Methicillin-Resistant Staphylococcus aureus (MRSA), 2004

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 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 (if patient admitted); 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 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-resistent Staphylococcus aureus in Minnesota. JAMA. 2003;290(22):2976-84). In a recent study comparing the results from three different states conducting MRSA surveillance, 12% of all MRSA reported in Minnesota from 2001-2003 were CA-MRSA compared to 20% of cases in Atlanta, Georgia and 8% of all cases in Baltimore, Maryland. Additionally, this study found that in Atlanta and Baltimore, children less than 2 were overrepresented among CA-MRSA cases (population based surveillance was not conducted in Minnesota). (Fridkin, S., et al. Methicillin-resistant Staphylococcus aureus disease in three communities. N Engl J Med. 2005;352(14):1436-44).

In 2004, 2,411 cases of MRSA infection were reported. Twenty-one percent of these cases were classified as CA-MRSA; 77% were classified as HA-MRSA, and <2% could not be classified. Isolates were received from 452 (89%) of the 508 CA-MRSA cases. To date, antimicrobial susceptibility testing has been completed on 155 (34%) and molecular subtyping by PFGE has been completed for 104 (23%) of these isolates. CA-MRSA patients continue to be younger than patients with HA-MRSA (median age, 33 years vs. 61 years) and more likely to have MRSA isolated from the skin (80% vs. 31%). Most CA-MRSA isolates belonged to one particular PFGE clonal group that is distinct from the most common HA-MRSA clonal group.

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. However, all 2004 CA-MRSA isolates tested to date have been susceptible to gentamicin, linezolid, synercid, trimethoprim-sulfamethoxazole, and vancomycin. Most CA-MRSA isolates (95%) were susceptible to tetracycline and rifampin (99%). Sixty-eight percent were susceptible to ciprofloxacin and 84% were susceptible to clindamycin Conversely, only 24% of CA-MRSA isolates were susceptible to erythromycin. Eighteen percent (17/93) of erythromycin-resistant, clindamycin susceptible isolates demonstrated inducible clindamycin resistance using the D test.

MDH also has received reports of serious illness and death due to community-associated methicillin-susceptible S. aureus infection. Critical illnesses or deaths due to community-associated S. aureus infection, regardless of susceptibility to methicillin, is now reportable in Minnesota.

• For up to date information see: Staphylococcus aureus
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2004