Methicillin-Resistant Staphylococcus aureus, 2001

Strains of Staphylococcus aureus that are resistant to methicillin and all beta-lactam antibiotics are referred to as methicillin-resistant Staphylococcus aureus (MRSA). Such strains first were recognized in the U.S. in the late 1960s. 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 from health care facilities in Minnesota describing increasing numbers of healthy young patients presenting with infections caused by MRSA. These patients had onset of their MRSA infections in the community and appeared to have none of the established risk factors for MRSA. Although most of the reported infections were not severe, some resulted in serious illness or death.1

In 1999, Minnesota Rules Governing Communicable Diseases were amended to require designated sentinel hospitals to report cases of MRSA. In addition, cases of community-onset MRSA (CO-MRSA) that cause serious illness or death were made reportable.

To be classified as a case of CO-MRSA, a patient must have no history of any of the following: a positive culture for MRSA obtained more than 48 hours after admission to a hospital (if admitted); prior MRSA infection or colonization; or hospitalization, surgery, residency in a long-term care facility, hemodialysis, peritoneal dialysis, or indwelling percutaneous devices or catheters within 1 year prior to the positive MRSA culture.

MDH initiated active surveillance for CO-MRSA in January 2000 at 12 sentinel hospital laboratories statewide. The laboratories (six in the seven-county Twin Cities metropolitan area and six in greater Minnesota) were selected to represent various geographic regions of the state. Currently, sentinel sites report all cases of MRSA identified at their facilities and send all CO-MRSA isolates to MDH. The purpose of this surveillance system is to monitor the incidence of CO-MRSA infections in Minnesota, to identify possible risk factors for CO-MRSA, and to identify the antibiotic susceptibility patterns and molecular subtypes of CO-MRSA isolates. Additionally, MDH has begun testing selected CO-MRSA isolates for inducible clindamycin resistance and possible S. aureus virulence factors, such as Panton-Valentine leukocidin.

During 2001, 1,374 cases of MRSA infection were reported by sentinel sites. Twelve percent of these cases were classified as CO-MRSA; 87% were classified as health care-associated MRSA (HA-MRSA), and 1% could not be classified. The MDH Public Health Laboratory received CO-MRSA isolates from 155 (92%) cases; to date, antimicrobial susceptibility testing and molecular subtyping by PFGE has been completed for 152 (98%) of these isolates. CO-MRSA patients were, on average, younger than patients with HA-MRSA (29 years vs. 61 years) and more likely to have MRSA isolated from the skin (73% vs. 28%). CO-MRSA isolates typically belonged to one particular PFGE clonal group that is distinct from the clonal group most common to HA-MRSA isolates.

All CO-MRSA isolates submitted in 2001 were susceptible to rifampin, trimethoprim-sulfamethoxazole, and vancomycin; 98% were susceptible to gentamicin; 94% were susceptible to tetracycline; 83% were susceptible to clindamycin; 76% were susceptible to ciprofloxacin, and 43% were susceptible to erythromycin. Drug susceptibility data reported from retrospective studies (1996-1998)2 and sentinel surveillance (2000-2001) have demonstrated a significant decrease in CO-MRSA susceptibility to ciprofloxacin, clindamycin, and erythromycin over time. Additionally, inducible clindamycin resistance has been demonstrated in many erythromycin-resistant/clindamycin-sensitive isolates.

In 2001, MDH received two reports of fatal necrotizing CO-MRSA pneumonia in young, previously healthy persons. MDH is interested in receiving all reports of necrotizing pneumonia due to S. aureus (methicillin-sensitive or-resistant).

References:
1. Centers for Disease Control and Prevention. Four pediatric deaths from community-acquired methicillin-resistant Staphylococcus aureus: Minnesota and North Dakota, 1997-1999. MMWR 1999; 48:707-10.
2. Naimi TS, LeDell KH, Boxrud DJ, et al. Epidemiology and clonality of community-acquired methicillin-resistant Staphylococcus aureus in Minnesota, 1996-1998. Clin Infect Dis 2001; 33:990-6.

• For up to date information see: Staphylococcus aureus
• Full issue>> Annual Summary of Communicable Diseases Reported to the Minnesota Department of Health, 2001