Context Methicillin-resistant Staphylococcus aureus (MRSA)
has traditionally been considered a health care–associated pathogen
in patients with established risk factors. However, MRSA has emerged in patients
without established risk factors (community-associated MRSA).
Objective To characterize epidemiological and microbiological characteristics
of community-associated MRSA cases compared with health care–associated
Design, Setting, and Patients Prospective cohort study of patients with MRSA infection identified
at 12 Minnesota laboratory facilities from January 1 through December 31,
2000, comparing community-associated (median age, 23 years) with health care–associated
(median age, 68 years) MRSA cases.
Main Outcome Measures Clinical infections associated with either community-associated or health
care–associated MRSA, microbiological characteristics of the MRSA isolates
including susceptibility testing, pulsed-field gel electrophoresis, and staphylococcal
exotoxin gene testing.
Results Of 1100 MRSA infections, 131 (12%) were community-associated and 937
(85%) were health care–associated; 32 (3%) could not be classified due
to lack of information. Skin and soft tissue infections were more common among
community-associated cases (75%) than among health care–associated cases
(37%) (odds ratio [OR], 4.25; 95% confidence interval [CI], 2.97-5.90). Although
community-associated MRSA isolates were more likely to be susceptible to 4
antimicrobial classes (adjusted OR, 2.44; 95% CI, 1.35-3.86), most community-associated
infections were initially treated with antimicrobials to which the isolate
was nonsusceptible. Community-associated isolates were also more likely to
belong to 1 of 2 pulsed-field gel electrophoresis clonal groups in both univariate
and multivariate analysis. Community-associated isolates typically possessed
different exotoxin gene profiles (eg, Panton Valentine leukocidin genes) compared
with health care–associated isolates.
Conclusions Community-associated and health care–associated MRSA cases differ
demographically and clinically, and their respective isolates are microbiologically
distinct. This suggests that most community-associated MRSA strains did not
originate in health care settings, and that their microbiological features
may have contributed to their emergence in the community. Clinicians should
be aware that therapy with β-lactam antimicrobials can no longer be relied
on as the sole empiric therapy for severely ill outpatients whose infections
may be staphylococcal in origin.