Methicillin-Resistant Staphylococcus aureus (MRSA), 2009

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 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 surveillance for CA-MRSA at 12 sentinel hospital laboratories in January 2000; thus, 2009 was the tenth year of surveillance. The laboratories (six in the metropolitan area and six in Greater Minnesota) were selected to represent various geographic regions of the state. Infection preventionists at the sites have had the huge burden to report all cases of MRSA identified at their facilities, and for the first 6 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 2009, 3,401 cases of MRSA infection were reported by the 12 sentinel laboratories. 56% of these cases were classified as CA-MRSA, 42% were classified as HA-MRSA, and 3% could not be classified. CA-MRSA infections increased from 131 cases (12% of all MRSA infections reported) in 2000 to 1,898 cases in 2009.

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 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 (pages 28-29).

In 2005, as part of the EIP 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 and was 19.4, 18.5 and 19.9 per 100,000 in 2006, 2007, and 2008 respectively. In 2008, surveillance was expanded to include Hennepin County. The incidence rate for MRSA infection in Ramsey and Hennepin Counties in 2009 was 17.0 per 100,000 (Ramsey 22.9/100,000 and Hennepin 14.4/100,000). MRSA was most frequently isolated from blood (76%), and 13% (35/279) of cases died. Thirteen percent (37/279) of cases had no reported health care-associated risk factors in the year prior to infection.

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

S. aureus that have developed resistance mechanisms to vancomycin are called vancomycin-intermediate (VISA) or vancomycin-resistant S. aureus (VRSA), as detected and defined according to Clinical and Laboratory Standards Institute (CLSI) approved standards and recommendations (Minimum Inhibitory Concentration [MIC]=4-8 ug/ml for VISA and MIC≥16 ug/ml for VRSA). Patients at risk for VISA and VRSA generally have underlying health conditions such as diabetes and end-stage renal disease requiring dialysis, previous MRSA infections, recent hospitalizations, and recent exposure to vancomycin.

VISA infections are rare but in the past 2 years there has been an increase in the reported number of cases. MDH confirmed 1 case in 2000 and 3 cases in 2008. All of these cases had traditional risk factors for VISA infection including histories of diabetes, non-healing MRSA-positive leg ulcers, end-stage renal disease requiring renal dialysis, and vancomycin use. In 2009, 3 cases were reported. Interestingly, 2 cases were methicillin-susceptible SA (MSSA) and 1 was MRSA. The 2 MSSA cases had no reported recent history of vancomycin use though both had prolonged exposure to other antibiotics. All 3 case-isolates were susceptible to daptomycin. None of the cases had a history of dialysis and 1 MSSA case was diabetic. Of note, 4 of the 6 2008-2009 cases were clustered near the Minnesota-Wisconsin border. Three of these were MSSA and none had traditional VISA risk factors except longstanding antibiotic use.

• 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, 2009