive, methicillin-sensitive, hospital-onset infectionsC. More than half of all noninvasive infections are likely to be skin and soft tissue infectionsD. Hospital-onset invasive MRSA infections are likely to significantly increase3. As the infection-control officer described in question 2, which of the following hospital infection-control practices do you think would be most likely to reduce incidence of invasive S. aureus infections, as based on their use in the VA hospital studied by Dr. Tracy and colleagues?A. Hand-washing with soap and water rather than using alcohol-based hand gelsB. Central-line bundlesC. Surveillance cultures for methicillin-sensitive S. aureusD. Soap-and-water rather than chlorhexidine bathing for surgical patients