High prevalence of Methicillin-Resistant Staphylococcus aureus

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by Evan Schwarz
It’s a normal night in the department and you’ve just left the room of a patient that appears to have cellulitis.  You are about to write your normal prescription for keflex and your mind wanders back to the three abscesses that you drained earlier in the shift.  You were sure that those abscesses were caused by MRSA and wonder if this cellulitis could also be the result of MRSA.  However as you don’t know the prevalence of MRSA in soft tissue infections, you do not know if you should consider changing your practice concerning antibiotic usage in soft tissue infections.

Review by Evan Schwarz
Column arranged by Evan Schwarz
Division of Emergency Medicine
Washington University in St. Louis
Frazee BW, LynnJ, Charlebois ED, et al.  High prevalence of Methicillin-resistant Staphylococcus aureus in emergency department skin and soft tissue infections.  Ann Emerg Med. 2005; 45: 311-320.
In this study a convenience sample consisting of 137 patients presenting to a northern California ED was prospectively enrolled.  Any patient with cellulitis, wound infection, ulcer, septic bursitis, an abscess, or a necrotizing soft tissue infection was eligible.  Patients with odontogenic infections and Bartholin gland abscess were excluded as were children.  A nasal swab was taken from all patients (to detect MRSA colonization) as well as a culture from the wound site if possible.  MRSA is also known to colonize the axilla or groin but these areas were not cultured.  Organisms that grew were then identified and tested for antimicrobial resistance.
Nearly 75% of all Staphylococcus aureus isolates were MRSA.  Overall 50% of the enrolled patients had a culture that grew MRSA.  While many of the nasal swabs were negative for Staph (71%), 40 samples were positive with 28 (70%) being MRSA.  Also 31 subjects had Staph identified on both their nasal swab and wound infection.  Of these 26 (84%) were concordant for methicillin susceptibility.  Streptococcal species and Proteus species were also commonly isolated from wound cultures.  All Staph isolates were susceptible to vancomycin and bactrim.  However for the MRSA isolates, oxacillin (or its equivalent) were completely ineffective just as would be a first generation cephalosporin such as keflex.
Antibiotic MSSA (N=30) MRSA (N=89)
Vancomycin 100 100
Bactrim 100 100
Oxacillin 93
100 94
Tetracycline 96 85
Levofloxacin 97 57
Erythromycin 58 4
Demographic data was also collected from all the patients that were enrolled.  Surprisingly, white race and furuncles were the only characteristics associated with MRSA colonization and infection.  Other risk factors such as homelessness and a history of multiple abscesses were only associated with colonization.  Other studies did not find the same demographic data as presented by these authors. 
While concerning because of the large amount of Staphylococcal isolates, this data may not be generalizable as all participants came from a single population.  Most importantly it may be difficult to use the demographic results to try to predict which patients are at the highest risk for MRSA infections as the results may not have external validity.   Also since only patients with soft tissue infection had nasal swabs done, the actual MRSA colonization rate is unknown since patients with colonization and without infection would not have been enrolled.  However if the prevalence of MRSA is found to be this high in future studies, the authors conclude that there may be very little utility in identifying predictor variables.   Of note, no follow up was done concerning antibiotic efficacy.  Of the patients that received antibiotics in this study, seventy-eight percent were given an ineffective (BETA)-lactam! 
Since these patients were not followed, it is not known if their infections got worse as they were on an inadequate antibiotic regimen or if there was a difference between in vitro and in vivo antibiotic susceptibilities.  Also of note in this study is that more than 98% of MRSA isolates possessed the SCCmecIV allele on genetic testing.  This allele is considered a marker for community acquired MRSA.  Also 94% of the MRSA isolates tested positive for the Panton-Valentine leukocidin gene that is also associated with community acquired MRSA.  This gene is also associated with spontaneous soft tissue infections.
In summary these authors find community-acquired MRSA to be a highly prevalent pathogen in skin and soft tissue infections.  They also question the usage of predictors to identify MRSA infections as the prevalence is so high.  When starting empiric therapy, the authors recommend antibiotics that cover community-acquired MRSA as well as Streptococcus pyogenes:  triimethoprim/sulfamethoxazole + cephalexin for moderate to severe skin infections or doxycycline (non-pregnant adults) or clindamycin (pediatrics) for minor skin infections. 


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