Estimating the US burden of methicillin-resistant Staphylococcus aureus (MRSA) infections is important for planning and tracking success of prevention strategies. OBJECTIVE To describe updated national estimates and characteristics of health care-and community-associated invasive methicillin-resistant Staphylococcus aureus (MRSA) infections in 2011. DESIGN, SETTING, AND PARTICIPANTS Active laboratory-based case finding identified MRSA cultures in 9 US metropolitan areas from 2005 through 2011. Invasive infections (MRSA cultured from normally sterile body sites) were classified as health care-associated community-onset (HACO) infections (cultured Յ3 days after admission and/or prior year dialysis, hospitalization, surgery, long-term care residence, or central vascular catheter presence Յ2 days before culture); hospital-onset infections (cultured >3 days after admission); or community-associated infections if no other criteria were met. National estimates were adjusted using US census and US Renal Data System data. MAIN OUTCOMES AND MEASURES National estimates of invasive HACO, hospital-onset, and community-associated MRSA infections using US census and US Renal Data System data as the denominator. RESULTS An estimated 80 461 (95% CI, 69 515-93 914) invasive MRSA infections occurred nationally in 2011. Of these, 48 353 (95% CI, 40 195-58 642) were HACO infections; 14 156 (95% CI, 10 096-20 440) were hospital-onset infections; and 16 560 (95% CI, 12 806-21 811) were community-associated infections. Since 2005, adjusted national estimated incidence rates decreased among HACO infections by 27.7% and hospital-onset infections decreased by 54.2%; community-associated infections decreased by only 5.0%. Among recently hospitalized community-onset (nondialysis) infections, 64% occurred 3 months or less after discharge, and 32% of these were admitted from long-term care facilities. CONCLUSIONS AND RELEVANCE An estimated 30 800 fewer invasive MRSA infections occurred in the United States in 2011 compared with 2005; in 2011 fewer infections occurred among patients during hospitalization than among persons in the community without recent health care exposures. Effective strategies for preventing infections outside acute care settings will have the greatest impact on further reducing invasive MRSA infections nationally.

N ESTIMATED 1.7 MILLION health care-associated infections are associated annually with 99 000 deaths in US hospitals. 1 Although many pathogens can cause health care-associated infections, about 16% of those recently reported to the Centers for Disease Control and Prevention's (CDC's) National Healthcare Safety Network were associated with pathogens that were resistant to the antimicrobials traditionally used to treat them. 2 These multidrugresistant organisms pose important treatment challenges, so preventing their transmission in health care facilities is an important patient safety priority. Perhaps no organism has garnered more attention than methicillin-resistant Staphylococcus aureus (MRSA), and preventing health care-associated MRSA infections has become a goal for public health agencies and policy makers. 3 Although several studies have demonstrated reductions in health careassociated MRSA infections following the implementation of prevention in-For editorial comment see p 687.

This study characterizes 1,984 methicillin-resistant Staphylococcus aureus (MRSA) isolates collected in 2005and 2006 from normally sterile sites in patients with invasive MRSA infection. These isolates represent a convenience sample of all invasive MRSA cases reported as part of the Active Bacterial Core surveillance system in eight states in the United States. The majority of isolates were from blood (83.8%), joints (4.1%), and bone (4.2%). Isolates were characterized by pulsed-field gel electrophoresis (PFGE); SCCmec typing; susceptibility to 15 antimicrobial agents; and PCR analysis of staphylococcal enterotoxin A (SEA) to SEH, toxic shock syndrome toxin 1, and Panton-Valentine leukocidin. Thirteen established PFGE types were recognized among these isolates, although USA100 and USA300 predominated, accounting for 53.2% and 31.4% of the isolates, respectively. As expected, isolates from hospital onset cases were predominantly USA100, whereas those from community-associated cases were predominantly USA300.

To determine frequency of methicillin-resistant Staphylococcus aureus infections caused by strains typically associated with community-acquired infections (USA300) among persons with healthcare-related risk factors (HRFs), we evaluated surveillance data. Of patients with HRFs, 18%–28% had a "community-associated" strain, primarily USA300; of patients without HRFs, 26% had a "healthcare-associated" strain, typically USA100.

Septic arthritis is a rheumatologic emergency as joint destruction occurs rapidly and can lead to significant morbidity and mortality. Accurate diagnosis can be particularly challenging in patients with underlying inflammatory joint disease. This review outlines the risk factors for septic arthritis and summarizes the causative bacterial organisms. We highlight advances in antibiotic management with a focus on new drugs for methicillin-resistant Staphylococcus aureus (MRSA) and discuss the use of adjunctive therapies for treatment of septic arthritis in adults.

WHAT'S KNOWN ON THIS SUBJECT: Invasive methicillin-resistantStaphylococcus aureus (MRSA) in children is associated with high morbidity and mortality. Although reductions in health careassociated MRSA infection among adults are documented, it is unclear if a similar trend is occurring among children.WHAT THIS STUDY ADDS: Data from population-based surveillance were analyzed to assess changes in invasive MRSA infection incidence over time. This analysis describes the epidemiology and trends of invasive MRSA infections among children in 9 US metropolitan areas and estimates national burden. METHODS:We evaluated reports of invasive MRSA infections in pediatric patients identified from population-based surveillance during 2005-2010. Cases were defined as isolation of MRSA from a normally sterile site and classified on the basis of the setting of the positive culture and presence or absence of health care exposures. Estimated annual changes in incidence were determined by using regression models. National age-and race-specific incidences for 2010 were estimated by using US census data. RESULTS:A total of 876 pediatric cases were reported; 340 (39%) were among infants. Overall, 35% of cases were hospital-onset, 23% were health care-associated community-onset, and 42% were community-

This is the first population-based study of ReA following infections due to bacterial enteric pathogens in the USA. These data will help determine the burden of illness due to these pathogens and inform clinicians about potential sequelae of these infections.

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