Methicillin-Resistant Staphylococcus aureus (MRSA), 2002

Strains of Staphylococcus aureus that are resistant to methicillin and all betalactam 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 MRSA infections. 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.

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

MDH initiated active surveillance for CA-MRSA at 12 sentinel hospital laboratories in January 2000. 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. Sentinel sites report all cases of MRSA identified at their facilities and send all CA-MRSA isolates to MDH. The purpose of this surveillance system 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 antibiotic susceptibility patterns and molecular subtypes of CA-MRSA isolates.

Exclusion criteria for cases of CAMRSA include: a positive culture for MRSA from a specimen obtained more than 48 hours after admission to a hospital (if admitted); prior MRSA infection or colonization; and hospitalization, surgery, residence in a longterm care facility, hemodialysis, peritoneal dialysis, or indwelling percutaneous devices or catheters within 1 year prior to the positive MRSA culture.

During 2002, 1,649 cases of MRSA infection were reported by sentinel sites. Thirteen percent of these cases were classified as CA-MRSA, 86% were classified as health care-associated MRSA (HA-MRSA), and 1% could not be classified. The MDH Public Health Laboratory received CA-MRSA isolates from 200 (90%) of the 223 CAMRSA cases; to date, antimicrobial susceptibility testing and molecular subtyping by PFGE has been completed for 122 (61%) of these isolates. CA-MRSA patients were, on average, younger than patients with HA-MRSA (median age, 31 years vs. 64 years) and more likely to have MRSA isolated from the skin (59% vs. 18%). Most CA-MRSA isolates belonged to one particular PFGE clonal group that is distinct from the clonal group most common among HA-MRSA isolates.

All CA-MRSA isolates submitted in 2002 were susceptible to rifampin and vancomycin. Most CA-MRSA isolates were susceptible to trimethoprimsulfamethoxazole (99%), gentamicin (98%), tetracycline (89%), clindamycin (88%), and ciprofloxacin (80%). Conversely, only 39% of isolates were susceptible to erythromycin. Drug susceptibility data reported from retrospective studies (1996-1998) and sentinel surveillance (2000-2001) have demonstrated a significant decrease in the percentage of CA-MRSA isolates that are susceptible to ciprofloxacin, clindamycin, and erythromycin. Additionally, inducible clindamycin resistance has been demonstrated in 47 (85%) of 55 erythromycin-resistant/ clindamycin-sensitive isolates from 2000-2001.

In 2002, MDH received reports of two cases of fatal necrotizing CA-MRSA pneumonia in young, previously healthy persons. MDH 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, 2002