Methicillin-Resistant Staphylococcus aureus (MRSA), 2000
Strains of Staphylococcus aureus that are resistant to methicillin (and possibly other antibiotics) are referred to as methicillin-resistant Staphylococcus aureus (MRSA). Such strains were first recognized in the United States in the late 1960s. Established risk factors for MRSA include recent hospitalization or surgery and residence in a long-term care facility.
In 1997, MDH began receiving unsolicited reports from several health care facilities in Minnesota describing increasing numbers of healthy young patients presenting with infections caused by MRSA. Many of these patients appeared to have none of the established risk factors for MRSA infection. Although most of the reported infections were not severe, some resulted in hospitalization or death. A review of the scientific literature indicated that MRSA infection among persons with no apparent risk factors was unusual. The infections being reported in Minnesota appeared to have been acquired in the community rather than in the health care setting, thereby suggesting a possible change in the epidemiology of MRSA. The phenomenon of community-acquired MRSA (CA-MRSA) also was being reported elsewhere in the U.S. and the world. An article by MDH authors and others reported the deaths of four children from Minnesota and North Dakota due to CA-MRSA [MMWR 1999;48(32):707-710].
In an effort to determine the incidence of CA-MRSA in Minnesota, MDH initiated active surveillance in January 2000 at 12 sentinel hospitals statewide. The hospitals (six in the seven-county metropolitan area and six in greater Minnesota) were selected to represent various geographic regions of Minnesota. These facilities were asked to submit case reports and MRSA isolates to MDH for all cases of MRSA (both community-acquired and health care-associated) identified at their facilities during 2000. The purpose of this surveillance system included monitoring the incidence of CA-MRSA infections in Minnesota, identifying possible risk factors for CA-MRSA, and identifying the antibiotic susceptibility patterns and genetic subtypes of MRSA isolates submitted to MDH.
To meet the provisional diagnosis of CA-MRSA as defined for the MDH surveillance system, a patient must not have had: a positive culture for MRSA from any specimen obtained more than 48 hours after admission to a hospital (if admitted); history of MRSA infection or colonization; or, hospitalization, surgery, residency in a long-term care facility, hemodialysis or peritoneal dialysis, or indwelling percutaneous devices or catheters within the past year.
During 2000, 1,164 cases of MRSA infection were reported by the 12 sentinel hospitals. Eleven percent of these cases were determined to be community-acquired, 85% were health care-associated, and 3% were of unknown origin. The MDH Public Health Laboratory received MRSA isolates from 932 (80%) cases and completed genetic subtyping on 338 (29%) of these isolates. CA-MRSA patients were, on average, younger than patients with health care-associated MRSA (23 years vs. 68 years, respectively) and more likely to have MRSA isolated from a skin site (74% vs. 40%, respectively). In addition, very few of the CA-MRSA patients were health care workers or had household members who were health care workers. The data also confirmed that most CA-MRSA isolates had genetic subtype and antibiotic susceptibility patterns that were distinct from health care-associated MRSA isolates. CA-MRSA isolates typically belonged to a particular clonal group and generally were susceptible to all of the following drugs: ciprofloxacin, clindamycin, gentamicin, tetracycline, and trimethoprim-sulfamethoxazole. These findings support the theory that CA-MRSA bacteria have not "escaped" from the healthcare setting, but rather have evolved independently and represent a change in the epidemiology of MRSA.