Cellulitis is a bacterial infection of the skin and the tissues beneath it, which usually affects the lower legs but can occur anywhere on the body. Cellulitis is most commonly caused by streptococci bacteria, which enter the skin via a wound.


There may be fever and chills; and the affected area is hot, red and swollen. Cellulitis is more severe in people with reduced immunity, such as those who have an immunodeficiency disorderUntreated cellulitis at the site of a wound may progress to bacteraemia (bacterial infection of the blood) and septicaemia (blood poisoning). Facial infections may spread to the eye socket.


Treatment of cellulitis is with an antibiotic drug such as a penicillin drug or erythromycin. (See also erysipelas.)

Cellulitis in more detail - non-technical


Cellulitis is a spreading bacterial infection just below the skin surface. It is most commonly caused by Streptococcus pyogenes or Staphylococcus aureus.


The word ‘‘cellulitis’’ actually means ‘‘inflammation of the cells.’’ Specifically, cellulitis refers to an infection of the tissue just below the skin surface. In humans, the skin and the tissues under the skin are the most common locations for microbial infection. Skin is the first defense against invading bacteria and other microbes. An infection can occur when this normally strong barrier is damaged due to surgery, injury, or a burn. Even something as small as a scratch or an insect bite allows bacteria to enter the skin, which may lead to an infection. Usually, the immune system kills any invading bacteria, but sometimes the bacteria are able to grow and cause an infection.

Once past the skin surface, the warmth, moisture, and nutrients allow bacteria to grow rapidly. Disease causing bacteria release proteins called enzymes which cause tissue damage. The body’s reaction to damage is inflammation which is characterized by pain, redness, heat, and swelling. This red, painful region grows bigger as the infection and resulting tissue damage spread. An untreated infection may spread to the lymphatic system (acute lymphangitis), the lymph nodes (lymphadenitis), the bloodstream (bacteremia), or into deeper tissues. Cellulitis most often occurs on the face, neck, and legs.

Orbital cellulitis

A very serious infection, called orbital cellulitis, occurs when bacteria enter and infect the tissues surrounding the eye. In 50–70% of all cases of orbital cellulitis, the infection spreads to the eye(s) from the sinuses or the upper respiratory tract (nose and throat). Twenty-five percent of orbital infections occur after surgery on the face. Other sources of orbital infection include a direct infection from an eye injury, from a dental or throat infection, and through the bloodstream.

Infection of the tissues surrounding the eye causes redness, swollen eyelids, severe pain, and causes the eye to bulge out. This serious infection can lead to a temporary loss of vision, blindness, brain abscesses, inflammation of the brain and spinal tissues (meningitis), and other complications. Before the discovery of antibiotics, orbital cellulitis caused blindness in 20% of patients and death in 17% of patients. Antibiotic treatment has significantly reduced the incidence of blindness and death.

Causes and symptoms

Although other kinds of bacteria can cause cellulitis, it is most often caused by Streptococcus pyogenes (the bacteria that causes strep throat) and Staphylococcus aureus. Streptococcus pyogenes is the so-called ‘‘flesheating bacteria’’ and, in rare cases, can cause a dangerous, deep skin infection called necrotizing fasciitis. Orbital cellulitis may be caused by bacteria which cannot grow in the presence of oxygen (anaerobic bacteria). In children, Haemophilus influenzae type B frequently causes orbital cellulitis following a sinus infection.

Streptococcus pyogenes can be picked up from a person who has strep throat or an infected sore. Other cellulitis-causing bacteria can be acquired from direct contact with infected sores. Persons who are at a higher risk for cellulitis are those who have a severe underlying disease (such as cancer, diabetes, and kidney disease), are taking steroid medications, have a reduced immune system (because of AIDS, organ transplant, etc.), have been burned, have insect bites, have reduced blood circulation to limbs, or have had a leg vein removed for coronary bypass surgery. In addition, chicken pox, human or animal bite wounds, skin wounds, and recent surgery can put a person at a higher risk for cellulitis.

The characteristic symptoms of cellulitis are redness, warmth, pain, and swelling. The infected area appears as a red patch that gets larger rapidly within the first 24 hours. A thick red line which progresses toward the heart may appear indicating an infection of the lymph vessels (lymphangitis). Other symptoms which may occur include fever, chills, tiredness, muscle aches, and a general ill feeling. Some people also experience nausea, vomiting, stiff joints, and hair loss at the infection site.

The characteristic symptoms of orbital cellulitis are eye pain, redness, swelling, warmth, and tenderness. The eye may bulge out and it may be difficult or impossible to move. Temporary loss of vision, pus drainage from the eye, chills, fever, headaches, vomiting, and a general ill feeling may occur.


Cellulitis may be diagnosed and treated by a family doctor, an infectious disease specialist, a doctor who specializes in skin diseases (dermatologist), or in the case of orbital cellulitis, an eye doctor (ophthalmologist). The diagnosis of cellulitis is based mainly on the patient’s symptoms. The patient’s recent medical history is also used in the diagnosis.

Laboratory tests may be done to determine which kind of bacteria is causing the infection but these tests are not always successful. If the skin injury is visible, a sterile cotton swab is used to pick up a sample from the wound. If there is no obvious skin injury, a needle may be used to inject a small amount of sterile salt solution into the infected skin, and then the solution is withdrawn. The salt solution should pick up some of the bacteria causing the infection. A blood sample may be taken from the patient’s arm to see if bacteria have entered the bloodstream. Also, a blood test may be done to count the number of white blood cells in the blood. High numbers of white blood cells suggest that the body is trying to fight a bacterial infection.

For orbital cellulitis, the doctor may often perform a special x-ray scan called computed tomography scan (CT). This scan enables the doctor to see the patient’s head in cross-section to determine exactly where the infection is and see if any damage has occurred. A CT scan takes about 20 minutes.


Antibiotic treatment is the only way to battle this potentially life-threatening infection. Mild to moderate cellulitis can be treated with the following antibiotics taken every four to eight hours by mouth:

  • penicillin v (Bicillin, Wycillin, Pen Vee, V-Cillin)
  • erythromycin (E-Mycin, Ery-Tab)
  • cephalexin (Biocef, Keflex)
  • flucloxacillin (Floxapen) cloxacillin (Tegopen)
  • co-amoxiclav (Augmentin)

Other medications may be recommended, such as acetaminophen (Tylenol) or ibuprofen (Motrin, Advil) to relieve pain, and aspirin to decrease fever.

A normally healthy person is usually not hospitalized for mild or moderate cellulitis. General treatment measures include elevation of the infected area, rest, and application of warm, moist compresses to the infected area. The doctor will want to see the patient again to make sure that the antibiotic treatment is effective in stopping the infection.

Persons at high risk for severe cellulitis will probably be hospitalized for treatment and monitoring. Antibiotics may be given intravenously to patients with severe cellulitis. Complications such as deep infection, or bone or joint infections, might require surgical drainage and a longer course of antibiotic treatment. Extensive tissue destruction may require plastic surgery to repair. In cases of orbital cellulitis caused by a sinus infection, surgery may be required to drain the sinuses.


Over 90% of all cellulitis cases are cured after seven to ten days of antibiotic treatment. Persons with serious disease and/or those who are taking immunosuppressive drugs may experience a more severe form of cellulitis which can be life threatening. Serious complications include blood poisoning (bacteria growing in the blood stream), meningitis (brain and spinal cord infection), tissue death (necrosis), and/or lymphangitis (infection of the lymph vessels). Severe cellulitis caused by Streptococcus pyogenes can lead to destructive and life-threatening necrotizing fasciitis.


Cellulitis may be prevented by wearing appropriate protective equipment during work and sports to avoid skin injury, cleaning cuts and skin injuries with antiseptic soap, keeping wounds clean and protected, watching wounds for signs of infection, taking the entire prescribed dose of antibiotic, and maintaining good general health. Persons with diabetes should try to maintain good blood sugar control.

Key terms 

  • Inflammation—A local, protective response to tissue injury. It is characterized by redness, warmth, swelling, and pain.
  • Necrotizing fasciitis—A destructive infection which follows severe cellulitis and involves the deep skin and underlying tissues.
  • Sinuses—Air cavities found in the bones of the head. The sinuses which are connected to the nose are prone to infection.

Cellulitis - technical 

Cellulitis is a superficial infection that spreads laterally in the upper dermis or along the subcutaneous fascia is known as cellulitis. The main cause is infection with β-haemolytic streptococci (also see: streptococci and enterococci), but other common causes include S. aureus. Cellulitis starts as a zone of spreading erythema and tenderness with other signs of inflammation such as increased surface temperature. With streptococcal infections the patient is often systemically unwell with fever and chills. The main sites for infection are the face or the lower legs. Cellulitic infections affecting the dermis and upper subcutaneous tissue are sometimes known as erysipelas. Non-group A streptococci are also sometimes responsible.

Complications include the development of septicaemia or encroachment on adjacent structures such as the orbital cavernous sinus leading to thrombosis. Rare causes of the same syndrome include zygomycete fungi. Cellulitis requires immediate antibiotic therapy with an oral penicillin such as flucloxacillin or a macrolide antibiotic, and in systemically unwell patients with an intravenous regime. Recurrent attacks are also seen, particularly on the limbs and are often associated with lymphoedema. Management is difficult as local antisepsis and improved drainage by themselves do not appear to prevent recurrences and long-term oral penicillin V (phenoxymethylpenicillin) is often necessary for recurrent disease. However, in lower limb cellulitis there is an association with skin lesions such as the cracks caused by athlete’s foot and treatment aimed to heal local skin defects, e.g. antifungals, is indicated.

In the early phases, these infections are easily confused with early necrotizing fasciitis (see: streptococci and enterococci) where the surrounding erythema spreads slowly but the patient remains unwell and the overlying skin becomes hypoaesthetic. Necrotizing fasciitis can be caused by group A streptococci but also by mixed bacterial infections after surgery or a compound fracture. The spreading erythema, dull ache, systemic reaction, and the reduction in overlying sensation should alert the physician. Surgical exploration is warranted, which then reveals necrotic and oedematous reaction in the underlying dermis and deeper fascial planes. Treatment is by early surgical debridement, which is an essential part of management in addition to systemic antibiotics.