Cellulitis is a spreading bacterial infection of the skin and the tissues immediately beneath the skin.
Cellulitis may be caused by many different bacteria. The most common are those of the Streptococcus species. Streptococci spread rapidly in the skin because they produce enzymes that hinder the ability of the tissue to confine the infection. Staphylococcus bacteria can also cause cellulitis, as can many other bacteria, especially after bites by humans or animals or after injuries in water or dirt.
Bacteria usually enter through small breaks in the epidermis that result from scrapes, punctures, burns, and skin disorders. Areas of the skin that become swollen with fluid (edema) are especially vulnerable. Cellulitis is more common in people with poor blood circulation (chronic venous insufficiency). However, cellulitis can also occur in skin that is not obviously injured.
Cellulitis most commonly develops on the legs but may occur anywhere. The first symptoms are redness, pain, and tenderness over an area of skin. These symptoms are caused both by the bacteria themselves and by the body's attempts to fight the infection. The infected skin becomes hot and swollen and may look slightly pitted, like an orange peel. Fluid-filled blisters, which may be small (vesicles) or large (bullae), sometimes appear on the infected skin. The borders of the affected area are not distinct, except in a form of cellulitis called erysipelas.
Most people with cellulitis feel only mildly ill, but some may have a fever, chills, rapid heart rate, headache, low blood pressure, and confusion.
As the infection spreads, nearby lymph nodes may become enlarged and tender (lymphadenitis), and the lymphatic vessels may become inflamed (lymphangitis—see Bacterial Skin Infections: Lymphangitis). Sometimes, bacteria spread through the blood (bacteremia—see Bacteremia, Sepsis, and Septic Shock: Bacteremia), which can cause more serious illness.
When cellulitis affects the same site repeatedly, especially the leg, lymphatic vessels may be damaged, causing permanent swelling of the affected tissue.
Diagnosis and Treatment
A doctor usually diagnoses cellulitis based on its appearance and symptoms. Laboratory identification of the bacteria from blood, pus, or tissue specimens usually is not necessary unless a person is seriously ill or the infection is not responding to drug therapy. Sometimes, doctors need to perform tests to differentiate cellulitis from a blood clot in the deep veins of the leg (deep vein thrombosis—see Venous Disorders: Deep Vein Thrombosis (DVT)), because the symptoms of these disorders are similar.
Prompt treatment with antibiotics can prevent the infection from spreading rapidly and reaching the blood and internal organs. Antibiotics that are effective against both streptococci and staphylococci (such as dicloxacillin or cephalexin) are used. People with mild cellulitis may take antibiotics by mouth. Those with rapidly spreading cellulitis, high fever, or other evidence of serious infection often receive intravenous antibiotics (such as oxacillin or nafcillin). Also, the affected part of the body, when possible, is kept immobile and elevated to help reduce swelling. Cool, wet dressings applied to the infected area may relieve discomfort.
Symptoms of cellulitis usually disappear after a few days of antibiotic therapy. However, symptoms often get worse before they get better probably because, with the death of the bacteria, substances that cause tissue damage are released. When this occurs, the body continues to react even though the bacteria are dead. Antibiotics are continued for 10 days or longer even though the symptoms may disappear earlier.
Last full review/revision October 2007 by A. Damian Dhar, MD, JD