(See also Overview of Bacterial Skin Infections.)
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 and Staphylococcus species. Streptococci spread rapidly in the skin because they produce enzymes that hinder the ability of the tissue to confine the infection. Cellulitis that is caused by Staphylococcus bacteria usually occurs around open wounds and pus-filled pockets (abscesses). Many other bacteria can cause cellulitis. Recently, a strain of Staphylococcus that is resistant to previously effective antibiotics has become a more common cause of cellulitis. This strain is called methicillin-resistant Staphylococcus aureus (MRSA). People who are exposed in a hospital or nursing facility commonly acquire a particular strain of MRSA that may respond differently to antibiotics than other strains of MRSA.
Bacteria usually enter through small breaks in the skin that result from scrapes, punctures, surgery, burns, fungal infections (such as athlete's foot), and skin disorders. Areas of the skin that become swollen with fluid (edema) are especially vulnerable. However, cellulitis can also occur in skin that is not obviously injured.
Cellulitis most commonly develops on the legs but may occur anywhere. Cellulitis usually only affects one side of the body, such as one hand or one leg. The first symptoms of cellulitis 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. Some may have a fever, chills, rapid heart rate, headache, low blood pressure, and confusion, which usually indicates a severe infection.
A doctor usually diagnoses cellulitis based on its appearance and symptoms. Laboratory identification of the bacteria from skin, blood, pus, or tissue specimens (called a culture) usually is not necessary unless a person is seriously ill or has a weakened immune system or the infection is not responding to drug therapy. Sometimes, doctors need to do tests to differentiate cellulitis from a blood clot in the deep veins of the leg (deep vein thrombosis) because the symptoms of these disorders are similar.
Most cellulitis resolves quickly with antibiotic therapy. Occasionally, people develop abscesses. Serious but rare complications include severe skin infections that rapidly destroy tissue (called necrotizing skin infections) and spread of bacteria through the blood (bacteremia).
When cellulitis affects the same site repeatedly, especially the leg, lymphatic vessels may be damaged, causing permanent swelling of the affected tissue.
Prompt treatment with antibiotics can prevent the bacterial 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.
If doctors suspect methicillin-resistant Staphylococcus aureus (MRSA) infection, such as when pus is draining from under the skin or when other serious symptoms develop, treatment may include antibiotics such as trimethoprim with sulfamethoxazole, clindamycin, or doxycycline by mouth. People with mild cellulitis may take antibiotics by mouth.
People with rapidly spreading cellulitis, high fever, or other evidence of serious infection are hospitalized and given antibiotics by vein (such as oxacillin or nafcillin). Also, the affected part of the body is kept immobile and elevated to help reduce swelling. Cool, wet dressings applied to the infected area may relieve discomfort. Disorders that increase risk of developing cellulitis in the future (for example, athlete's foot) are treated.
Symptoms of cellulitis usually disappear after a few days of antibiotic therapy. However, cellulitis 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 release 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.
Abscesses are cut open and drained.