Methicillin-Resistant Staphylococcus aureus (MRSA), 2011

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 2005, as part of the EIP Active Bacterial Core surveillance (ABCs) system, we 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 was 17.0, 14.0, and 18.2 per 100,000 in 2009, 2010, and 2011, respectively (2011: Ramsey 19.9/100,000 and Hennepin 17.4/100,000). MRSA was most frequently isolated from blood (66%), and 14% (42/302) of the cases died. The rate of invasive MRSA infection acquired in hospitals (hospital-onset or nosocomial) decreased from 5.4 per 100,000 in 2005 to 1.8 in 2011. Twelve percent (37/302) of 2011 reported cases had no reported health care-associated risk factors in the year prior to infection. Please refer to the MDH Antibiogram for details regarding antibiotic susceptibility testing results.

Vancomycin-intermediate (VISA) and vancomycin-resistant S. aureus (VRSA) are reportable in Minnesota, as detected and defined according to Clinical and Laboratory Standards Institute approved standards and recommendations: a 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. There have been no VRSA cases in Minnesota. We confirmed 1 VISA case in 2000, 3 cases in 2008, 3 cases in 2009, and 2 cases in 2010. In 2011, 5 VISA cases were reported; 2 were methicillin-susceptible SA (MSSA) and 3 were MRSA. The MSSA cases had a history of immunosuppression; interestingly, there was no prior history of MRSA or recent exposure to vancomycin. The MRSA cases had a history of diabetes or chronic renal insufficiency. One MRSA isolate was daptomycinnon susceptible. Critical illnesses or deaths due to community-associated (CA) S. aureus infection (both methicillin-susceptible and-resistant) are reportable in Minnesota. From 2005-2011, 106 cases of critical illness or death due to community- associated S. aureus infection were reported: 8 (2005), 14 (2006), 16 (2007), 19 (2008), 20 in 2009, 17 (2010), and 12 (2011); 56 (53%) were MRSA and 50 (47%) MSSA. Twenty-six (46%) MRSA cases were male and the median age was 35 years (12 days-88 years); 28 (56%) MSSA cases were male and the median age was 16 years (1 day-78 years). Multifocal infections occurred in 25 cases; 17 MRSA, 8 MSSA. Pneumonia was most frequent with 31 MRSA and 20 MSSA cases, and accounted for 21 (68%) deaths. One MRSA and 8 MSSA had TSS; 3 MRSA and 8 MSSA had endocarditis (5/11 fatal); 20 MRSA and 7 MSSA had skin structure infections. Death occurred in 15 (28%) MRSA and 16 (32%) MSSA cases.

PFGE typing and toxin PCR were performed on 45 MRSA and 42 MSSA isolates. Most MRSA isolates belonged to clonal groups associated with CA USA types (80% USA300). There was no change in the number of USA300 MRSA cases over time. MSSA isolates were in clonal groups associated with CA and health care-associated USA types.

• 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, 2011