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The Epidemiology of MRSA

The latest word on this worrisome infection

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).

Comment: For those interested in the statistics of invasive MRSA infections, this paper provides the most reliable data available. The important point for clinicians is that most invasive MRSA infections now occur in the community, rather than in the hospital. About 10% of the MRSA infections in this study were cellulitis, a reminder that skin and soft tissue are the sites of a significant number of MRSA infections. Several studies indicate that most invasive infections from S. aureus — both methicillin susceptible and methicillin resistant — originate from organisms that patients carry in their own nares or on their skin.

— Jan V. Hirschmann, MD

Published in Journal Watch Dermatology October 26, 2007

Citation(s):

Klevens RM et al. Invasive methicillin-resistant Staphylococcus aureus infections in the United States. JAMA 2007 Oct 17; 298:1763.

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