High Prevalence of Methicillin-Resistant Staphylococcus aureus in Emergency Department Skin and Soft Tissue Infections

Study objective

We sought to determine the prevalence of methicillin-resistant Staphylococcus aureus (MRSA) among emergency department (ED) patients with skin and soft tissue infections, identify demographic and clinical variables associated with MRSA, and characterize MRSA by antimicrobial susceptibility and genotype.

Methods

This was a prospective observational study involving a convenience sample of patients who presented with skin and soft tissue infections to a single urban public hospital ED in California. Nares and infection site cultures were obtained. A health and lifestyle questionnaire was administered, and predictor variables independently associated with MRSA were determined by multivariate logistic regression. All S aureus isolates underwent antibiotic susceptibility testing. Eighty-five MRSA isolates underwent genotyping by pulsed field gel electrophoresis, staphylococcal chromosomal cassette mec (SCCmec) typing, and testing for Panton-Valentine leukocidin genes.

Results

Of 137 subjects, 18% were homeless, 28% injected illicit drugs, 63% presented with a deep or superficial abscess, and 26% required admission for the infection. MRSA was present in 51% of infection site cultures. Of 119 S aureus isolates (from infection site and nares), 89 (75%) were MRSA. Antimicrobial susceptibility among MRSA isolates was trimethoprim/sulfamethoxazole 100%, clindamycin 94%, tetracycline 86%, and levofloxacin 57%. Among predictor variables independently associated with MRSA infection, the strongest was infection type being furuncle (odds ratio 28.6). Seventy-six percent of MRSA cases fit the clinical definition of community associated. Ninety-nine percent of MRSA isolates possessed the SCCmec IV allele (typical of community-associated MRSA), 94.1% possessed Panton-Valentine leukocidin genes, and 87.1% belonged to a single clonal group (ST8:S).

Conclusion

In this urban ED population, MRSA is a major pathogen in skin and soft tissue infections. Although studies from other practice settings are needed, MRSA should be considered when empiric antibiotic therapy is selected for such infections.

Introduction

Methicillin-resistant Staphylococcus aureus (MRSA) has been recognized as a nosocomial pathogen since the 1960s. Cases of MRSA infection presenting from the community were first described in the early 1980s. However, until recently, the majority of such cases were associated with known risk factors for MRSA, particularly recent contact with a health care facility.¹ From an epidemiologic perspective, such cases were believed to represent temporary circulation into the community of a nosocomial strain.

In the mid 1990s, reports began to appear in the United States, particularly among children, of community-associated MRSA infection, defined as occurring in patients without identifiable risk factors.2, 3 Although most were relatively minor skin and soft tissue infections, deaths were soon described after treatment failure with β-lactam antibiotics.⁴

Recent studies, involving genetic typing and antimicrobial susceptibility testing of community-associated MRSA, seem to confirm that strains are spread within the community and are genetically and phenotypically distinct from hospital-associated MRSA.5, 6 The mecA gene codes for S aureus methicillin resistance. In community-associated MRSA, mecA is carried within a genetic element called staphylococcal chromosomal cassette mec (SCCmec) type IV, which is distinct from the SCCmec (types I, II, and III) typically found in hospital-associated MRSA. Compared with hospital-associated MRSA, community-associated MRSA tends to be susceptible to a broader array of antibiotics. Furthermore, community-associated MRSA may be more virulent than typical hospital-associated MRSA. Many community-associated MRSA strains have been found to carry genes for Panton-Valentine leukocidin, a virulence factor that is associated with skin and soft tissue infections and necrotizing pneumonia.5, 6, 7, 8

Despite the well-documented emergence of community-associated MRSA in the United States, evidence about its importance in emergency department (ED) patients remains indirect. The prevalence of MRSA colonization in the community has ranged from 0.2% to 2.8% in recent US studies,⁹ with the highest rates seen among the poor and in urban populations where injection drug use is common.10, 11 Most recent reports of community-associated MRSA infection have concerned clustered outbreaks, such as within isolated Native American communities, within prisons, and among athletes who share equipment.12, 13, 14, 15, 16 In these settings, as much as 55% to 80% of S aureus infections were due to community-associated MRSA. In a study of all S aureus isolates at 12 medical center laboratories in Minnesota during 2000, 12% were community-associated MRSA, and 75% of community-associated MRSA isolates were from skin and soft tissue infections.⁶ How many of these were ED cases was not reported. There has been no published study that prospectively examines the importance of community-associated MRSA in the ED setting.

We sought to investigate the prevalence of MRSA colonization and infection in ED patients with community-acquired skin and soft tissue infections and to identify demographic and clinical variables associated with MRSA in our population. Antimicrobial susceptibility testing and genotyping were performed to assess the relatedness among MRSA strains and determine whether they were likely health care or community associated.