Medical Author : Melissa Conrad Stöppler, MD
Medical Editor : William C. Shiel Jr., MD, FACP, FACR
- Cellulitis is a spreading bacterial infection of the skin and tissues beneath the skin.
- Staphylococcus and Streptococcus are the types of bacteria that are usually responsible for cellulitis, although many types of bacteria can cause the condition.
- Sometimes cellulitis appears in areas where the skin has broken open, such as the skin near ulcers or surgical wounds.
- Symptoms and signs include redness, tenderness, swelling, and warmth of the affected area.
- Cellulitis is not contagious.
- Cellulitis is treated with oral or intravenous antibiotics.
What is cellulitis?
Cellulitis is a bacterial infection of the skin and tissues beneath the skin. Unlike impetigo, which is a very superficial skin infection, cellulitis is an infection that also involves the skin's deeper layers: the dermis and subcutaneous tissue.
The main bacteria responsible for cellulitis are Streptococcus and Staphylococcus ("staph"), the same bacteria that can cause impetigo. MRSA (methicillin-resistant Staph aureus) can also cause cellulitis. Sometimes, other bacteria (for example, Hemophilus influenzae, Pneumococcus, and Clostridium species) may cause cellulitis as well.Cellulitis is fairly common and affects people of all races and ages. Men and women appear to be equally affected. Although cellulitis can occur in people of any age, it is most common in middle-aged and elderly people. Cellulitis is not contagious.
What are cellulitis symptoms and signs?
Where does cellulitis occur?
Cellulitis may occur anywhere on the body; the legs are a common location. The lower leg is the most common site of the infection (particularly in the area of the tibia or shinbone and in the foot; see the illustration below), followed by the arm, and then the head and neck areas. In special circumstances, such as following surgery or trauma wounds, cellulitis can develop in the abdomen or chest areas. People with morbid obesity can also develop cellulitis in the abdominal skin. Special types of cellulitis are sometimes designated by the location of the infection. Examples include periorbital (around the eye socket) cellulitis, buccal (cheek) cellulitis, facial cellulitis, and perianal cellulitis.
What does cellulitis look like?
The signs of cellulitis include redness, warmth, swelling, tenderness, and pain in the involved tissues. Any skin wound or ulcer that exhibits these signs may be developing cellulitis.
Other forms of noninfectious inflammation may mimic cellulitis. People with poor circulation in the legs, for instance, often develop scaly redness on the shins and ankles; this is called "stasis dermatitis" and is often mistaken for the bacterial infection of cellulitis.
What are cellulitis risk factors?
Most commonly, cellulitis develops in the area of a break in the skin, such as a cut, small puncture wound, or insect bite. In some cases when cellulitis develops without an apparent skin injury, it may be due to microscopic cracks in the skin that are inflamed or irritated. It may also appear in the skin near ulcers or surgical wounds.
In other circumstances, cellulitis occurs where there has been no skin break at all, such as with chronic leg swelling (edema). A preexisting skin infection, such as athlete's foot (tinea pedis) or impetigo can predispose to the development of cellulitis. Likewise, inflammatory conditions of the skin like eczema, psoriasis, or skin damage caused by radiation therapy can lead to cellulitis.
People who have diabetes or conditions that compromise the function of the immune system (for example, HIV/AIDS or those receiving chemotherapy or drugs that suppress the immune system) are particularly prone to developing cellulitis.
Conditions that reduce the circulation of blood in the veins or that reduce circulation of the lymphatic fluid (such as venous insufficiency, obesity, pregnancy, or surgeries) also increase the risk of developing cellulitis.
What causes cellulitis? Is cellulitis contagious?
The majority of cellulitis infections are caused by infection with either strep (Streptococcus) or staph (Staphylococcus) bacteria.
The most common bacteria that cause cellulitis are beta-hemolytic streptococci (groups A, B, C, G, and F). A form of rather superficial cellulitis caused by strep is called erysipelas and is characterized by spreading hot, bright red circumscribed area on the skin with a sharp, raised border. Erysipelas is more common in young children. The so-called "flesh-eating bacteria" are, in fact, also a strain of strep bacteria that can sometimes rapidly destroy tissues underneath the skin.
Staph (Staphylococcus aureus), including methicillin-resistant strains (MRSA), is another common type of bacteria that causes cellulitis. There is a growing incidence of community-acquired infections due to methicillin-resistant S. aureus (MRSA), a particularly dangerous form of this bacteria that is resistant to many antibiotics, including methicillin, and is therefore more difficult to treat.
Cellulitis can be caused by many other types of bacteria. In children under 6 years of age, H. flu (Hemophilus influenzae) bacteria can cause cellulitis, especially on the face, arms, and upper torso. Cellulitis from a dog or cat bite or scratch may be caused by the Pasteurella multocida bacteria, which has a very short incubation period of only four to 24 hours. Aeromonas hydrophilia, Vibrio vulnificus, and other bacteria are causes of cellulitis that develops after exposure to freshwater or seawater. Pseudomonas aeruginosa is another type of bacteria that can cause cellulitis, typically after a puncture wound.
Cellulitis is not contagious because it is an infection of the skin's deeper layers (the dermis and subcutaneous tissue), and the skin's top layer (the epidermis) provides a cover over the infection. In this regard, cellulitis is different from impetigo, in which there is a very superficial skin infection that can be contagious.
How is cellulitis diagnosed, and what is the treatment for cellulitis?
First, it is crucial for the doctor to distinguish whether or not the inflammation is due to an infection. The history and physical exam can provide clues in this regard, as can sometimes an elevated white blood cell count. A culture for bacteria may also be of value, but in many cases of cellulitis, the concentration of bacteria may be low and cultures fail to demonstrate the causative organism. In this situation, cellulitis is commonly treated with antibiotics that are designed to eradicate the most likely bacteria to cause the particular form of cellulitis.
When it is difficult or impossible to distinguish whether or not the inflammation is due to an infection, doctors sometimes treat with antibiotics just to be sure. If the condition does not respond, it may need to be addressed by different methods dealing with types of inflammation that are not infected. For example, if the inflammation is thought to be due to an autoimmune disorder, treatment may be with a corticosteroid.
Antibiotics, such as derivatives of penicillin or other types of antibiotics that are effective against the responsible bacteria, are used to treat cellulitis. If the bacteria turn out to be resistant to the chosen antibiotics, or in patients who are allergic to penicillin, other appropriate antibiotics can be substituted. Sometimes the treatment requires the administration of intravenous antibiotics in a hospital setting, since oral antibiotics may not always provide sufficient penetration of the inflamed tissues to be effective. In certain cases, intravenous antibiotics can be administered at home.
In all cases, physicians choose a treatment based upon many factors, including the location and extent of the infection, the type of bacteria causing the infection, and the overall health status of the patient.
Can cellulitis be prevented?
Under some circumstances, cellulitis can be prevented by proper hygiene, treating chronic swelling of tissues (edema), care of wounds or cuts. In other cases, microscopic breaks in the skin may not be apparent and infection may develop. In general, cellulitis in a healthy person with an intact immune system is preventable by avoiding skin surface wounds. In people with predisposing conditions (see above) and/or weakened immune systems, cellulitis may not always be preventable.
What is the outlook/prognosis for cellulitis? What are complications of cellulitis?
Cellulitis is a treatable condition, but antibiotic treatment is necessary to eradicate the infection and avoid complications and spread of the infection. Most cellulitis can be effectively treated with oral antibiotics at home. Sometimes hospitalization and intravenous antibiotics are required if oral antibiotics are not effective. Sepsis is a serious complication of cellulitis. If not properly treated, cellulitis can occasionally spread to the bloodstream and cause a serious bacterial infection of the bloodstream that spreads throughout the body (sepsis).
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