Friday, November 25, 2005

The emerging threat of community acquired methicillin resistant Staphylococcus aureus

Methicillin resistant Staphylococcus aureus (MRSA) has been well known for decades as a nosocomial pathogen. More recently MRSA has been increasingly reported in the community. Community acquired MRSA (CA-MRSA) tends to present as a new strain with features which distinguish it from the more familiar nosocomial MRSA. I found this helpful review in September’s Mayo Clinic Proceedings. (The abstract is linked here. Full text open access will be available in March 2006).

Although some community isolates appear to have escaped from hospitals, the new strains (“true” CA-MRSA) arise de novo in the community. Here are some important distinctions pointed out in the review:

1) CA-MRSA has a unique genetic determinant of resistance---the type IV Staphylococcal cassette cartridge (SCC).
2) Unlike the “old” MRSA which has multiple drug resistance, CA-MRSA is typically sensitive to many non beta lactam antibiotics. Sensitivity patterns have regional variation and tend to show a typical pattern in a given region, which may be the clinician’s principal clue that CA-MRSA rather than the older strain is present.
3) CA-MRSA appears to be more virulent than other S. aureus strains, in part due to expression of the Panton-Valentine leukocidin.
4) CA-MRSA appears to have increased transmissibility and infectivity compared to other S. aureus strains.
5) Skin infections, soft tissue infections and necrotizing pneumonia are characteristic compared to other strains, and necrotizing fasciitis has been reported. Bacteremia and endocarditis are less characteristic.

Perhaps the most troubling aspect of CA-MRSA is its emergence as a cause of community acquired pneumonia. The authors state: “It is now prudent to consider CA-MRSA as an etiology of severe CAP in the correct clinical context. Severe necrotizing pneumonia with or without hemoptysis after an influenzalike illness in high-risk patients warrants therapy directed against MRSA.” The antibiotics of choice for pneumonia are vancomycin and linezolid.

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