Cellulitis is strictly an acute, subacute, or chronic infection of the subcutaneous tissues, whereas erysipelas is an infection of the dermis and superficial subcutis. Infection of the more superficial layers gives rise to superficial edema and inflammation, with the consequent development of a palpable, often advancing edge. The causative organism is usually regarded as Streptococcus, though many organisms have been isolated, including Haemophilus influenzae, and more rarely staphylococci, Aeromonas hydrophilia, and Pseudomonas aeruginosa, as well as fungi and Gram-negative rods. Fulminating and necrotic cellulitis and fasciitis may occur rarely, usually in relation to immunosuppression or atypical organisms. These are rare, but necrotizing fasciitis may have a mortality of up to 50%.
The management of cellulitis and erysipelas should initially be directed to trying to identify the organism responsible for the infection, and then directing appropriate antimicrobial therapy. Any underlying and predisposing condition should be identified and treated to prevent subsequent recurrence. Perhaps the commonest condition that is not identified and treated is toe web tinea pedis, which provides a portal of entry for infection.
Uncomplicated cellulitis and erysipelas may be managed without admission if the patient does not exhibit signs of systemic toxicity. In such cases oral broad-spectrum antibiotics, chosen to cover group A streptococci and staphylococci, may be sufficient, supplemented with a single parenteral loading dose or long-acting preparation. The drug of choice is oral penicillin V (phenoxymethylpenicillin) with or without flucloxacillin, or erythromycin, if the patient has a known penicillin allergy. Newer macrolides, such as clarithromycin, may be more acceptable with fewer side effects. Some authorities have recommended the use of clindamycin rather than a macrolide because of apparent increased tissue penetration, but this may be associated with an increased incidence of Clostridium difficile superinfection. Further, although most group A β-hemolytic streptococci are sensitive to this, increasing numbers of MRSA are displaying resistance to clindamycin, and widespread use may exacerbate this resistance.
Immunocompromised patients, those with signs of systemic toxicity, and otherwise debilitated patients should be treated as inpatients with intravenous antimicrobials (e.g., penicillin G – benzylpenicillin) or one of the newer antibiotics (e.g., ciprofloxacin, ticarcillin, teicoplanin, or imipenem/cilastatin). If there is evidence of head and neck disease or sinus infection, amoxicillin combined with clavulanic acid should be considered to cover H. influenzae infection.
Blood cultures may be positive and significant in only about 25% of cases, but should always be taken if there is any evidence of systemic toxicity. Swabs of wounds and broken skin may be helpful, but surface swabs of unbroken skin provide little or no useful information. If available, aspirate of bullae may yield positive cultures. Slightly better rates for isolation than those of needle aspirates have been achieved with punch skin biopsies.
In the case of cellulitis or erysipelas of the lower leg, skin scrapings from toe webs should be taken for mycological examination. Facial erysipelas should warrant sinus radiographs to exclude underlying sinusitis. Crepitus should prompt the clinician to the presence of either clostridia or non-spore-forming anaerobes, either alone or mixed with other bacteria such as Pseudomonas, Escherichia coli, or Klebsiella spp.