Clinical Overview

Cellulitis is an acute non-suppurative bacterial infection of dermis and subcutaneous tissue characterized by poorly demarcated erythema, edema, warmth, and tenderness. This common infection affects 34 million people annually in US alone, with 14 million requiring hospitalization. Streptococcus pyogenes (Group A Streptococcus, GAS) and Staphylococcus aureus predominate; however, MRSA (methicillin-resistant S. aureus) prevalence increased 30-fold over past decade, altering empiric therapy algorithms.

Epidemiology

Cellulitis annual incidence 24-33 per 1000 population-years. Predisposing factors: lymphedema (48% of lower extremity cases), venous insufficiency (25%), obesity, diabetes (3-fold increased risk), immunosuppression (HIV CD4 <50, chemotherapy), previous cellulitis (30% recurrence risk), skin barrier disruption (trauma, athlete's foot, venous ulcers). GAS accounts for 50-70% of cases; S. aureus 20-30%; MRSA 5-10% depending on geography/risk factors. MRSA prevalence increases in: prior antibiotics (5-fold), IV drug use (4-fold), healthcare exposure, chronic wounds, recurrent cellulitis (25% of recurrent cases MRSA+). Mortality 0.4% in immunocompetent, 4-6% in septic shock/immunocompromised. Complications: deep space infection (necrotizing fasciitis in 2-5%), sepsis (1-2%), hematogenous spread. Risk factors for complications: male gender, immunosuppression, diabetes, obesity, extreme age.

Pathophysiology

Cellulitis results from bacterial invasion through skin barrier disruption (trauma, intertrigo, venous/lymphatic insufficiency) or hematogenous seeding. S. pyogenes produces numerous virulence factors: hyaluronic acid capsule (molecular mimicry prevents immune recognition), streptolysins (tissue destruction), streptopeptidases (fibrin degradation), and M protein (antiphagocytic). S. aureus produces alpha-toxins (pore formation), Panton-Valentine leukocidin (PVL, leukocyte destruction), and phenol-soluble modulins. MRSA resistance mediated by altered penicillin-binding proteins (PBPs) with mecA gene encoding PBP2a. Innate immune response: IL-6, TNF-alpha, and complement activation create inflammation manifesting as erythema, edema, warmth. Poorly demarcated borders distinguish cellulitis from erysipelas (sharply demarcated, raised borders, more superficial).

Clinical Presentation

Cellulitis presents acutely with poorly demarcated erythematous plaque with edema and warmth. Systemic symptoms: fever (70%), chills, malaise. Pruritus or pain common (65%). Lymphadenopathy/lymphangitis in 30-50%. Distribution: lower extremities (70%), upper extremities (20%), face (5%), trunk (rare). Associated findings: cellulitis may follow traumatic inoculation (visible wound), occur in area of pre-existing skin disease (intertrigo, athlete's foot, eczema), or develop in setting of lymphedema/venous insufficiency without obvious portal of entry. Rapid progression: can extend 5-10cm per 24 hours in fulminant cases. Systemic toxicity: fever >38.5°C, tachycardia, hypotension in severe cases indicating sepsis.

Diagnosis

Diagnosis primarily clinical based on characteristic poorly demarcated erythema with edema and warmth. Blood cultures positive in 5-10% of non-bacteremic cases, higher in severe cellulitis/sepsis. Wound/blister fluid culture if suppuration present (increases sensitivity to 25-30% if aspirate obtained from leading edge at margin). Avoid unnecessary cultures in typical non-severe cases (low yield, does not alter empiric therapy initiation). CBC: mild leukocytosis common (WBC 10-15k), though absence does not exclude infection. CRP/ESR elevated, supporting inflammation but non-specific. Imaging (ultrasound, CT) indicated for: assessment of deep space involvement, abscess detection, or diagnostic uncertainty. MRI most sensitive for necrotizing fasciitis detection (fascial enhancement, subcutaneous fat stranding).

Treatment Algorithm

Outpatient Management (Non-Severe Cellulitis, No Systemic Toxicity, Reliable Patient): Empiric oral antibiotics covering GAS + S. aureus: cephalexin 500mg four times daily x 7-10 days, or amoxicillin-clavulanate 875-125mg twice daily. MRSA risk factors present (prior antibiotics, healthcare exposure, severe disease trend): add TMP-SMX 1-2 DS tablets twice daily OR clindamycin 300-450mg 3-4 times daily. Clinical improvement expected by 48-72 hours; if worsening, switch to parenteral therapy. Leg elevation, NSAIDs for comfort. Follow-up within 24-48 hours essential.

Hospitalization Criteria (Inpatient Parenteral Therapy): Severe cellulitis, systemic toxicity (fever >38.5°C, hypotension, altered mental status), immunocompromised status (CD4 <50, chemotherapy, transplant), facial/periorbital cellulitis (risk of ophthalmic involvement), rapid progression, failed oral therapy, or unreliable outpatient follow-up. Initial empiric regimen: cefazolin 1-2g IV q6-8h (covers GAS/MSSA, first-line). If beta-lactam allergy: clindamycin 600-900mg IV q6-8h. If MRSA risk: vancomycin 15-20mg/kg IV q8-12h (target trough 15-20 mcg/mL) OR daptomycin 4mg/kg IV daily (superior in some MRSA studies, though not FDA-approved for cellulitis). MRSA prevalence >10% in region: consider empiric vancomycin + cefazolin (dual therapy) vs. vancomycin monotherapy. Duration: switch to oral therapy after 48-72 hours clinical improvement, completing 7-10 day total course.

Severe/Septic Cellulitis: Broad-spectrum empiric therapy covering gram-positive (streptococci, staphylococci including MRSA) and gram-negative organisms. Vancomycin 15-20mg/kg IV q8-12h PLUS ceftriaxone 1-2g IV q12h OR cefepime 2g IV q12h (covers gram-negatives). Add clindamycin 600-900mg IV q6-8h for MRSA/anaerobic coverage if concern for necrotizing infection. Blood cultures, lactate level, CT imaging essential. Sepsis management: fluid resuscitation, vasopressor support if hypotensive, ICU consultation. Consider infectious diseases consultation. Escalate to vancomycin if clinical failure on cephalosporin within 48 hours.

Recurrent Cellulitis Management: Investigate predisposing factors (lymphedema, venous insufficiency, athlete's foot) and address. Prophylactic therapy: benzathine penicillin G 1.2 million units IM monthly or oral penicillin V 250mg twice daily continuous for 1-2 years reduces recurrence by 70%. Indicated after ≥2 episodes in 12 months.

Prognosis

Cellulitis prognosis excellent with prompt antibiotics: 85-90% clinical cure with appropriate therapy within 7-10 days. Mortality <1% in immunocompetent non-septic patients, 4-6% with sepsis/shock. Complications (necrotizing fasciitis, deep space abscess) develop in 2-5%; early recognition critical. Recurrence rate 8-28% depending on underlying predisposing factors (lymphedema increases recurrence 2-fold). Prophylactic antibiotics reduce recurrence 70% in high-risk patients.

When to See a Dermatologist

Dermatologists recognize cellulitis and coordinate with primary care/infectious diseases for empiric therapy initiation. Refer for diagnostic uncertainty, recurrent cellulitis investigation, or management of underlying predisposing skin conditions (eczema, athlete's foot).

Frequently Asked Questions

Q: Is cellulitis contagious?
A: Cellulitis itself is not directly contagious person-to-person. However, the causative bacteria (streptococci, staphylococci) can spread if open wounds/drainage contact others. Maintain good wound hygiene and hand hygiene to prevent spread.

Q: How quickly should I start antibiotics?
A: Immediately. Cellulitis progression rapid (5-10cm per 24 hours). Start empiric antibiotics within first 24 hours of symptom onset. Delayed therapy increases risk of complications (abscess, sepsis, necrotizing infection).

Q: Will I get sepsis?
A: Sepsis develops in 1-2% of cellulitis cases with prompt antibiotics. Risk increases with: immunosuppression, diabetes, extreme age, delayed treatment, or MRSA infection. Early recognition of fever, chills, rapid progression warrants hospitalization.

Q: Can cellulitis return after cure?
A: Yes, recurrence rate 8-28% depending on predisposing factors. Lymphedema, venous insufficiency, previous cellulitis increase recurrence risk. Address underlying causes (compression therapy for lymphedema, wound care). Prophylactic antibiotics reduce recurrence 70% in high-risk patients.

References

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