Concurrent Epidemics of Skin and Soft Tissue Infection and Bloodstream Infection Due to Community-Associated Methicillin-Resistant Staphylococcus aureus

Since 2000, a dramatic increase in the number of methicillin-resistant Staphylococcus aureus (MRSA) infections has been observed in North America, mostly related to the emergence of the USA300 clone as a primary cause of community-associated disease [1–3]. More recent studies revealed that USA300 has spread from the community to hospitals where it has become the predominant clone isolated in various settings [4, 5]. Most USA300 infections manifest as skin and soft tissue infections (SSTIs) [6], although life-threatening infections have also been documented among persons with comorbid conditions as well as healthy community-dwelling individuals [7, 8]. Inasmuch as SSTIs can result in dissemination of staphylococci into the bloodstream, the impact of this focus of infection—especially in the context of the USA300 epidemic—on the burden and outcome of BSI has not been well studied. Staphylococcus aureus is one of the most common etiologies of bloodstream infection (BSI) [9, 10]. MRSA BSI is a serious complication of MRSA colonization and/or infection, associated with substantial morbidity and mortality [11–14] and unresolved treatment issues [15, 16]. Bacteremia was present in 75% of invasive MRSA diseases in a population-based multicentric study performed in 2004–2005 in the United States [17], with an estimated standardized incidence rate of MRSA BSI at 23.8 per 100 000 inhabitants per year. In this study, results of molecular typing of a subset of isolates (11.3% of reported cases) suggested that USA300 accounted for 67% (100/150) of community-associated invasive MRSA disease, but only 16% (34/216) of healthcare-associated and 22% (108/485) of nosocomial invasive MRSA diseases [17], which led to the authors’ statement that “most invasive MRSA disease in the United States is still caused by MRSA strain of healthcare origin” [17]. However, this may not be the case in the most affected areas, such as San Francisco, where USA300 emerged in 2000 and dramatically changed MRSA epidemiology [18–20]. Indeed, it has been shown that the prevalence of USA300 strains among cases of healthcare-associated MRSA BSI may vary dramatically among geographically clustered hospitals [21]. The purpose of our study was to (1) assess temporal trends of MRSA BSI and (2) determine risk factors associated with BSI due to the USA300 clonal type from 2000 to 2008 in the Community Health Network of San Francisco, which includes San Francisco General Hospital, a public tertiary care center, and 13 citywide outpatient clinics.