Skin and Soft Tissue Infections

Skin infections are among the most common problems treated by emergency physicians. In 2005 bacterial skin infections accounted for 3% of all emergency department (ED) visits in the United States, with more than 3,400,000 visits in that year alone. While treating many patients with mild skin infections, the emergency physician is vigilant for the rare occurrence of life threatening variants, such as necrotizing fasciitis. The epidemiology of skin infections has changed in the past decade with the emergence of a new organism, community-associated methicillinresistant Staphylococcus aureus (CA-MRSA). Unlike most streptococci, S. aureus has usually been resistant to penicillin since penicillin was first commercialized because of the production of a β-lactamase (also known as penicillinase) enzyme that cleaves the penicillin molecule. Penicillinase-resistant penicillins typified by methicillin were developed in 1959, but strains of S. aureus resistant to these agents (MRSA) were identified by 1960. Resistance is due to an altered bacterial target, the cell wall building enzyme known as penicillin-binding protein. A new strain was identified in Chicago in the mid-1990s. It was characterized by resistance to methicillin and related antimicrobials but susceptibility to agents ineffective against the older “nosocomial MRSA”; it could not be traced to the hospital environment. The DNA fingerprinting technique known as pulsed-field gel electrophoresis identified the predominant CA-MRSA strain as USA300. In contrast, the older nosocomial strain of MRSA is known as USA-100. A large, nationwide, ED-based study found CA-MRSA to be the leading identifiable cause of skin infections among ED patients. The number of ED visits for skin and soft tissue infections increased from 1.2 million in 1993 to more than 3.4 million in 2005; whether this was due to an increase in abscess, cellulitis, or both is unknown. Intriguingly, whereas CA-MRSA is now found in all populations of patients and in health care and community settings alike, the epidemic of skin infections it has caused is most apparent among ED patients for reasons that remain unclear. These data are summarized in Figure 137-1.