Summary

Cellulitis and erysipelas are common acute bacterial skin and soft tissue infections (SSTIs). Cellulitis involves the deeper dermis and subcutaneous fat with ill-defined spreading margins, while erysipelas is more superficial, involving the upper dermis and lymphatics with characteristically well-demarcated, raised edges. Both typically present with erythema, warmth, swelling, and tenderness, often with systemic symptoms. Group A Streptococcus (Streptococcus pyogenes) is the most common causative organism, followed by Staphylococcus aureus. Most cases respond to oral antibiotics (flucloxacillin), but severe infections require IV therapy. Critically, cellulitis must be differentiated from necrotising fasciitis, a surgical emergency.

Key Facts

• Definition: Acute spreading bacterial infection of skin and subcutaneous tissue
• Incidence: 200 per 100,000 person-years; 2-4% of emergency department presentations
• Most common organisms: Streptococcus pyogenes (Group A Strep) and Staphylococcus aureus
• Most common site: Lower limb (70-80%)
• Key differentiator: Erysipelas has raised, sharply demarcated edges; cellulitis has ill-defined margins
• First-line treatment: Flucloxacillin 500mg-1g QDS orally (or IV if severe)
• Critical mimic: Necrotising fasciitis (pain out of proportion, rapid spread)

Clinical Pearls

Eron Classification: Use this to guide oral vs IV antibiotics and disposition. Class I-II can be managed as outpatients; Class III-IV need admission.

Draw the Line: Mark the leading edge of erythema with permanent marker and timestamp it. This is the most reliable way to track progression or response to treatment.

Treat the Portal: Most lower limb cellulitis occurs due to tinea pedis (athlete's foot) or leg ulcers providing entry for bacteria. Always check between the toes and treat any interdigital maceration.

Why This Matters Clinically

Cellulitis is the 4th most common reason for emergency hospital admission for infection. While most cases are straightforward, failure to recognise necrotising fasciitis leads to delayed surgery and high mortality (30-40%). Recurrent cellulitis occurs in 20-30% of patients, often due to untreated risk factors like lymphoedema or tinea pedis.

Structured Approach

General:

• Observations: Temperature, HR, BP, RR, SpO2 (sepsis screening)
• General appearance: Ill/toxic vs well
• NEWS score if in hospital setting

Local Skin Examination:

• Inspect: Define extent of erythema, demarcation, surface changes
• Palpate: Warmth, tenderness, oedema (pitting?), crepitus
• Mark the edge: Use permanent marker with date/time
• Measure: Size of area affected

Portal of Entry Search:

• Interdigital spaces: Look for tinea pedis (maceration, scaling)
• Leg ulcers: Venous or arterial
• Wounds, insect bites, abrasions
• Postoperative incisions (if recent surgery)

Regional Examination:

• Lymph nodes: Inguinal (lower limb), axillary (upper limb), cervical (facial)
• Lymphangitis: Red streaking toward lymph nodes

Special Tests

Natural History

• Without treatment: Progressive spread, systemic sepsis, potential for necrotising fasciitis
• With treatment: Resolution within 5-10 days; most respond to oral antibiotics

Outcomes with Treatment

Prognostic Factors

Good Prognosis:

• Young, fit patient
• First episode
• No comorbidities
• Portal of entry addressed (tinea treated)
• Early presentation

Poor Prognosis:

• Diabetes mellitus
• Immunosuppression
• Lymphoedema
• Prior cellulitis at same site
• Delay in treatment
• Advancing age

Key Guidelines

1. NICE NG141 (2019) — Cellulitis and erysipelas: antimicrobial prescribing. Recommends flucloxacillin first-line; 5-7 day course. NICE NG141
2. CREST Guidelines (2005) — Guidelines on the management of cellulitis in adults. Early UK regional guidance, widely influential. CREST
3. PHE/NICE Antimicrobial Prescribing (2019) — Antibiotic choice based on local resistance patterns. PHE

Landmark Trials

PATCH I Trial (Thomas et al., 2011) — Prophylactic antibiotics for recurrent cellulitis

• 274 patients with recurrent cellulitis randomised to penicillin V or placebo
• Key finding: Penicillin reduced recurrence by 45% while on treatment
• Clinical Impact: Supports prophylactic antibiotics in recurrent cases

PATCH II Trial (2017) — Follow-up of PATCH I

• Extended analysis showed effect wanes after stopping prophylaxis
• Clinical Impact: Raises question of appropriate duration of prophylaxis

Kilburn et al. (2010) — Eron classification validation

• Demonstrated that Eron classification predicts need for hospitalisation and IV therapy
• Clinical Impact: Widely adopted for severity stratification

Evidence Strength

What is Cellulitis?

Cellulitis is a skin infection caused by bacteria that have got under the skin, usually through a small cut, insect bite, or cracked skin (like athlete's foot between your toes). The skin becomes red, swollen, warm, and painful. It can spread if not treated and sometimes makes you feel generally unwell with a fever.

Is it serious?

For most people, cellulitis is uncomfortable but treatable with antibiotics. However, if it spreads quickly, causes severe pain, or you become very unwell, it can be a sign of a more serious infection that needs urgent hospital treatment.

How is it treated?

1. Antibiotics: Most people take tablets (usually flucloxacillin) for 5-7 days. If the infection is more severe, you may need antibiotics through a drip in hospital.
2. Resting the area: Keeping the affected limb raised (e.g., leg on a stool) helps reduce swelling.
3. Treating the cause: If you have athlete's foot or a skin break that let the bacteria in, this should also be treated to prevent recurrence.
4. Marking the edge: The doctor may draw around the red area with a pen to track whether it's getting better or worse.

What to expect

• The area may look worse initially before improving (redness may deepen as inflammation resolves)
• Improvement is usually seen within 2-3 days of starting antibiotics
• Full resolution takes 5-10 days
• There may be some residual discolouration or swelling for weeks

When to seek help

Go back to your doctor or seek urgent medical attention if:

• The redness is spreading rapidly (faster than you can mark it)
• The pain is getting much worse despite painkillers
• You develop new blisters, blackish discolouration, or the skin looks dusky
• You feel increasingly unwell (fever, shivering, confusion)
• The antibiotics aren't helping after 2-3 days