Methicillin-Resistant Staphylococcus aureus (MRSA), 2007

Strains of Staphylococcus aureus that are resistant to methicillin and all available beta-lactam antibiotics are referred to as methicillin-resistant S. aureus (MRSA). Traditional risk factors for health care-associated (HA) MRSA include recent hospitalization or surgery, residence in a long-term care facility, and renal dialysis.

In 1997, MDH began receiving reports of healthy young patients with MRSA infections. These patients had onset of their MRSA infections in the community and appeared to lack the established risk factors for MRSA. Although most of the reported infections were not severe, some resulted in serious illness or death. Strains of MRSA cultured from persons without HA risk factors for MRSA are now known as community-associated MRSA (CA-MRSA). CA-MRSA is defined as: a positive culture for MRSA from a specimen obtained <48 hours of admission to a hospital in a patient with no history of prior MRSA infection or colonization; no presence of indwelling percutaneous devices or catheters at the time of culture; and no history of hospitalization, surgery, residence in a long-term care facility, hemodialysis, or peritoneal dialysis in the year prior to the positive MRSA culture.

MDH initiated active surveillance for CA-MRSA at 12 sentinel hospital laboratories in January 2000. The laboratories (six in the metropolitan area and six in Greater Minnesota) were selected to represent various geographic regions of the state. Sentinel sites report all cases of MRSA identified at their facilities and for the first six years of surveillance submitted all CA-MRSA isolates to MDH. The purpose of this surveillance is to determine demographic and clinical characteristics of CA-MRSA infections in Minnesota, to identify possible risk factors for CA-MRSA, and to identify the antimicrobial susceptibility patterns and molecular subtypes of CA-MRSA isolates. A comparison of CA- and HA-MRSA using sentinel site surveillance data from 2000 demonstrated that CA- and HA-MRSA differ demographically and clinically, and that their respective isolates are microbiologically distinct.

In 2007, 3,495 cases of MRSA infection were reported by the 12 sentinel hospital laboratories. Fifty percent (1,761/3,495) of these cases were classified as CA-MRSA; 47% (1,644/3,495) were classified as HA-MRSA; and 3% (90/3,495) could not be classified. CA-MRSA infections increased from 131 cases (12% of all MRSA infections reported) in 2000 to 1,761 cases (50% of total MRSA infections reported) in 2007.

The CDC classifies MRSA isolates into pulsed-field types (PFTs) (currently USA100-1200) based on genetic relatedness. CA-MRSA isolates are most often classified as PFT USA300 or USA400. In Minnesota, the predominant CA-MRSA PFT has changed dramatically over time. In 2000, 63% of CA-MRSA isolates were USA400 and 4% were USA300. In 2006, only 10% of CA-MRSA isolates were USA400 and 78% were USA300. Because USA400 isolates are much more likely than USA300 isolates to demonstrate inducible clindamycin resistance (ICR) on disk diffusion testing, the change in the predominant CA-MRSA PFT has also been associated with a decrease in the proportion of erythromycin-resistant, clindamycin-sensitive CA-MRSA isolates demonstrating ICR, from 93% in 2000 to 10% in 2006.

In 2005, as part of the CDC Active Bacterial Core surveillance (ABCs) system, MDH initiated population-based invasive MRSA surveillance in Ramsey County. In 2005 the incidence of invasive MRSA infection in Ramsey County was 19.8 per 100,000 population and was 19.4 and 18.5 in 2006 and 2007, respectively. In 2007, most MRSA was isolated from blood (82% in 2007), and 23% of the cases (21/93) died. Of the invasive isolates that underwent molecular subtyping, 79% (45/57) were of a known HA-MRSA PFT. Ramsey County had a somewhat lower invasive MRSA infection incidence rate than other participating surveillance sites (Klevens RM, et al. Invasive methicillin-resistant Staphylococcus aureus infections in the United States. JAMA. 2007;298(15):1763-71).

In 2007, MDH started collecting isolates from CA-MRSA and HA-MRSA invasive (isolated from a normally sterile body site) infections. Antimicrobial susceptibility and PFGE testing were performed on submitted isolates. Please refer to the MDH antibiogram for details. (see pp. 25-26).

Critical illnesses or deaths due to community-associated S. aureus infection (both methicillin-susceptible and-resistant) are now reportable in Minnesota, as is vancomycin-intermediate and vancomycin-resistant S. aureus.

• For up to date information see: Methicillin-resistant Staphylococcus aureus (MRSA)
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2007