Background: Methicillin-resistant Staphylococcus aureus (MRSA), which accounted for 64.4 percent of S. aureus infections in U.S. intensive care units in 2003, has become prevalent in the community setting. In addition to skin and soft tissue disease, MRSA can cause potentially fatal systemic infection. Klevens and colleagues evaluated the incidence and scope of community-based MRSA to assess the burden of invasive infection.

The Study: The study included active population-based surveillance of MRSA infections in nine U.S. communities from July 2004 to December 2005. The total population was approximately 16.5 million persons. MRSA was identified by reviewing records and by contacting hospital laboratories. Based on health care risk factors for MRSA, infections were designated as health care–associated (hospital or community onset) or community-associated. Demographic and mortality data were collected, and MRSA isolates from invasive infections were obtained.

Results: Most of the 8,987 MRSA infections identified during the study were health care–associated. There were 5,250 (58.4 percent) community-onset infections; 2,389 (26.6 percent) hospital-onset infections; 1,234 (13.7 percent) community-associated infections; and 114 (1.3 percent) infections that could not be classified. The overall infection rate was 20 to 50 per 100,000 persons with a mortality rate of 6.3 per 100,000. The incidence of MRSA was greatest in older persons, blacks, and men, and was lowest in children five to 17 years of age. The most common documented risk factors for MRSA were a history of hospitalization, surgery, long-term care residency, and MRSA infection or colonization. Invasive disease was associated with bacteremia (75.2 percent), pneumonia (13.3 percent), cellulitis (9.7 percent), osteomyelitis (7.5 percent), endocarditis (6.3 percent), and septic shock (4.3 percent).

Conclusion: The authors estimate that the incidence of invasive MRSA infection was 94,360 in 2005 with 18,650 fatalities. The rate of invasive MRSA infections was 33.0 per 100,000 in Atlanta, Ga., up from the 2001 to 2002 rate of 19.3 per 100,000. The rate in Baltimore, Md., was 116.7 per 100,000, up from the 2001 to 2002 rate of 40.4 per 100,000. Although molecular analysis indicates that most strains of MRSA originate in health care facilities, the authors speculate that the occurrence of invasive MRSA infection in other settings may be increasing.