Cellulitis: Causes, Symptoms, Diagnosis, and Treatment

Learn about cellulitis, a common bacterial skin infection, its symptoms, risk factors, diagnosis, and treatment approaches.

By Medha deb

Created on Oct 3, 2025

Learn about cellulitis, a common bacterial skin infection, its symptoms, risk factors, diagnosis, and treatment approaches.

What is Cellulitis?

Cellulitis is a bacterial skin infection characterized by localized redness, warmth, swelling, and tenderness. It occurs when bacteria enter the deeper layers of the skin and underlying subcutaneous tissues, leading to inflammation and sometimes rapid spread. Cellulitis is a common condition, responsible for over 14 million cases and around 650,000 hospitalizations annually in the United States, accounting for billions in healthcare costs.

Key Facts About Cellulitis

Type of infection: Acute, usually affecting the deep dermis and subcutaneous tissue.
Main bacteria: Group A streptococcus (Streptococcus pyogenes) and Staphylococcus aureus.
Risk: Can affect anyone, but certain risk factors increase susceptibility.
Contagiousness: Cellulitis itself isn’t typically contagious, but the bacteria causing it can spread via direct contact if there is exposed, damaged skin.

Causes of Cellulitis

Cellulitis usually results from a bacterial infection entering through a break in the skin. The most common causative agents are:

Group A Streptococcus (Streptococcus pyogenes) — Most frequent cause.
Staphylococcus aureus — Common in purulent cellulitis (associated with abscesses or wounds).
Occasional bacteria: Group G streptococcus and, less often, other Gram-negative bacteria (especially in immunocompromised patients or animal bites).

How Do Bacteria Enter the Skin?

The skin provides a protective barrier. When breached, bacteria can penetrate and cause infection. Common entry points include:

Cuts, scrapes, or puncture wounds
Surgical incisions
Ulcers or chronic wounds
Insect or animal bites
Cracks between toes (often seen in athletes)
Injection sites (in medical treatment or substance use)

Risk Factors for Cellulitis

Some individuals are at higher risk due to underlying health issues or environmental factors:

Diabetes mellitus — impairs immune function and skin healing
Venous insufficiency or lymphedema — poor circulation increases infection risk
Peripheral arterial disease
Chronic edema
Prior radiation therapy or surgery (e.g., mastectomy, saphenous vein harvest)
Skin conditions: Eczema, psoriasis, athlete’s foot (dermatophytic infections)
Immunocompromised status: chemotherapy, neutropenia, HIV/AIDS
Injection drug use

Symptoms and Signs of Cellulitis

Typical symptoms of cellulitis develop rapidly and may include:

Red, hot, tender, swollen skin
Pain in the affected area — often significant
Warmth over the infected region
Edema (swelling)
Poor definition of the affected area, less sharp demarcation than erysipelas
Fever and chills — may accompany moderate or severe cases
Enlarged lymph nodes (adenopathy)
Rarely: Bullae (fluid-filled blisters) develop in severe cases
Potential development of lymphangitis — streaking redness along lymphatic vessels

Diagnosis of Cellulitis

Cellulitis is diagnosed primarily through clinical evaluation rather than laboratory testing. Physicians will:

Assess symptoms: redness, swelling, warmth, and pain
Evaluate the patient’s medical history and risk factors
Rule out mimicking conditions via physical examination
For severe or atypical cases, may order blood tests or imaging (like CT scans for suspected orbital cellulitis)

Conditions That Mimic Cellulitis

Condition	Features
Allergic reactions	Rapid onset, often associated with itching; may not be warm/tender
Gout	Acute joint pain and redness, usually of the big toe
Herpes zoster (shingles)	Painful, band-shaped rash with blisters
Erythroderma	Widespread skin redness and scaling
Insect bite reactions	Localized redness and swelling, puncture marks usually present
Panniculitis	Inflammation of subcutaneous fat tissue
Lyme disease (erythema migrans)	Bull’s-eye rash, tick exposure history
Sweet’s syndrome	Painful skin lesions with fever; neutrophilia
Pyoderma gangrenosum	Ulcerative skin lesions, often linked to autoimmune conditions
Fixed drug reaction	Sudden rash after medication use; clearly demarcated
Thrombophlebitis	Redness, tenderness along veins, history of varicose veins
Necrotizing fasciitis	Rapidly spreading, intense pain, systemic toxicity, skin necrosis

Classification and Severity of Cellulitis

The severity of cellulitis guides treatment strategy. Guidelines (IDSA, 2014) classify cellulitis as:

Non-purulent cellulitis — No abscess, necrotizing fasciitis, or purulent discharge. Typical presentation for most cases.
Purulent cellulitis — Associated with a wound, abscess, carbuncle, or furuncle, with pus present. Staph infections more common here.

Severity	Key Features
Mild	Localized infection, no systemic symptoms or purulence
Moderate	Local infection plus systemic signs (fever, increased WBCs)
Severe	Failed oral antibiotics, signs of sepsis (SIRS), end-organ damage, immunocompromised status, skin sloughing or bullae, hypotension

Complications of Cellulitis

If left untreated, cellulitis can cause severe complications:

Abscess formation — pockets of pus require drainage
Transition to severe infection — systemic symptoms, sepsis
Spread to bloodstream (bacteremia) — can become life-threatening
Necrotizing fasciitis — rapid progression to tissue death, surgical emergency
Lymphangitis — spread along lymph vessels
Chronic swelling or damage — especially with repeat infections or poor circulation
Orbital cellulitis — infection of tissues around the eyes, requires urgent intervention

Treatment and Management of Cellulitis

Early and effective treatment of cellulitis protects against complications. The primary therapies include:

Antibiotics

First-line antibiotics: Beta-lactams (e.g., cephalexin, amoxicillin, penicillin VK) for non-purulent cellulitis
Coverage for MRSA: Clindamycin, doxycycline, or trimethoprim-sulfamethoxazole (if risk factors for resistant bacteria are present)
Duration: Typically 5 to 10 days depending on clinical response

Purulent Cellulitis Management

Incision and drainage (I&D) — mandatory for abscesses, carbuncles, or furuncles
Oral antibiotics — used alongside I&D for mild cases
Hospital admission — for severe cases or failed outpatient therapy

Supportive Care

Pain relief: NSAIDs, acetaminophen
Elevation of affected limb: reduces swelling
Hydration: adequate fluids for skin healing
Monitoring for response: follow up with healthcare provider if symptoms worsen

When to Hospitalize?

Severe infection: systemic toxicity, spread beyond initial site, underlying immunocompromise
Failed outpatient therapies
Complications present — bullae, necrotizing fasciitis, orbital involvement

Prevention of Cellulitis

Preventing breaks in the skin is key to avoiding cellulitis. Consider these strategies:

Good skin hygiene: regular cleaning and moisturizing
Quick treatment of skin injuries: cleanse and cover wounds promptly
Management of chronic skin conditions: eczema, athlete’s foot, psoriasis
Care during surgery or IV therapy: sterile technique
Diabetes management: tight glycemic control reduces risk
Prompt attention to swelling: treat lymphedema and venous insufficiency

Frequently Asked Questions (FAQs)

What does cellulitis look like?

Cellulitis commonly appears as a red, swollen, warm area with poorly defined edges, sometimes accompanied by pain, fever, and tenderness.

Is cellulitis contagious?

While cellulitis itself is not typically contagious, the bacteria causing it may spread via direct contact if another person’s skin is broken.

How is cellulitis different from erysipelas?

Cellulitis involves deeper skin layers and has less defined edges, while erysipelas is a more superficial infection with sharply demarcated edges and is also usually caused by streptococcus.

When should I seek emergency care for cellulitis?

If you or someone you know develops fever, rapidly spreading redness, severe pain, skin blisters/bullae, confusion, or signs of systemic illness, immediate medical care is needed to prevent complications.

What are the long-term risks if cellulitis recurs?

Repeated episodes of cellulitis can result in chronic swelling, lymphedema, and damage to lymphatic vessels, making future infections more likely.

Key Takeaways

Cellulitis is a common, potentially serious bacterial skin infection.
Early recognition and prompt antibiotic therapy are crucial.
Prevention relies on skin care and management of underlying health issues.
Complications may be serious and can include abscess formation, sepsis, or necrotizing fasciitis.
Consult healthcare professionals for any new, rapidly spreading skin redness or if systemic symptoms occur.

References

Author

medha deb

Medha Deb is an editor with a master's degree in Applied Linguistics from the University of Hyderabad. She believes that her qualification has helped her develop a deep understanding of language and its application in various contexts.

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