What is cellulitis?
Cellulitis is an infection of the deeper layers of skin causing infected areas to look red and swollen and to feel painful and hot.
Causes of cellulitis include streptococcus bacteria.
Cellulitis image: Dr P. Marazzi/Photo Researchers Inc
Where does cellulitis occur?
Some cases of cellulitis appear on areas of trauma, 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. In such cases, it is difficult to know where the bacteria came from. Patients who have diabetes or impairment of the immune system - for example, from HIV/AIDS or from drugs that depress the immune system - are particularly prone to developing cellulitis.
What does cellulitis look like?
The signs are those of any inflammation - redness, warmth, swelling, and pain. Any skin wound or ulcer that exhibits these signs may be developing cellulitis.
Other forms of non-infected 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 cellulitis.
What causes cellulitis?
Cellulitis can be caused by many types of bacteria. The most common is staph (Staphylococcus aureus), followed by strep (Group A Streptococcus). A form of cellulitis caused by strep is called erysipelas - it is characterised by a spreading hot, bright red circumscribed area on the skin with a sharp raised border. Cellulitis from a dog or cat bite or scratch may be caused by the Pasturella multocida bacteria, which has a short incubation period of between four and 24 hours. Cellulitis after an injury from a saltwater fish or shellfish - like a fish bite or a crab pinch - can be due to the Erysipelothrix rhusiopathiae bacteria. These same bacteria can also cause cellulitis after a skin injury on a farm, especially if working with pigs or poultry.
Is cellulitis contagious?
Cellulitis is not contagious because it is an infection of the skin's deeper layers, the dermis and subcutaneous tissue. The skin's top layer (the epidermis) provides a cover over the infection. In this regard, cellulitis is different from impetigo, which is a superficial infection and is contagious.
How is cellulitis treated?
First, it is crucial for the doctor to distinguish whether or not the skin inflammation is due to an infection. The history and physical examination can provide clues in this regard, as can the white blood cell count. A culture for bacteria may also be of value.
When it is difficult to know if the inflammation is caused by 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. For example, if the inflammation is thought to be due to an autoimmune disorder, treatment may be with a corticosteroid.