creation date: 2026-11-02 11:08
tags: PathologiesIncomplete
Cellulitis
Background
Definitions
Cellulitis is a type of skin and soft tissue infection in which bacteria enters through a breach in the skin barrier.
Erysipela refers to a superficial skin infection that involves the upper dermis/superficial lymphatic and nonpurulent.
Skin abscess refers to a collection of pus within the dermis or subcutaneous space.
Microbiology
Cellulitis
The most common causes are:
- Beta-hemolytic streptococci, group A, B, C, G, and F
- Most common: group A Streptococcus (Streptococcus pyogenes)
- Staphylococcus aureus (including MRSA)
Less commonly:
- Haemophilus influenzae type b
- Clostridia
- Non-spore-forming anaerobes
- Streptococcus pneumoniae
- Neisseria meningitidis
There are also microbes associated with specific clinical circumstances:
| Circumstance | Pathogen |
|---|---|
| Animal bites | Pasteurella multocida, Capnocytophaga canimorsus |
| Water exposure | Aeromonas hydrophila, Vibrio vulnificus |
| Immunocompromised, diabetic foot | Pseudomonas aeruginosa |
| Neonates/infants, pelvic lymph node dissection | Group B Streptococcus |
| Gas gangrene | Clostridium |
| Orbital cellulitis | Streptococcus pneumoniae |
Skin abscess
The most common cause is:
- Staphylococcus aureus (MSSA or MRSA) - 75% of cases
In multi-organism abscesses, may include:
- S. pyogene
- Gram-negative bacilli with anaerobes
Risk Factors
Factors that predispose to cellulitis/skin abscess include:
- Skin barrier disruption due to trauma (such as abrasion, penetrating wound, pressure ulcer, venous leg ulcer, insect bite, injection drug use)
- Skin inflammation (such as eczema, radiation therapy, psoriasis)
- Edema due to impaired lymphatic drainage
- Edema due to venous insufficiency
- Obesity
- Immunosuppression (such as diabetes or HIV infection)
- Skin breaks between the toes (“toe web intertrigo”); these may be clinically inapparent
- Pre-existing skin infection (such as tinea pedis, impetigo, varicella)
- Prior saphenous vein harvesting for coronary artery bypass graft surgery
Clinical Presentation
Signs & Symptoms
Cellulitis
- Skin erythema, edema, and warmth
- Petechiae, hemorrhage
- Fever and other systemic infectious symptoms
- May or may not be purulent
- Almost always unilateral, often lower extremities
- Lymphangitis and enlargement of regional lymph nodes
Skin abscess
- Painful
- Fluctuant, erythematous nodule
- May have surrounding cellulitis
- Spontaneous drainage of purulent material may occur
History & Physical Exam
History should include:
- Risk factors and manifestations for DVT
- Medical history that may predispose skin infection
- History of trauma/wounds that may allow for pathogen entry
Diagnosis
Criteria
Diagnosis is made clinically.
Work-up
Rule out DVT
Based on clinical suspicion, consider further investigations for DVT.
Cultures
Cultures of the wound or debrided material in an abscess drain are not routine. Note, swabs of intact skin is entirely not helpful and should not be performed.
Blood cultures prior to initiating antibiotics is warranted if:
- Severe local infection
- Systemic signs of infection (eg. fever, sepsis)
- History of recurrent or multiple abscesses
- Failure of initial antibiotic therapy
- Extremes of age
- Presence of underlying comorbidities (lymphadema, malignancy, neutropenia, immunodeficiency, splenectomy, diabetes)
- Special exposures (animal bites, water-associated injury)
- Presence of indication for prophylaxis against infective endocarditis
- Community patterns of S. aureus susceptibility are unknown or rapidly changing
Imaging
Ultrasound may be useful to determine if skin abscess is present. MRI may distinguish cellulitis from osteomyelitis.
Differential
Rapidly progressive erythema with systemic toxicity
- Necrotizing fasciitis
- Toxic shock syndrome
- Clostridial myonecrosis (gas gangrene)
Other infections
- Erythema migrans (Lyme disease)
- Erythrasma - brown or orange-hued erythema with distinct borders in an intertriginous area
- Herpes zoster
- Pyomyositis - localized muscle pain with movement or palpation
- Septic arthritis
- Septic bursitis
- Osteomyelitis
- Mycotic aneurysm
- Medical device infection
Non-infectious mimics
Unilateral:
- Contact dermatitis - pruritic
- Acute gout
- Drug reaction
- Vasculitis
- Hematoma
- Insect bite
- Deep vein thrombosis
- Panniculitis
- Vaccination site reaction
- Calciphylaxis - patients with CKD
- Malignancy
- Eosinophilic cellulitis - recurrent erythematous patches/plaques, pruritic and nontender
- Erythema ab igne - repeated exposure to heat/infrared radiation
- Radiation recall - sites of prior radiation therapy
Bilateral:
- Stasis dermatitis - context of chronic venous insufficiency
- Lipodermatosclerosis
- Lymphedema
Skin abscess
- Epidermoid cyst
- Folliculitis
- Hidradenitis suppurativa
- Nodular lymphagitis
- Botryomycosis
- Myiasis
Red Flags / Complications
Complications include:
- Bacteremia and sepsis
- Infective endocarditis
- Septic arthritis
- Osteomyelitis
- Metastatic infection
- Toxic shock syndrome
Management
Antibiotic Therapy
Antibiotic selection is based on most likely pathogen and empirically covering the most common microbes.
- Dicloxacillin 500 mg PO q6h
- Cephalexin 500 mg PO q6h
Empiric MRSA coverage is indicated for:
- Systemic signs of toxicity (eg. fever >38, hypotension, sustained tachycardia)
- Cellulitis with purulent wound drainage
- Known MRSA colonization or infection
- Injection drug use
- High-risk neutropenia
Options include:
- TMP-SMX 1-2 double strength tab PO BID
Specific coverage is warranted for certain causes:
| Cause | Example therapy |
|---|---|
| Diabetic foot ulcer | See separately |
| Animal bites | |
| Human bites | |
| Puncture wounds (non-bite) | |
| Pressure injury/ulcer | |
| Surgical wound | |
| Water exposure | |
| Soil exposure | |
| Travel-related skin infections |