What is it?

Cellulitis is a common bacterial infection of the deeper layers of skin (dermis) and the tissue beneath (subcutaneous tissue). Unlike superficial skin infections, cellulitis spreads through these deeper layers, causing the affected area to become red, swollen, warm, and painful. While it most commonly affects the lower legs, cellulitis can occur anywhere on the body and, if left untreated, can spread rapidly and become life-threatening. Fortunately, most cases respond well to antibiotic treatment when caught early.

Who is affected?

Cellulitis is one of the most common bacterial skin infections, affecting millions annually.

Prevalence: Approximately 14.5 million cases annually in the United States; accounts for 2-3% of emergency department visits; incidence of 200 per 100,000 person-years
Age distribution: Can affect any age; peak incidence in middle-aged and elderly adults; more severe complications in elderly and very young
Gender: Slightly more common in males (55-60% of cases)
Risk factors: Disrupted skin barrier (cuts, insect bites, eczema, athlete's foot); lymphedema or chronic venous insufficiency; obesity (BMI >30 doubles risk); diabetes mellitus; weakened immune system; previous cellulitis (recurrence rate 8-20% within one year)

What causes it?

Cellulitis results from bacterial invasion through compromised skin barriers:

Common bacteria: Beta-hemolytic streptococci (Groups A, B, C, G) - most common cause; Staphylococcus aureus including MRSA - increasing prevalence; most cases are monomicrobial (single bacterium)
Entry points: Visible breaks - cuts, wounds, insect bites, injection sites; microscopic breaks - dry cracked skin, athlete's foot (tinea pedis); medical procedures - surgery, IV lines, dialysis
Predisposing conditions: Lymphatic damage - lymphedema increases risk 70-fold; venous insufficiency - chronic leg swelling; toe web intertrigo - fungal infection between toes is major portal
Risk multiplication: Diabetes - impairs healing and immunity; obesity - skin fold infections, poor lymphatic drainage; immunosuppression - medications, HIV, cancer
Rare causes: Water exposure - Vibrio species, Aeromonas; animal bites - Pasteurella, Capnocytophaga; immunocompromised - fungal, mycobacterial

What are the clinical features?

Cellulitis typically develops over hours to days with characteristic features:

Classic signs: Red (erythematous) area that spreads; skin feels warm or hot to touch; swelling (edema) of affected area; pain and tenderness, often before visible changes; borders typically irregular and not sharply defined (unlike erysipelas)
Common locations: Lower legs and feet (70-80% of cases); arms and hands; face (facial cellulitis); around surgical wounds or injury sites
Systemic symptoms: Fever and chills (present in 20-70% of cases); malaise and fatigue; enlarged lymph nodes near affected area; red streaks extending from area (lymphangitis - suggests spreading infection)
Severity progression: Mild - localized redness and swelling without systemic symptoms; moderate - larger area involved with fever; severe - rapid spread, high fever, confusion, low blood pressure (sepsis)
Special presentations: Periorbital cellulitis - around eye, emergency in children; perianal cellulitis - common in children, often streptococcal; bilateral lower leg - rare, consider other diagnoses

How is it diagnosed?

Diagnosis is primarily clinical, with limited role for testing:

Clinical diagnosis: Based on classic signs and symptoms; rapid onset and progression typical; response to antibiotics supports diagnosis
Laboratory tests: Blood tests (CBC, CRP) - elevated white cells and inflammatory markers in 50%; blood cultures - positive in only 2-5% of cases, reserved for severe cases; not routinely recommended for uncomplicated cellulitis
Imaging: Usually unnecessary; ultrasound can detect abscess if suspected; MRI/CT only if necrotizing fasciitis or osteomyelitis concern
Differential diagnosis (important): Venous stasis dermatitis - bilateral, chronic, less acute; deep vein thrombosis - use Wells score, D-dimer, ultrasound; contact dermatitis - itchy, clear exposure history; erysipelas - superficial, sharply demarcated, raised borders; necrotizing fasciitis - severe pain out of proportion, rapid progression
"Pseudocellulitis": Up to 30% of diagnosed cellulitis is actually another condition; bilateral presentation unusual for cellulitis (<10% of cases)

What treatment options are available?

Treatment centers on antibiotics and supportive care:

Oral antibiotics (uncomplicated cases): First-line - cephalexin 500mg four times daily or 1000mg twice daily for 5-10 days; alternative - amoxicillin-clavulanate 875/125mg twice daily; MRSA coverage if suspected - trimethoprim-sulfamethoxazole plus cephalexin or doxycycline
Intravenous antibiotics (severe cases): Indications - failed oral therapy, systemic toxicity, immunocompromised; cefazolin or ceftriaxone for streptococcal/MSSA; vancomycin or daptomycin for MRSA
Duration of therapy: Typical course 5-10 days; improvement expected within 24-48 hours; extend if slow response or complications
Supportive measures: Elevation of affected limb reduces swelling; rest and immobilization initially; adequate analgesia for pain; mark borders to monitor progression/improvement
Prevention of recurrence: Treat predisposing factors - athlete's foot, eczema; daily washing and moisturizing; prompt wound care; compression stockings for venous insufficiency; prophylactic antibiotics if ≥3-4 episodes per year (penicillin V or erythromycin)
Prognosis: Excellent with appropriate treatment - 90% cure rate; most improve within 24-48 hours of antibiotics; hospitalization needed in 10-15% of cases; mortality <1% with treatment, but up to 30% if untreated sepsis; recurrence in 8-20% within one year

Key points for patients

Cellulitis is a serious bacterial infection that requires prompt antibiotic treatment
Early treatment prevents complications and reduces risk of hospitalization
Most cases improve within 2-3 days of starting antibiotics
When to seek immediate medical attention: Rapid spreading of redness; high fever, confusion, or feeling very unwell; redness near eyes or on face; no improvement after 48 hours of antibiotics; signs of sepsis - rapid heart rate, difficulty breathing, low blood pressure
Prevention strategies: Keep skin clean and moisturized; treat athlete's foot and eczema promptly; clean all wounds properly; manage chronic conditions like diabetes; wear protective footwear in public areas

Cellulitis