Cellulitis

Pathogenesis: Bacteria gain access to the epidermis through breaks in the skin, whether accidental (e.g., cuts, scratches, burns) or iatrogenic (e.g., surgical incisions, IV catheters). The expanding area of erythema may be due to extracellular toxins and/or the host immune response rather than to increasing bacterial numbers.
Microbiology: Etiologic causes include commensal flora (e.g., S. aureus, S. pyogenes) or a wide variety of exogenous flora. With the latter, a thorough history and epidemiologic data may help identify the cause.
- - Examples of exogenous bacteria causing cellulitis include the following: Pasteurella multocida after a cat or dog bite; Capnocytophaga canimorsus after a dog bite; Eikenella corrodens after a human bite; P. aeruginosa in association with ecthyma gangrenosum in neutropenic pts, a penetrating injury (stepping on a nail), or hot-tub folliculitis; Aeromonas hydrophila after a laceration sustained in fresh water; or Erysipelothrix rhusiopathiae after contact with domestic swine and fish.
Clinical manifestations: This acute inflammatory condition of the skin is characterized by localized pain, erythema, swelling, and heat.
- - Cellulitis due to S. aureus often spreads from a central site of localized infection, such as an abscess or an infected foreign body, and is referred to as “purulent cellulitis.”
- - S. pyogenes can cause nonpurulent cellulitis, a rapidly spreading, diffuse process that often occurs with fever and lymphangitis.
Diagnosis: If there is drainage, an open wound, or an obvious portal of entry, Gram's staining and culture may identify the etiology. Aspiration or biopsy of the leading edge of the cellulitic tissue yields a diagnosis in only 20% of cases.
Treatment: See Table 84-1.

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