MRSA: More Common Than You Think
MRSA: More Common Than You Think
Abstract & Commentary
By Donna Woods, DO, Southwest Arizona Regional Medical Director, NextCare Urgent Care, Tucson, AZ, is Associate Editor for Urgent Care Alert.
Dr. Woods reports no financial relationships relevant to this field of study.
Synopsis: Methicillin-resistant Staphylococcus aureus (MRSA) is common in patients presenting to the emergency department with cellulitis or abscess. All MRSA isolates were sensitive to rifampin and bactrim.
Source: Moran G, et al. Methicillin-resistant S. aureus infections among patients in the emergency department. N Engl J Med. 2006:355:666-674.
This prospective study on the prevalence of CA-MRSA (community-acquired methicillin-resistant Staphylococcus aureus) in skin infections examined in cultures from 422 patients treated in university-affiliated emergency departments in 11 US cities. Patients were at least 18 years old and had a diagnosis of abscess, wound infection, or cellulitis, with purulent drainage for less than one week duration. Swabs were obtained and sent to hospital laboratories for culture and sensitivities, and then forwarded to the CDC for further genetic characterization. Specifically, genes for the production of the Panton-Valentine Leukocidin (PVL), (a cytotoxin that may serve as a virulence factor and is associated with CA-MRSA abscesses and necrotizing pneumonia) and SCCmec (a gene complex that confers methicillin resistance) were examined.
S. aureus was isolated in 76% (320/422) of patients, and MRSA was isolated in 59% (249/422) of patients. When analyzed by each individual hospital, the prevalence of MRSA ranged from 15%-74%. MRSA was found to be the most common infecting organism in 10 of 11 hospitals. Other types of bacteria isolated included methicillin-susceptible S. aureus (MSSA) (17%), coag-negative staphylococcus (3%), streptococcus (7%), and Proteus mirabilis (1%). Polymicrobial infections were identified in 31 patients, 10 of which contained MRSA. No organism was identified in 38 patients.
All MRSA isolates in this study were sensitive to bactrim and rifampin. Most strains showed in-vitro sensitivity to clindamycin (95%), 92% were sensitive to tetracyclines, 60% were sensitive to fluoroquinolones, and 7% were sensitive to erythromycin.
Type IV SCCmec (which is characteristic of CA-MRSA) and genes for PVL were isolated in 98% of MRSA samples.
Using multivariate logistic-regression analysis, the following characteristics were associated with an increased risk of having MRSA: taking an antibiotic in the past month, having an abscess, reporting a spider bite, having a history of MRSA infection, and close contact with a person with similar infection. Having an underlying illness and racial classification of 'other' were actually associated with a decreased risk of having MRSA in this study, compared with other bacteria.
Complete treatment information was available in 96% of the patients. An incision and drainage (I+D) and antibiotic were administered to 66% of patients; 10% received antibiotics alone, 19% received I+D alone, and 5% received neither I+D nor antibiotic therapy.
In 57% (100/175) of patients who were treated with an antibiotic, the prescribed therapy was not in concordance with culture and sensitivity results. Despite this finding, no significant differences in outcome were found at a follow-up of 15-21 days between patients with MRSA and patients with other types of bacteremia. Case-finding audits revealed that only 42% of eligible patients were enrolled. Unenrolled patients were found to have similar characteristics (eg, age, sex, and ethnicity) to those enrolled, and MRSA was isolated in 57% of unenrolled patients.
Commentary
It is not surprising that this study demonstrates that the incidence of CA-MRSA is rising. Prior to 2000, reports of CA-MRSA were rare. Only 3% of cultures from skin and soft-tissue infections submitted to laboratories in Minnesota contained MRSA.1 Numerous studies during the past 6 years have demonstrated an increase in MRSA.2-4
What is striking about these results is that CA-MRSA was the most common organism isolated. These results were not associated with independent outbreaks, and the results were fairly consistent at 10 of 11 sites. This information is compelling because the use of empiric antibiotics in this setting should be directed at covering 'the most common bug.' Often, an anti-staphylococcal β-lactam is prescribed as a first-line treatment, but this study suggests that this may not be the most appropriate choice of therapy.
Although the incidence of complications from I+D is low, and complete resolution of an abscess is usually possible with I+D alone,5-8 there are many instances where the standard of care suggests empiric antibiotics should be prescribed. Tintinalli's Emergency Medicine textbook suggests that "in patients with diabetes, alcoholism, or other underlying immunocompromising illnesses, or in those on immunosuppressant medications such as steroids or chemotherapeutics, the threshold of antibiotic use should be much lower. Furthermore, patients who present with signs of systemic disease such as fever and chills and those with cellulitis extending beyond the abscess borders also should be considered for antibiotic therapy."9 This text also suggests that infections on the hands and face be treated more aggressively due to their higher morbidity rate.9
Bacteremia associated with I+D of an abscess is also controversial. One study showed bacteremia in 11 of 30 patients who underwent I+D of abscesses.10 In another study, blood cultures were obtained at 2 minutes and 10 minutes after an I+D, and none of them were positive.11 Ultimately, the American Heart Association recommends treating patients with structural heart disease with endocarditis prophylaxis. Such prophylaxis in the past usually consisted of an anti-staph β-lactam (or vancomycin if a known MRSA infection). Perhaps the initial choice of antibiotics in this setting also should be revisited.
Finding a balance between evidence-based medicine and the standard of care in this arena remains a challenge. Due to the multitude of factors that affect these decisions, and the lack of clear guidelines on antibiotic use in this setting, clinical judgment is relied on heavily. When an antibiotic is considered — for either treatment of an abscess or endocarditis prophylaxis for an I+D — anti-staph β-lactams should not be the first line of therapy because they would not be active against what seems to be the 'most common bug' in this setting.
References
- Naimi TS, et al. Epidemiology and clonality of community-acquired methicillin-resistant Staphylococcus aureus in Minnesota, 1996-1998. Clin Infect Dis. 2001;33:990-996.
- Moran GJ, et al. Methicillin-resistant Staphylococcus aureus in community-acquired skin infections. Emerg Infect Dis. 2005;11:928-930.
- King MD, et al. Emergence of community-acquired methicillin-resistant Staphylococcus aureus USA-300 clone as the predominant cause of skin and soft-tissue infections. Ann Intern Med. 2006;144:309.
- Sattler CA, et al. Prospective comparison of risk factors and demographic and clinical characteristics of community-acquired methicillin-resistant versus methicillin-susceptible Staphylococcus aureus infection in children. Pediatr Infect Dis. J 2002;21:910-917.
- Meislin HW, et al. Cutaneous abscesses. Anaerobic and aerobic bacteriology and outpatient management. Ann Intern Med. 1977;87:145-147.
- Llera JL, et al. Treatment of cutaneous abscess: A double-blind clinical study. Ann Emerg Med. 1985; 14:15-19.
- Macfie J, et al. The treatment of acute superficial abscesses: A prospective clinical trial. Br J Surg. 1977;64:264-266.
- Lee MC, et al. Management and outcome of children with skin and soft tissue abscesses caused by community-acquired methicillin-resistant Staphylococcus aureus. Pediatr Infect Dis J. 2004;23:123-127.
- Tintinalli JE, et al. Emergency Medicine, A Comprehensive Study Guide. 6th ed. New York: McGraw-Hill;2004:985-986.
- Fine BC, et al. Incision and drainage of soft-tissue abscesses and bacteremia [letter]. Ann Intern Med. 1985;103:645.
- Bobrow BJ, et al. Incision and drainage of cutaneous abscesses is not associated with bacteremia in afebrile adults. Ann Emerg Med. 1997;29:404-408.
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