What is cellulitis?
Cellulitis is a common bacterial infection of the skin, which can affect all ages. It usually affects a limb but can occur anywhere on the body. Symptoms and signs are usually localised to the affected area but patients can become generally unwell with fevers, chills and shakes (sepsis or bacteraemia).
Severe or rapidly progressive cellulitis may lead to septicaemia (blood poisoning), necrotising fasciitis (a more serious soft tissue infection), or endocarditis (heart valve infection).
Who gets cellulitis?
Cellulitis is more common in some situations. The following lists those who may be at greater risk of developing cellulitis.
- Previous episode(s) of cellulitis
- Venous disease (eg gravitational eczema, leg ulceration) and/or lymphoedema
- Current or prior injury (eg trauma, surgical wounds, radiotherapy)
- Tinea pedis (or athlete's foot) in the toes of the affected limb
Although many people attribute an episode of cellulitis to an unseen spider bite, they are falsely blamed. Documented spider bites have not led to the disease.
What are the clinical features of cellulitis?
The patient usually feels quite unwell and has fever and chills. Some or all of the following features may be seen over the affected skin.
- Increased warmth
- Erosions and ulceration
If there is no increased warmth over the skin it is unlikely to be cellulitis.
Lymphangitis is a red line originating from the cellulitis and leading to tender swollen lymph glands draining the affected area (eg in the groin with a leg cellulitis). It is caused by infection within the lymph vessels.
After successful treatment, the skin may flake or peel off as it heals.
What causes cellulitis?
Cellulitis is caused by bacterial infection. It can occur by itself, or complicate an underlying skin condition or wound. The most common infecting organisms are Streptococcus pyogenes (two thirds of cases) and Staphylococcus aureus (one third). Rare causes of cellulitis include:
- Pseudomonas aeruginosa, particularly following a puncture wound involving the foot or hand
- Haemophilus influenzae in children with facial cellulitis
- Anaerobes, Eikenella, Streptococcus viridans from human bites
- Pasteurella multocida from cat or dog bites
- Vibrio vulnificus from salt water exposure eg following coral injury
- Aeromonas hydrophila from fresh or salt water exposure eg following leech bites
- Erysipelothrix (erysipeloid) affecting a butcher
How is the diagnosis made?
The diagnosis of cellulitis is based on the clinical features. If any pustules, crusts or erosions are present, a swab should be taken for culture. A complete blood count is likely to show leukocytosis (raised white cell count). Elevated C-reactive protein is usual. Blood cultures may be of use if a patient has a high fever or is otherwise very unwell.
Occasionally further investigations are required to rule out other possible diagnoses such as deep vein thrombosis of the leg, radiation damage following radiotherapy, or inflammatory breast cancer (carcinoma erysipeloides).
The patient should be assessed for signs of sepsis, recognised by fever, malaise, loss of appetite, nausea, lethargy, headache, aching muscles and joints. Serious infection leads to hypotension (low blood pressure, collapse), reduced capillary circulation, heart failure, diarrhoea, gastrointestinal bleeding, renal failure and loss of consciousness.
Bacterial infection below the skin is called necrotising fasciitis and is potentially very serious. It is recognised by severe pain, skin pallor, local anaesthesia, purpura, ulceration and necrosis.
Blood tests show raised white cell count (mainly neutrophils) and raised C-reactive protein. On the whole, the greater the count, the more serious the infection.
What is the differential diagnosis of cellulitis?
It is not infrequent that cellulitis is diagnosed and treated when an inflammatory skin disease is actually responsible for redness and swelling. Common conditions causing "pseudocelluitis" include:
- Eczema/dermatitis due to stasis, contact factors
- Fungal infection eg tinea corporis, tinea pedis
- Drug eruption
- Insect bites and stings
What is the treatment for cellulitis?
Cellulitis is potentially serious and should be assessed by a medical practitioner promptly. The management of cellulitis is becoming more complicated due to rising rates of methicillin-resistant Staphylococcus aureus (MRSA) and macrolide- or erythromycin-resistant Streptococcus pyogenes.
Oral antibiotics used commonly are penicillin, flucloxacillin, dicloxacillin, cefuroxime or erythromycin. The usual intravenous antibiotics used are penicillin-based antibiotics (eg penicillin G or flucloxacillin) or cephalosporins (eg ceftriaxone, cefotaxime or cefazolin) for a few days. Sometimes oral probenecid is added to maintain antibiotic levels in the blood. Knowledge of local organisms and resistance patterns is essential in selecting appropriate antimicrobial therapy.
Most patients can be treated with oral antibiotics at home, usually for 5 to 10 days. However if there are signs of systemic illness or extensive cellulitis, treatment may require intravenous antibiotics either as an outpatient or in hospital. Treatment for uncomplicated cellulitis is usually for 10 to 14 days but antibiotics should be continued until all signs of infection have cleared (redness, pain and swelling) – sometimes for several months.
Oral antibiotics used commonly are penicillin, flucloxacillin, dicloxacillin, cefuroxime or erythromycin. The usual intravenous antibiotics used are penicillin-based antibiotics (eg penicillin G or flucloxacillin) or cephalosporins (eg cefotaxime, ceftriazone or cefazolin) for a few days. Sometimes oral probenecid is added to maintain antibiotic levels in the blood.
In situations where a broader antibiotic cover is required, for example a diabetic patient with a foot ulcer complicated by cellulitis, amoxicillin and clavulanic acid may be used. Clindamycin, sulfamethoxazole/trimethoprim, doxycycline and vancomycin are alternative antibiotics in patients with serious penicillin or cephalosporin allergy, or where infection with methicillin-resistant Staphylococcus aureus is suspected.
The affected limb should be elevated, and the patient should rest.
If infection is serious, the patient should be admitted to hospital urgently and resuscitated with fluids, oxygen and intraveonous antibiotics.
What should I do if I have recurrent cellulitis?
Patients with recurrent cellulitis should
- Avoid trauma, wear long sleeves and pants in high risk activities eg gardening
- Keep skin clean and well moisturised, with nails well tended
- Avoid having blood tests taken from the affected limb
- Treat fungal infections of hands and feet early
- Keep swollen limbs elevated during rest periods to aid lymphatic circulation. Those with chronic lymphoedema may benefit from compression garments.
On DermNet NZ:
- Streptococcal skin infections
- Wound infection
- Bacterial infections online course for health professionals
- Cellulitis – Medline Plus
- Cellulitis – Medscape Reference
- Cellulitis – emedicinehealth
- Patient information: Cellulitis and erysipelas (skin infections) (The Basics) – UpToDate (for subscribers)
- Patient information: Skin and soft tissue infection (cellulitis) (Beyond the Basics) – UpToDate (for subscribers)
Books about skin diseases:
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