Here it is, Cellulitis in a nutshell

By Dr. Sanjaya Senanayake

Cellulitis is an infection that is the “bread and butter” of Infectious Diseases Physicians and other doctors worldwide. While it doesn’t evoke the spine-tingling tension of an Ebola outbreak or SARS, cellulitis is an important condition that affects countless numbers of people every year.

What is cellulitis?

First of all, cellulitis has got nothing to do with cellulite. So if you are seeking information on “thunder thighs” and their reduction, read no further! Cellulitis is infection of the tissue under the skin - the “subcutaneous” tissue. The infection tends to spread through special blood vessels in the body known as lymphatics.

Is it rare?

Not at all. Cellulitis would be one of the most common reasons for hospital admission amongst Infectious Diseases units.

Where do you get cellulitis?

By definition, cellulitis can affect any part of the body covered by skin. But, in practice, the most common part of the body affected by cellulitis is the leg; therefore, the emphasis of this article will be cellulitis in that part of the body.

Which bugs cause cellulitis?

Cellulitis is usually caused by bacteria that live on our skin. For the vast majority of our lives, these skin bacteria have been peacefully co-existing on us. But on some occasions, they can stir up trouble and cause cellulitis. Out of all our skin bacteria, the two most frequent culprits are Group A Streptococcus (also known as Streptococcus pyogenes) and Staphylococcus aureus (also known as “golden staph”). There are other causes which will be discussed below (see “Severe forms of cellulitis”).

How do you get cellulitis?

Sometimes, patient with cellulitis will recall a break in the skin that has become infected e.g. a cut, a scratch or a surgical wound. But more often than not, there is no history of trauma to the affected part of the body; however, there are a number of risk factors that predispose people to cellulitis.

What are the risk factors for cellulitis?

Tinea pedis

This is a common fungal infection also known as “Athlete’s Foot”. It causes a break in the skin between the toes but usually causes no symptoms. The fungus itself doesn’t cause cellulitis; but, the breach in the skin provides a portal of entry for skin bacteria to wreak their microbial mayhem, causing cellulitis.

Peripheral vascular disease (blocked arteries in the leg)

A healthy blood supply to the extremities is vital to prevent infection. People with blocked arteries in the leg are at higher risk of cellulitis. The two most common groups in which blocked arteries in the leg are seen are chronic smokers and diabetics.

Chronic venous insufficiency

Damaged veins in the leg can increase the risk of cellulitis of the leg. This is often a consequence of aging and other conditions, such as varicose veins. People with chronic venous hypertension usually have thickened skin, dermatitis and small star-like veins over the leg.


Lymphatics are vessels that carry a clear liquid to the lymph nodes. The lymphatic system is a vital part of our immune system so any disruption to it increases the risk of infection. When disruption to the lymphatic vessels occurs, it results in swelling of the affected limb called lymphoedema. This can occur spontaneously, be a complication of surgery (e.g. lymphoedema of the arm following breast cancer surgery) or infection (e.g. filariasis).

Generalized reduction in the body’s immunity Any condition that reduces the body’s ability to fight infection increases the risk of cellulitis e.g. in diabetes and in people who take immune-lowering medications.

What happens when you get cellulitis of the leg?

Typically, your foot or leg will start hurting and then become red, hot and swollen - but the order of these symptoms is variable. With time, the affected area of the leg will extend upwards. The infection spreads via the lymphatic vessels which run up the inside thigh and into the lymph nodes at the very top of the thigh; therefore, patients may have a streak of redness and pain on the inside and top of the thigh. Some patients can become generally unwell with fevers, uncontrollable shivering and sweating.

Serious complications of cellulitis Septicaemia Amputation Death

How do you diagnose it?

Cellulitis is purely a clinical diagnosis. Blood and radiological tests are only useful to clarify the severity and extent of the condition.

What is the usual treatment of cellulitis?

Antibiotics (usually penicillin-based) in oral or intravenous form.

Elevation of the affected leg and bed rest Treat the underlying cause e.g. give antifungal cream for tinea pedis, encourage better control of diabetes and refer to a Vascular Surgeon if there is a history of blocked arteries.

Severe forms of cellulitis Although cellulitis is generally not life-threatening, there are a few exceptions:

Necrotizing skin and soft tissue infections

These are fulminant conditions associated with a very high death rate that include “necrotizing fasciitis”, “synergistic gangrene” and “gas gangrene”. You may sometimes read in the media of cases described as being caused by “flesh eating bacteria”. These are technically not just cellulitis because they end up spreading much deeper than the subcutaneous tissue. People with these conditions have severe pain in the affected areas, often associated with large blisters. They also look very unwell from the end of the bed. The only cure is surgery in conjunction with antibiotics, sometimes involving many operations. Often the operated area will be completely exposed e.g. an open abdomen with the intestines visible or an open leg with the bones on full display.

Dog and cat bites

In Sri Lanka, animal bites, especially from stray dogs, rightly make us worry about rabies. But they also harbour a bacterium called Pasteurella in their mouths. If introduced into our skin through a bite or scratch, they can cause an aggressive form of cellulitis that loves to invade the tendons and joints. These patients often need surgery in addition to antibiotics.

Cellulitis from water

When cellulitis occurs following an injury in water, different bacteria have to be considered. Aeromonas and Vibrio are two bacteria that we worry about, in particular. They can both cause severe blistering infections. Also it is vital to ask the patient about exposure to water as these bacteria require different antibiotics compared to standard cellulitis. For those readers who like the outdoors, it is worth remembering that leech bites complicated by cellulitis are often due to Aeromonas and require special antibiotics.

So there you have it - cellulitis in a nutshell. The take home message is that it is a common condition which has the potential to become very serious if not attended to promptly.

(The writer is an Infectious Diseases Physician and Associate Professor of Medicine at the Australian National University)

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Here it is, Cellulitis in a nutshell


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