A 42-year-old man with morbid obesity, diabetes mellitus, congestive heart failure, and venous insufficiency presented with fever, pain, and swelling of his left lower extremity (LLE). He reported trivial trauma to his LLE after bumping into a table 2 weeks prior; the affected area progressed from mild redness to an open ulcer at the ankle. It eventually developed increased redness, warmth, and pain extending from the left ankle to the knee. At admission, he had an open ulcer with purulent drainage along with excoriation of the superficial layer of the skin (Figure 67-1). Given the purulent nature of the cellulitis and concern for methicillin-resistant Staphylococcus aureus (MRSA), he was started on intravenous vancomycin and received appropriate wound care. After initial improvement, he was switched to oral clindamycin to complete a total of 10 days of therapy. On a 2-week follow-up visit, his cellulitis had resolved.

Cellulitis is a rapidly spreading infection of the skin involving the deeper dermis and the subcutaneous tissue.^1,2 It extends deeper than erysipelas,³ which is in the differential diagnosis.

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  A common infection seen by both hospital-based and primary care physicians.⁴

  
  
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  Contributes to more than 600,000 hospitalizations each year.⁴

  
  
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  Annual office visits for cellulitis and cutaneous abscess increased from 4.6 million to 9.6 million in 2005.⁵

  
  
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  A lesion with exudate and purulent drainage, without an underlying drainable abscess, is defined as purulent cellulitis; it is predominantly due to S aureus.^4,6

  
  
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  Lesions without exudate, purulent drainage, or an underlying drainable abscess are defined as nonpurulent cellulitis, which is predominantly due to streptococcal species.⁴

Common Pathogens

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  Over the past decade, there has been an increase in purulent skin and soft tissue infections related to MRSA.⁴

  
  
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  Most commonly caused by group A βhemolytic S aureus and Staphlycoccus aureus.^3,4

Uncommon Pathogens

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  Other βhemolytic streptococci including groups B, C, and G⁴

  
  
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  Fresh water exposure: Aeromonas hydrophila^1,4

  
  
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  Salt water exposure: Vibrio vulnificus^1,4 in those with cirrhosis

  
  
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  Exposure to saltwater fish, shellfish, poultry, meat, and hides: Erysipelothrix rhusiopathiae, which can cause erysipeloid, mostly in the upper extremity³

  
  
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  Cat or dog bites: Pasteurella multocida or Capnocytophaga canimorsus^1,4

  
  
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  Neutropenic hosts: Pseudomonas aeruginosa or other gram negatives^1,4

  
  
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  Human immunodeficiency virus (HIV): Helicobacter cinaedi,¹ rare cause, atypical appearance with no warmth, can be recurrent, multifocal, and can be associated with bacteremia

  
  
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  Defective cell-mediated immunity: Cryptococcus neoformans¹

Lower extremity cellulitis occurs when there is a portal of entry for micro-organisms through a disrupted cutaneous barrier.

Predisposing Factors

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  Trauma (Figure 67-1) to the lower extremity including puncture wound and abrasion.³

  
  
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  Ulcers (Figure 67-1) and other skin lesions such as ecthyma, impetigo.^1,3

  
  
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  Fungal infection resulting in fissured toe webs (tinea pedis),¹ especially if it is complicated with bacterial colonization (Figure 67-2).⁴

  
  
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  Obesity⁴ and lower extremity edema from various other causes (Figure 67-3).³

  
  
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  Saphenous venectomy for coronary artery bypass grafting resulting in changes to venous and lymphatic drainage.³

  
  
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  Any underlying subcutaneous abscess or osteomyelitis with fistula tracking to the skin.³

  
  
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  Alterations due to surgery, radiation, and neoplastic agents involving the pelvic lymph nodes leading to lower extremity lymphedema³ (Figure 67-4).

  
  
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  Rarely, secondary cellulitis from bacteremia can occur.³