Further Developments in Community-Associated MRSA
Abstract & Commentary
Commentary by Stephanie Abbuhl, MD, FACEP, Medical Director, Department of Emergency Medicine, The Hospital of the University of Pennsylvania; Associate Professor of Emergency Medicine, University of Pennsylvania School of Medicine, Philadelphia, PA. Dr. Abbuhl is on the Editorial Board of Emergency Medicine Alert.
Source: Frazee BW et al. High prevalence of methicillin-resistant Staphylococcus aureus in emergency department skin and soft tissue infections. Ann Emerg Med 2005;45:311-320.
Although community-associated methicillin-resistant Staphylococcus aureus (MRSA) has been described since the mid 1990s, this is the first study to look prospectively at the prevalence of MRSA among emergency department (ED) patients with skin and soft-tissue infections. This observational study involved a convenience sample of 137 patients at a single urban ED who presented with a chief complaint of a skin or soft-tissue infection. Nares and infection site cultures were obtained, except in cases of pure cellulitis where no site culture was obtained. A health history and lifestyle questionnaire was administered to all subjects and clinical data were taken from the patient’s medical record; outcome information was not collected. A multivariate logistic regression analysis was done to determine predictor variables independently associated with MRSA. All S. aureus isolates were tested for antibiotic susceptibility, and the majority of isolates underwent various genotyping and chromosomal testing.
MRSA was isolated from 51.3% (95%CI, 42.3% to 60.2%) of infection site cultures. Of the 119 total S. aureus isolates (from infection site and nares), 75% were MRSA. The majority (71%) of nares cultures were negative for S. aureus, however, of the 40 nares S aureus isolates, 28 (70.0%; 95% CI 55.8% to 84.2%) were MRSA. Antimicrobial susceptibility among MRSA isolates was trimethoprim/sulfamethoxazole 100%, clindamycin 94%, tetracycline 86%, and levofloxacin 57%. 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.
Among predictor variables independently associated with MRSA infection, the strongest was infection type being furuncle (odds ratio 28.6). Of the MRSA cases, 76% fit the clinical definition of community-associated, and all but one MRSA isolate possessed the SCCmec IV allele, a genetic marker of community acquisition.
Commentary
This important study makes clear that, at least in this particular urban ED population, MRSA is a surprisingly common cause of skin and soft-tissue infections. The first question raised is: Can these results can be generalized to other ED populations? Given that other studies have documented an increasing incidence of community-associated MRSA across the United States from various sources and sites, it is quite likely that MRSA may be replacing methicillin-susceptible S. aureus as the typical community staphylococcal strain.
Further research is needed to determine the prevalence of MRSA in other geographic locations and in patients with different demographic profiles.
The second question that this study raises is one that won’t wait for further research: What empiric antibiotic regimen should emergency physicians use now that MRSA is a significant possibility? While this study did not assess outcomes, the authors do mention that among the 47 documented patients with MRSA infections receiving antibiotics, an ineffective beta-lactam—such as cephalexin—was prescribed in 79%. The findings have led to a major change in the initial empiric antibiotic choice in the study ED. For most cases, the authors have recommended a combination that includes covering for both community-associated MRSA with trimethoprim/sulfamethoxazole, and Streptococcus pyogenes with cephalexin.
Several key points are discussed in the article’s accompanying editorial.1 First, most skin abscesses can be cured with adequate incision and drainage. Second, in areas of high prevalence, empiric therapy for community-associated MRSA with a combination of trimethoprim/sulfamethoxazole and rifampin should be considered. The rifampin may be of benefit because of improved eradication of nasal MRSA carriage, but the magnitude of benefit gained from the addition of rifampin is unclear at this point. Oral clindamycin is a reasonable option to the combination of trimethoprim/sulfamethoxazole and rifampin. Most MRSA isolates are resistant to macrolides and quinolones, and many are resistant to tetracyclines. Third, culturing skin and soft-tissue infections is now more important than previously.
This not only provides information about the local prevalence of MRSA, but also ensures that patients who do not improve can be switched to an antibiotic with known in vitro activity. And finally, generalization of these data to the treatment of cellulitis is complex; S. pyogenes is believed to be a more common pathogen in cellulitis, and cephalexin works well for it, however, S. pyogenes is often resistant to trimethoprim/sulfamethoxazole. Once again, clindamycin therapy is a single-agent alternative to a combination of trimethoprim/sulfamethoxazole and cephalexin in these cases.
Finally, the editorialists make the excellent point that in hospitals where community-associated MRSA prevalence is high, the effect of contact isolation for known MRSA patients becomes less clear. Further research will help define the extent of community-associated MRSA, the patterns of antimicrobial susceptibility, the role of eliminating nasal colonization and the role of infection control practices on MRSA transmission. Multiple issues come into play with skin and soft-tissue infections in the community-associated MRSA era: If it is an abscess, is an antibiotic necessary? Which antibiotic(s) should be used? Wound culture is once again important; and close follow-up is more important than ever.
References
1. Moran GJ, et al. Community-associated methicillin-resistant Staphylococcus aureus: Is it in your community and should it change practice? Ann Emerg Med 2005;45:321-322.
Although community-associated methicillin-resistant Staphylococcus aureus (MRSA) has been described since the mid 1990s, this is the first study to look prospectively at the prevalence of MRSA among emergency department patients with skin and soft-tissue infections.
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