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  1. What are the common bacterial causes of cellulitis?

  2. How does the presentation help with diagnostic and therapeutic decisions?

  3. What is the expected response to treatment?

  4. What features suggest more complicated infection?

  5. What are strategies for patients with recurrent infections?

Cellulitis is an acute infection involving the epidermis, dermis, and subcutaneous tissues. Most cases are caused by Staphylococcus aureus or beta-hemolytic streptococci, although in many patients with presumed infectious cellulitis, a definite bacteriologic diagnosis cannot be made. Occasionally, cellulitis is due to nonbacterial pathogens or noninfectious inflammatory conditions. The costs to society of cellulitis are significant. Up to 2% of hospital admissions are due to cellulitis. While only 7% of patients with cellulitis are admitted to the hospital, hospitalized patients account for 80% of the total health care costs associated with cellulitis.

Healthy skin is resistant to infection. Skin breakdown is usually required for cellulitis to develop. This may be obvious or too subtle to detect. Tinea pedis is an especially common portal of entry for bacteria in lower-extremity cellulitis. Other local risk factors for cellulitis include common skin conditions such as eczema and psoriasis, trauma, intravenous drug use, animal and human bites, venous stasis, and lymphedema. Medical risk factors for cellulitis include diabetes, arterial insufficiency, cirrhosis, renal insufficiency, and neutropenia.

Cellulitis occasionally causes bacteremia. Rarely, cellulitis arises from systemic bacteremia, with secondary seeding of the skin. This is more common in neutropenic patients. These infections lead to bacterial proliferation in the vessel wall, tissue ischemia, and skin necrosis. Clinically, this presents as an area of inflamed skin, with a hemorrhagic pustule that develops into a necrotic ulcer (ecthyma gangrenosum).

Cellulitis presents with pain, swelling, and erythema of the skin. In most patients with cellulitis, the margins of erythema are not sharply demarcated. Fever is usually present in patients admitted to the hospital with cellulitis, although it is often absent in milder forms. A minority of patients with cellulitis have enlargement of the draining lymph nodes (lymphadenitis). The bacteriology of cellulitis usually cannot be inferred from the physical examination. However, suppuration and abscess formation is strongly suggestive of Staphylococcus aureus. Rapid clinical evolution, lymphangitis, and features of erysipelas (discussed later), such as peau d'orange and a raised, well-demarcated border, suggest beta-hemolytic streptococci. A number of epidemiologic features in cellulitis put patients at risk for particular pathogens. These are summarized in Table 203-1 and discussed in more detail later in this chapter. As well, several noninfectious conditions may mimic cellulitis. These are summarized in Table 203-2.

Table 203-1 Cellulitis Pathogens Associated with Particular Epidemiology*

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