Microbiology

β-haemolytic streptococci (group A, C, G) — most common; predominate in classic erysipelas-like cellulitis and recurrent disease.
Staphylococcus aureus — particularly with abscess / purulent component; MRSA in care-home or healthcare-associated settings.
Atypical / contact-exposure organisms:
- Vibrio vulnificus, Aeromonas hydrophila — fresh / salt water exposure.
- Pasteurella multocida, Capnocytophaga canimorsus — animal bites.
- Eikenella corrodens — human bites.
- Erysipelothrix rhusiopathiae — fish / meat handlers (erysipeloid).
- Mycobacterium marinum — fish-tank / aquarium granuloma.

Clinical features and assessment

Acute, hot, tender, poorly-defined erythema with rapid expansion; commonly unilateral; lower limb most frequent in adults.
Systemic features (fever, rigors, leucocytosis) in classes II-IV.
Mark the edge at presentation to monitor progression.
Eron / CREST classification:
- I — no systemic toxicity or comorbidity → oral antibiotics outpatient.
- II — systemic features or significant comorbidity → home IV via OPAT or short admission.
- III — systemic toxicity (SIRS), unstable comorbidity, limb-threatening → admission, IV antibiotics.
- IV — sepsis, necrotising fasciitis suspected → emergency surgical review.

Lymphoedema flare / acute on chronic — common after lymphadenectomy.
Inflamed epidermoid / sebaceous cyst — well-defined; central punctum; rubbery on resolution.
Erysipelas — sharply demarcated, raised, classically facial / lower-limb (separate page).
Acute contact dermatitis — pruritus dominates over pain; geometric distribution.
Stasis dermatitis / lipodermatosclerosis — chronic, bilateral, hyperpigmented; not erysipelas-like.
Deep vein thrombosis — calf tenderness, Homans sign; Wells score; D-dimer.
Inflammatory breast cancer or recurrent cutaneous metastasis — peau d'orange; firm.
Cutaneous angiosarcoma — particularly in older adults, post-mastectomy lymphoedema (Stewart-Treves) or after radiotherapy.
Necrotising fasciitis — pain out of proportion, dusky / bullous skin, crepitus, systemic toxicity — surgical emergency.
Inflammatory carcinoma erysipeloides — breast / gastric / lung metastasis simulating cellulitis.

Routine: FBC, CRP, U&E, LFT, lactate, blood cultures if class II+.
Swab any open wound or pustule; aspirate fluctuant collections.
Imaging: lower-limb venous Doppler if DVT possible; soft-tissue ultrasound for collection; MRI / CT if necrotising fasciitis or bone involvement suspected.
If recurrent: consider tinea pedis as portal of entry; assess for chronic lymphoedema, obesity, varicose eczema.
If unilateral, painless, slowly progressive in older adult — biopsy to exclude carcinoma erysipeloides or angiosarcoma.

NICE NG141 (2019):
- Class I (no MRSA risk): oral flucloxacillin 500 mg-1 g QDS for 5-7 days (or clarithromycin / doxycycline if penicillin-allergic).
- Facial cellulitis: co-amoxiclav 500/125 mg TDS.
- Class II-III: IV co-amoxiclav 1.2 g TDS or ceftriaxone 1-2 g OD; consider OPAT.
- MRSA risk: vancomycin / teicoplanin / linezolid per local protocol.
- Animal bite: co-amoxiclav.
Address portal of entry (interdigital tinea, fissures, eczema).
Limb elevation, analgesia, hydration; mark erythema edge daily.
Recurrence prophylaxis: penicillin V 250 mg BD long-term for ≥2 episodes in 12 months in same area (PATCH trials).