Cellulitis: Deep Skin Bacterial Infection

Clinical Overview

Cellulitis is an acute, spreading bacterial infection of the dermis and subcutaneous tissues, most commonly caused by Group A Streptococcus pyogenes and Staphylococcus aureus. The condition presents with rapidly spreading erythema, edema, warmth, and tenderness in affected area, often accompanied by systemic symptoms including fever, malaise, and occasionally chills. Cellulitis represents a medical emergency requiring prompt antibiotic therapy to prevent bacteremia, sepsis, and potential mortality. The condition typically develops in response to break in skin barrier (laceration, insect bite, surgical wound, dermatitis with excoriation) or entry point for bacteria. Predisposing factors including diabetes mellitus, immunocompromised status, venous insufficiency, and lymphedema increase cellulitis risk. Early recognition and appropriate systemic antibiotic therapy result in cure in >90% of cases; delayed treatment increases risk of severe complications including sepsis, necrotizing fasciitis, and systemic inflammatory response syndrome.

Epidemiology

Cellulitis is extremely common, affecting an estimated 24 million cases annually worldwide. Peak incidence occurs in older adults (age >60 years) and diabetic patients. The condition shows slight male predominance. Risk factors dramatically increase incidence: diabetes mellitus increases risk 2-3 fold, immunocompromised status (HIV, chemotherapy, immunosuppressive medications) increases risk 5-10 fold, venous insufficiency and lymphedema increase risk 5-10 fold, and prior cellulitis increases recurrence risk. Lower extremities are most commonly affected (70-80% of cellulitis cases), with face and upper extremities less common sites. Causative organisms vary by location: lower extremity cellulitis is most commonly caused by Group A Streptococcus (S. pyogenes) and Staphylococcus aureus, while facial cellulitis from Haemophilus influenzae has become rare since advent of H. influenzae vaccination.

Pathophysiology

Cellulitis develops when bacteria traverse skin barrier through break in epidermis and establish infection in dermis and subcutaneous tissue. Group A Streptococcus and Staphylococcus aureus produce virulence factors including hyaluronidase, proteases, and toxins that facilitate tissue invasion and breakdown of tissue barriers. The organisms evade initial immune defenses through production of anti-phagocytic substances and immunomodulatory toxins. Local inflammatory response includes recruitment of neutrophils and macrophages; however, the acute nature of cellulitis reflects rapid bacterial proliferation outpacing local immune containment. Systemic inflammatory response characterized by fever, elevated acute phase reactants (CRP, ESR), and leukocytosis develops. Bacteremia occurs in 5-15% of cellulitis cases, with risk of dissemination and sepsis if untreated. Host factors influencing disease severity include immunocompromised status (reduced bacterial clearance), vascular insufficiency (impaired antibiotic and immune cell delivery), diabetes (impaired neutrophil function and chemotaxis), and renal/hepatic failure (impaired antibiotic metabolism).

Clinical Presentation

Cellulitis presents acutely with spreading erythema, edema, warmth, and tenderness, typically with sharp demarcation between affected and unaffected skin. Associated systemic symptoms including fever (often 38-39°C), malaise, fatigue, and sometimes rigors are common. Regional lymphadenopathy and lymphangitis (streaking along lymphatic vessels) may be present. Pain and tenderness are prominent features. In severe cases, bullae, ecchymosis, or tissue necrosis may develop. The affected area shows poorly marginated erythema (distinguishing from cellulitis from erysipelas which has sharp demarcation). Lower extremities are most commonly affected. Systemic symptoms are often prominent, with fever preceding visible skin changes in some patients. Secondary complications including bacteremia (5-15% of cases), abscess formation, necrotizing fasciitis progression, and sepsis can develop if untreated.

Diagnosis

Diagnosis is primarily clinical based on characteristic presentation of acute, spreading erythema and edema with systemic symptoms. Key diagnostic features include: (1) acute onset of erythema, edema, warmth, tenderness; (2) spreading borders; (3) regional lymphadenopathy ± lymphangitis; (4) systemic symptoms (fever, malaise). Imaging (ultrasound, MRI, CT) may be helpful if diagnosis is unclear or if abscess is suspected. Blood cultures should be obtained in febrile patients or if systemic toxicity is present; cultures are positive in 5-15% of cellulitis cases. Needle aspiration or drainage culture is rarely recommended (low yield, <5% positive) unless abscess is suspected. Complete blood count typically shows leukocytosis. Comprehensive metabolic panel assesses organ function. Differential diagnosis includes: erysipelas (distinguished by sharper demarcation and superficial involvement), lymphedema (distinguished by chronic course and absence of acute inflammation), contact dermatitis, and deeper infections (abscess, necrotizing fasciitis).

Treatment Algorithm

Systemic antibiotic therapy is the cornerstone of cellulitis treatment. First-line oral agents for non-severe cellulitis (non-toxic appearing, outpatient candidate): cephalexin 500 mg four times daily for 10-14 days, or clindamycin 300-450 mg three to four times daily for 10-14 days (if MRSA risk is high). Alternative oral agents include amoxicillin-clavulanate 875-125 mg twice daily. For severe cellulitis requiring hospitalization or intravenous therapy: nafcillin 1-2 g IV every 4-6 hours for streptococcal/methicillin-sensitive staph infections, or vancomycin 15-20 mg/kg IV every 8-12 hours for MRSA risk or severe disease. For Streptococcus pyogenes monoinfection in non-severe disease, penicillin V 500 mg four times daily remains effective. Antibiotic selection should consider: MRSA prevalence in local region (if high, use vancomycin or clindamycin), severity of presentation (non-severe can be managed orally; severe requires IV therapy), and clinical response (improvement expected within 48-72 hours; lack of response suggests MRSA or other organism resistance warranting therapy modification).

Supportive care is critical: elevation of affected limb above heart level reduces edema, cool compresses provide symptomatic relief, and analgesia with acetaminophen or NSAIDs controls pain and fever (NSAIDs may provide benefit through anti-inflammatory action, though should be used cautiously in patients at risk for sepsis). Close monitoring for systemic progression (fever, hypotension, confusion, rapid progression of erythema) is essential; development of systemic toxicity warrants hospitalization and IV antibiotics.

Prognosis

The prognosis of cellulitis with appropriate antibiotic therapy is excellent: cure rates exceed 90% with standard therapy. However, mortality reaches 5-15% in untreated severe cellulitis or sepsis. Factors influencing prognosis include: timely initiation of appropriate antibiotics (early treatment associated with better outcomes), severity at presentation (non-severe cellulitis has better prognosis than systemic toxicity), underlying risk factors (immunocompromised patients require closer monitoring and potentially more aggressive therapy), and organism virulence (group A streptococcal infections can progress rapidly if untreated). Recurrent cellulitis occurs in 10-20% of patients within 5 years; higher recurrence rates are associated with lymphedema or venous insufficiency.

When to See a Dermatologist

Cellulitis requires immediate evaluation by primary care or emergency medicine (not specialist dermatology referral typically) given acute nature and need for urgent antibiotic initiation. Dermatology referral is appropriate for: (1) diagnosis uncertainty; (2) recurrent cellulitis requiring investigation for underlying predisposing factors; (3) atypical presentation.

Frequently Asked Questions

Q: Is cellulitis contagious? A: Cellulitis results from bacteria entering through broken skin barrier; it is not contagious from person to person as the causative organisms (streptococcus, staph) are ubiquitous in environment.

Q: How quickly does cellulitis improve with antibiotics? A: Clinical improvement with fever reduction and decreased erythema spreading is expected within 48-72 hours of appropriate antibiotic therapy initiation. Complete resolution typically requires 1-2 weeks.

Q: Can cellulitis progress to a serious infection? A: Yes, untreated cellulitis can progress to bacteremia, sepsis, necrotizing fasciitis, and systemic inflammatory response syndrome with potential mortality. Prompt antibiotic therapy prevents these complications.

Q: Why do I keep getting cellulitis? A: Recurrent cellulitis suggests underlying predisposing factor (diabetes, venous insufficiency, lymphedema, immunocompromised status) that should be investigated and managed. Prophylactic measures may reduce recurrence risk.

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