Cellulitis

What is cellulitis? What are cellulitis symptoms and signs?

Cellulitis is a bacterial infection of the skin and tissues beneath the skin. Cellulitis usually begins as a small area of tenderness, swelling, and redness that spreads to adjacent skin. As this red area begins to enlarge, the affected person may develop a fever -- sometimes with chills and sweats -- and swollen lymph nodes ("swollen glands") near the area of infected 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.

Where does cellulitis occur?

Cellulitis may occur anywhere on the body, but the lower leg is the most common site of the infection (particularly in the area of the tibia or shinbone and in the foot), 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, and pain in the involved tissues. Any skin wound or ulcer that exhibits these signs may be developing cellulitis.
Other forms of noninfected inflammation may mimic cellulitis. People with poor leg circulation, 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?

Sometimes cellulitis appears in areas where the skin has broken open, such as the skin near ulcers or surgical wounds. Many times, however, cellulitis occurs where there has been no break in the skin at all, such as with chronic leg swelling (edema). A pre-existing 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 or skin damage caused by radiation therapy can also increase a person's risk of contracting 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 depress the immune system) are particularly prone to developing cellulitis.
Although cellulitis can occur in people of any age, it is most common in middle-aged and elderly people.
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?

The majority of cellulitis infections are caused by 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 common in young children. The so-called "flesh-eating bacteria" are, in fact, also a strain of strep 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 and is 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.

Is cellulitis contagious?

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.
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. In many cases, treatment requires the administration of intravenous antibiotics in a hospital setting, since oral antibiotics may not always provide sufficient penetration of the injury 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.

Cellulitis At A Glance

• 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.
• Cellulitis is not contagious.
• Cellulitis is treated with oral or intravenous antibiotics.

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