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What is cellulitis?
Cellulitis is a bacterial infection of the skin and underlying layers. It can occur in anyone and on any area of the body, but is most common on the lower legs. Skin affected by cellulitis quickly becomes red, swollen, hot and tender.
The skin is made up of three main layers: the epidermis (outer layer), dermis (middle layer) and subcutis (bottom layer). Cellulitis occurs when bacteria enter the dermis through a break in the skin, such as a cut, sore, scratch or burn. The two most common types of bacteria that cause cellulitis are Streptococcus pyogenes and Staphylococcus aureus.
Cellulitis caused by Streptococcus pyogenes is able to spread easily because these bacteria produce a substance that prevents the skin from controlling the infection.
Staphylococcus aureus usually causes cellulitis through an infection of open or pus-filled wounds. In recent times, a strain of the bacteria, called methicillin-resistant Staphylococcus aureus (MRSA), has become a more common cause of cellulitis. The term methicillin-resistant means these bacteria are no longer destroyed by once-effective penicillin-based antibiotics.
In rare cases, cellulitis can also be caused by different types of bacteria that enter the skin in a specific way. For example, cellulitis can arise from an animal bite (Pasteurella multocida), exposure to contaminated salt water (Vibrio vulnificus) or fresh water (Aeromonas hydrophila), or a puncture wound, usually to the hands or feet (Pseudomonas aeruginosa).
Rarer still is cellulitis caused by infection of the skin with a fungus. However, fungal cellulitis usually only occurs in people with a weakened immune system.
Risk factors that increase the likelihood of developing cellulitis include:
- Existing skin wounds or ulcers;
- A history of cellulitis;
- Poor circulation;
- Intravenous drug use;
- A weakened immune system;
- A skin condition that causes dryness and cracking, such as athlete's foot or eczema;
- Certain other medical conditions, such as diabetes, chickenpox or shingles, and;
- Swelling from a build-up of lymph fluid (lymphoedema), particularly in the legs and arms.
Signs and symptoms
In general, the signs and symptoms of cellulitis include:
- Skin that is red, swollen, hot and tender;
- Small red spots appearing on top of the red skin;
- Fever and chills;
- Skin blisters that may leak fluid or pus, and;
- Swollen lymph nodes.
Methods for diagnosis
Your doctor will most likely diagnose cellulitis by looking closely at the skin and asking questions about symptoms. You may also have a line drawn with a pen around the affected area of skin to track the spread of infection.
If you are seriously ill or have a weakened immune system, your doctor may swab your wounds or collect a small amount of blood. These samples are then examined in a laboratory for the presence of Streptococcus pyogenes or Staphylococcus aureus bacteria.
Sometimes, an X-ray or imaging scan may be requested to rule out other possible causes of skin redness and swelling, such as deep vein thrombosis (DVT).
Types of treatment
Treatment of cellulitis will depend on how severe the symptoms are. Your doctor may recommend one or more of the following options:
To relieve pain and fever, your doctor may suggest over-the-counter medications, such as paracetamol and ibuprofen. Plenty of water and rest can help to boost the immune system to naturally fight the infection. If your leg is affected by cellulitis, keeping it raised higher than the hips, by using pillows or a chair, may reduce the swelling. Similarly, if your arm is affected, the forearm may be raised above the elbow.
In most cases, cellulitis is treated with oral antibiotics for about 10 days. Antibiotics commonly prescribed for both Streptococcus pyogenes and Staphylococcus aureus bacterial infections include flucloxacillin or dicloxacillin. Less frequently, cephalexin or erythromycin may be given, especially if you are allergic to penicillin.
For severe cases of cellulitis that have spread or that occur deep in the skin, admission to a clinic or hospital may be necessary for treatment with intravenous antibiotics. Some examples of intravenous antibiotics for cellulitis include flucloxacillin, cefotaxime, ceftriaxone or cefazolin. Once symptoms improve, you will most likely be sent home and prescribed oral antibiotics.
Less frequently, clindamycin, doxycycline, sulfamethoxazole/trimethoprim or vancomycin may be prescribed if an infection with methicillin-resistant Staphylococcus aureus (MRSA) is suspected.
If you have a pus-filled wound (abscess) from cellulitis, your doctor may cut open the affected skin to drain out the fluid and bacteria.
Although most cases of cellulitis are successfully treated with antibiotics, complications can arise when the condition is left untreated or keeps coming back. However, complications are rare and tend to be more common in the elderly, young children or people with a weakened immune system.
If left untreated, infection can spread to the following areas:
- Blood (sepsis);
- Bone (osteomyelitis);
- Lymphatic system (lymphangitis);
- Deep layers of skin (necrotising fasciitis), and;
- Brain (meningitis).
Some signs that the infection has spread include fever, increased heart rate, rapid breathing, vomiting and confusion. Seek urgent medical attention if you develop any of these symptoms.
Other possible complications of cellulitis can include development of an abscess, permanent swelling or permanent death of tissues (gangrene). If gangrene is not treated quickly, the affected limb may need to be amputated.
The outlook for cellulitis is good in most cases if it is diagnosed and treated early with oral antibiotics. If intravenous treatment in a hospital or clinic is required, this is also likely to be successful. But again, early treatment is an important factor in reducing the risk of complications.
Antibiotics may produce an initial increase in skin redness before symptoms begin to fade. While most mild to moderate cases of cellulitis clear within about a week of starting treatment, antibiotics may need to be continued for longer until all signs of infection have cleared.
A number of steps can be taken to reduce the chance of developing cellulitis, particularly if you are at increased risk or have had the condition before. For example, maintaining good handwashing habits and keeping wounds clean can help to limit the spread of bacteria. Similarly, applying moisturiser regularly and treating underlying skin conditions promptly can reduce the chances of bacteria entering the body.
If cellulitis keeps coming back, your doctor may prescribe oral antibiotics on a long-term basis or to keep on stand-by in case of infection. For all types of antibiotic treatment, taking the full course prescribed by your doctor is important to ensure that the infection is completely cleared.