Methicillin-Resistant Staphylococcus aureus (MRSA), 2003

Strains of Staphylococcus aureus that are resistant to methicillin and all betalactam antibiotics are referred to as methicillin-resistant Staphylococcus aureus (MRSA). Risk factors for 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 (if patient admitted); no history of prior MRSA infection or colonization; no presence of indwelling percutaneous devices or catheters at the time of culture; or 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.

In 1999, the Minnesota Rules governing communicable disease reporting were amended to require that designated sentinel hospitals report all cases of MRSA to MDH. In addition, cases of CA-MRSA causing serious illness or death were made reportable statewide.

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 community and health care-associated MRSA (HAMRSA) 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- 2984).

In 2003, 1,817 cases of MRSA infection were reported in 2003. Eighteen percent of these cases were classified as CA-MRSA; 81% were classified as HA-MRSA, and 1% could not be classified. Isolates were received from 298 (90%) of the 333 CA-MRSA cases. To date, antimicrobial susceptibility testing has been completed on 81 (24%) and molecular subtyping by PFGE has been completed for 161 (54%) of these isolates. CA-MRSA patients were younger than patients with HA-MRSA (median age, 29 years vs. 68 years) and more likely to have MRSA isolated from the skin (77% vs. 19%). Most CA-MRSA isolates belonged to one particular PFGE clonal group that is distinct from the clonal group most common among HA-MRSA isolates.

Clinicians should be aware that therapy with beta-lactam antimicrobials can no longer be relied upon as the sole empiric therapy for severely ill patients whose infections may be staphylococcal in origin. All CA-MRSA isolates submitted in 2003 were susceptible to gentamicin, linezolid, rifampin, synercid, trimethoprimsulfamethoxazole, and vancomycin. Most CA-MRSA isolates (94%) were susceptible to tetracycline. Sixty-seven percent were susceptible to ciprofloxacin and 69% were susceptible to clindamycin including 14 isolates demonstrating inducible clindamycin resistance. Conversely, only 26% of isolates were susceptible to erythromycin.

MDH has also received reports of serious illness and death due to community-associated methicillin-susceptible S. aureus (CA-MSSA) infection and is interested in receiving reports of all serious illnesses or deaths due to S. aureus infection, regardless of susceptibility to methicillin.

• For up to date information see: Staphylococcus aureus
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2003