The Graying of Methicillin-Resistant Staphylococcus aureus

Abstract

From Rush University Medical Center and Stroger Hospital of Cook County, Chicago, Illinois (both authors). Received August 12, 2008; accepted August 17, 2008; electronically published December 1, 2008. Infect Control Hosp Epidemiol 2008; 30:9-12 2008 by The Society for Healthcare Epidemiology of America. All rights reserved. 0899-823X/2009/3001-0003$15.00. DOI: 10.1086/592709 Not so long ago, methicillin-resistant Staphylococcus aureus (MRSA) was a quintessential nosocomial pathogen. Over the past few years, however, cases of infection with communityassociated MRSA (CA-MRSA) stricto sensu—that is, infections in patients with no healthcare exposures—have emerged. Most populations affected early appeared to share a characteristic of increased risk of disease spread by direct person-to-person contact (Figure). Since the emergence of CA-MRSA strains, several terms have been used to classify S. aureus strains on the basis of epidemiologic, genotypic, and phenotypic characteristics (Table). These classifications provide rough estimates of likely sources of strains and can be used for epidemiologic tracking. As more healthcare exposures occur among the general population, “healthcare-associated MRSA infection”—cases in which healthcare exposures or risk factors are present in patients whose MRSA infection manifests in the community— have gained attention. However, the lines between categories may be “graying,” with community-associated strains encroaching on hospitals, hospital-associated strains entering the community, and healthcare exposures increasing. The study by Johnson and colleagues in this issue of the journal evaluates MRSA colonization among hospitalized dialysis patients and among healthcare workers on an inpatient dialysis unit. As background, it is known that nasal colonization with S. aureus can be persistent, intermittent, or always absent; that it occurs more frequently in some populations, including hemodialysis patients; and that it usually precedes infection. Surveys of the general US population indicate that the prevalence of nasal colonization with MRSA rose from 0.8% in 2001–2002 to 1.5% in 2003–2004, while the prevalence of S. aureus colonization overall (ie, including both susceptible and resistant strains) decreased from 32.4% to 28.6%. On the basis of nasal culture results, Johnson et al. determined that 33% of dialysis patients in their study were colonized with S. aureus and 21.7% were colonized with MRSA—a 14-fold higher prevalence than national estimates. Johnson et al. identified isolates as being CA-MRSA on the basis of the presence of SCCmec IV genetic elements; colonization with such strains occurred in 38.5% of dialysis patients colonized with MRSA and in 8.3% of the total dialysis population. Among healthcare workers, 31% were colonized with S. aureus, 6% with MRSA, and 2% with CA-MRSA. In case-control analyses, there were few significant differences in demographic characteristics or risk factors between patients colonized with MRSA and patients colonized with methicillin-susceptible S. aureus (MSSA) or between patients colonized with MRSA strains that carried SCCmec II and patients colonized with strains that carried SCCmec IV. Because dialysis patients and healthcare workers were evaluated in different periods, inferences about patient-staff cross-transmission may be unreliable. What is the significance of the findings of Johnson et al.? The 6% prevalence of MRSA colonization among healthcare workers is consistent with the findings of recent studies that examined MRSA colonization among emergency-department