Cellulitis

Cellulitis describes a deep infection of connective tissue, usually skin and subcutaneous tissues, in which there is obvious oedema (1,2,3). Erysipelas is a form of cellulitis.

Aetiology

• Streptococcus pyogenes (Group A streptococcus) and Staphylococcus aureus are the primary causative organisms for cellulitis (1,2)
  • the microbiological spectrum of cellulitis is wider than erysipelas, occasionally including other Gram-positive, Gram-negative, or anaerobic micro-organisms (3)
• Streptococcus pyogenes is the major agent of erysipelas
  • Staphylococcus aureus is a minor cause

Clinical Features

• cellulitis presents as a hot, raised, tender area of skin whose margin is less well demarcated than in erysipelas
• presentation is commonly unilateral, and an abrasion or skin breakdown usually allows the infective organism to enter
• systemic features such as fever, rigors, and malaise may be present

Differential Diagnosis/Investigations

• exclude non-infectious causes and inflammatory reactions (1, 2)
  • up to a third of suspected lower-limb cellulitis cases are misdiagnosed mimics (e.g., stasis dermatitis or venous oedema) (3)
• full blood count and blood cultures may be considered if appropriate (e.g., severe infection or systemically unwell) (3)
• routine swab analysis is not indicated unless there is open purulence or broken skin (2)

Treatment

• offer systemic antibiotics, taking into account severity, site of infection, risk of uncommon pathogens, microbiological results, and MRSA status (2)
  • first-line oral therapy for uncomplicated cellulitis is typically flucloxacillin monotherapy (5–7 days) (1, 2)
  • alternative options for penicillin allergy include clarithromycin, erythromycin (in pregnancy), or doxycycline (1, 2)
• avoid routine dual coverage (e.g., adding MRSA agents to standard beta-lactams) for uncomplicated non-purulent cases, as it does not improve outcomes (3)
• severe cases may require parenteral antibiotics (e.g., IV flucloxacillin or co-amoxiclav) and hospital assessment (1, 2, 3)

Key Clinical Points

• consider marking the extent of infection with a single-use surgical marker pen (2)
• advise patients that completing a standard 5-day course of curative antibiotic treatment will not generally mean all clinical symptoms have resolved
  • full symptomatic resolution at 5 to 7 days is not expected, and residual erythema or inflammation can take up to 10–14 days to fully clear despite successful treatment (3)
  • trial data cited in the literature demonstrate that only around 25% of patients achieve complete resolution of symptoms by day 7, with up to 30% still showing residual erythema or warmth at day 14 despite effective antibiotic therapy (3)
• infection around the eyes (periorbital/orbital) or nose is more concerning due to the risk of serious intracranial complications (2)
• do not routinely offer antibiotics to prevent recurrent cellulitis or erysipelas after a single episode (2)
• address underlying predisposing factors (e.g., treating tinea pedis, managing chronic oedema, keeping skin moisturised) to prevent high recurrence rates (2, 3)

Reference:

1. Phoenix G, Das S, Joshi M. Diagnosis and management of cellulitis. BMJ. 2012 Aug 7;345
2. Public Health England (June 2021). Managing common infections: guidance for primary care
3. Long B, Yadav K, Rech M A, Gottlieb M. Advances in the diagnosis and management of skin and soft tissue infections. BMJ 2026; 394 :e100580