The Epidemiology of MRSA

Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of both nosocomial and community-acquired infections. Researchers delineated the epidemiologic patterns of MRSA infection in 2005, using data from nine predominantly urban areas around the country, comprising about 16.5 million people. They outlined the frequency of invasive MRSA infections, defined as the isolation of the organism from a normally sterile site, such as blood, cerebrospinal fluid, pleural effusion, or various tissues. They divided these infections into three types:

• Healthcare associated, with onset in the hospital

• Healthcare associated, with a community onset (including patients with an indwelling invasive device at the time of the infection; a history of MRSA infection or colonization; or a history of surgery, hemodialysis, hospitalization, or residence in a long-term-care facility during the last 12 months)

• Community associated (with none of the features included in the second type).

Most MRSA infections were healthcare associated; almost 60% of these had a community onset (annual incidence, about 17 per 100,000 persons), and 25% had a hospital onset (9 per 100,000). The incidence of community-associated infections was about 5 per 100,000. Rates of invasive MRSA infections were highest in those older than 65 and in males. The lowest rates were in young people aged 5 to 17. MRSA was more common in blacks than in whites, and mortality rates were higher in those older than 65, those who were black, and those who had healthcare-associated, community-onset infections. Given these statistics, the authors estimate that approximately 94,000 invasive MRSA infections occurred in 2005 in the U.S., with about 18,500 deaths (20% mortality rate).