Definition
Acute bacterial infection of the deep dermis and subcutaneous tissue with spreading erythema, warmth, swelling and tenderness. Erysipelas is the superficial form with a raised, sharply demarcated edge.
Aetiology
- Beta haemolytic streptococci and Staphylococcus aureus in most cases
- An organism is identified in about 15%
- Aeromonas and Vibrio after water exposure, Pasteurella after animal bites
- Risk factors are lymphoedema, venous insufficiency, tinea pedis, obesity, diabetes and previous cellulitis
Clinical Features
- Unilateral and lower limb in most
- Fever, lymphangitis and tender regional nodes
- Blisters and petechiae in severe cases
- Features suggesting necrotising infection
- Pain out of proportion to the skin signs
- Rapid spread despite antibiotics
- Crepitus, skin necrosis or cutaneous anaesthesia
- Systemic toxicity
Investigations
- Diagnosis is clinical
- Blood cultures with systemic toxicity, immunocompromise or animal bites
- LRINEC score of 6 or more warrants assessment for necrotising fasciitis
- Ultrasound for abscess, and venous duplex when DVT is possible
- Aspirate a suspected septic joint through uninvolved skin
- MRI for suspected osteomyelitis or deep collection
Differential Diagnosis
- Septic arthritis, with effusion and pain on any passive joint movement
- Necrotising soft tissue infection, with pain and toxicity beyond the visible erythema
- Deep vein thrombosis, with calf swelling and less erythema, confirmed on duplex
- Gout and pseudogout, with abrupt periarticular redness
- Acute osteomyelitis
- Prepatellar or olecranon bursitis
- Venous stasis dermatitis, often bilateral
Management
- Oral antibiotics. Flucloxacillin or cefalexin for mild cellulitis, 5 days if improving
- Intravenous antibiotics. Systemic features, rapid spread or failure of oral therapy
- MRSA cover. Purulent infection, penetrating trauma, injecting drug use or known colonisation
- Elevation. Speeds resolution of oedema
- Treat predisposing factors. Tinea pedis, oedema and venous disease to reduce recurrence
- Surgical drainage. Abscess, or urgent exploration for suspected necrotising infection
References
- Stevens DL, Bisno AL, Chambers HF, Dellinger EP, Goldstein EJ, Gorbach SL, Hirschmann JV, Kaplan SL, Montoya JG, Wade JC, Infectious Diseases Society of America. Practice guidelines for the diagnosis and management of skin and soft tissue infections. 2014 update by the Infectious Diseases Society of America. Clin Infect Dis. 2014, 59(2), e10-52. PubMed
- Raff AB, Kroshinsky D. Cellulitis. A review. JAMA. 2016, 316(3), 325-37. PubMed
- Wong CH, Khin LW, Heng KS, Tan KC, Low CO. The LRINEC (Laboratory Risk Indicator for Necrotizing Fasciitis) score. A tool for distinguishing necrotizing fasciitis from other soft tissue infections. Crit Care Med. 2004, 32(7), 1535-41. PubMed
Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 19 October 2026.