Methicillin-Resistant Staphylococcus aureus (MRSA), 2005

Strains of Staphylococcus aureus that are resistant to methicillin and all beta-lactam antibiotics are referred to as methicillin-resistant Staphylococcus 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 health care-associated 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 Twin Cities 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 submit 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 (Naimi, T., et al. Community-onset and health care-associated methicillin-resistant Staphylococcus aureus in Minnesota. JAMA. 2003;290(22):2976-84.)

In 2005, 2,955 cases of MRSA infection were reported to MDH by the 12 sentinel hospital laboratories. Thirty-four percent (1,004/2,955) of these cases were classified as CA-MRSA; 64% (1,904/2,955) were classified as HA-MRSA, and 2% (47/2,955) could not be classified. Isolates were received from 931 (93%) of the 1,004 CA-MRSA cases. To date, antimicrobial susceptibility testing has been completed on 514 (55%) and pulsed-field gel electrophoresis (PFGE) subtyping has been completed for 309 (33%) of these isolates.

Notable trends in total case numbers, PFGE subtypes, and antibiotic susceptibility patterns have been identified during the 6 years of CA-MRSA sentinel surveillance. CA-MRSA infections reported from the 12 sentinel surveillance sites have increased from 131 cases (12% of all MRSA infections reported) in 2000 to 1,004 cases (34% of total MRSA infections reported) in 2005.

MRSA is resistant to all beta-lactam antimicrobials and beta-lactams should no longer be used as the sole empiric therapy for severely ill patients whose infections may be staphylococcal in origin. However, all 2005 CA-MRSA isolates tested to date have been susceptible to linezolid, synercid, rifampin, and vancomycin and most CA-MRSA isolates were susceptible to trimethoprim-sulfamethoxazole (99%), gentamicin (99%), tetracycline (94%), clindamycin (90%), and ciprofloxacin (63%). Conversely, only 14% of CA-MRSA isolates in 2005 were susceptible to erythromycin.

The CDC classifies MRSA isolates into pulsed-field types (PFTs) (currently USA100-1200) based on genetic relatedness. (McDougal, L. et. al. Pulsed-field gel electrophoresis typing of oxacillin-resistant Staphylococcus aureus isolates from the United States: Establishing a national database. J Clin Microbiol. 2003;41:5113-20). 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 2005, only 14% of CA-MRSA isolates were USA400 and 66% 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 12% in 2005.

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: Staphylococcus aureus
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2005