Methicillin-resistant Staphylococcus aureus (MRSA) strains that typically cause community-acquired infections are infecting patients in hospitals, and strains considered health care associated are infecting patients with no health care-related risk factors, reported Dr. R. Monina Klevens of the Centers for Disease Control and Prevention (CDC) and her associates.
The Active Bacterial Core Surveillance program of the CDC is a population-based surveillance system for invasive organisms, including MRSA infections. The surveillance is ongoing in California, Colorado, Connecticut, Georgia, Maryland, Minnesota, New York, Oregon, and Tennessee (Emerging Infectious Diseases 2006;12:1991–3).
Surveillance reports are generated when positive MRSA cultures are detected in normally sterile sites, such as blood or cerebrospinal fluid.
Laboratory reports are linked to the patient's medical record, and health care-related risk factors (HRFs) are abstracted from the medical record for inclusion in a surveillance database.
HRFs include the presence of an invasive device, history of MRSA infection or colonization, dialysis, hospitalization, residence in a long-term care facility, and surgery.
Obtaining a bacterial culture more than 48 hours after hospital admission also suggests infection with a health care-associated strain.
MRSA strains are empirically typed, based on the separation characteristics seen using pulsed-field gel electrophoresis. In earlier MRSA outbreaks, strains USA300 and USA400 were the predominant community-associated strains. USA100 and USA 500 were considered health care-associated strains.
From January 2004 through February 2006, more than 9,100 cases of invasive MRSA were reported and classified as classic health care-associated infections, with cultures obtained more than 48 hours after admission; cases with one or more HRF but with community-onset infections (less than 48 hours); or cases with community-associated infections and no HRF.
Of the 9,147 cases, 2,535 (28%) were classic health care-associated infections, 5,353 (59%) were health care associated, but community acquired, and 1,259 (14%) were community-associated infections.
One hundred isolates were selected for typing by pulsed-field gel electrophoresis. Seven of 27 isolates (26%) from community-associated cases were USA100 or USA500, and 8 of 29 (28%) classic health care-associated cases were strain USA300. Overall, 18%–28% of patients with HRFs were infected with strains considered to be community-associated MRSA strains.
“The distinction between health care- and community-associated MRSA is rapidly blurring,” Dr. Klevens and her associates wrote.