You’ve noticed that the skin on one or both of your legs has become swollen and reddened, and it’s spreading. Perhaps it is also painful or rashlike, or you see a sore.
You wouldn’t be blamed for thinking you might have cellulitis, a common bacterial infection affecting the soft tissue, particularly the dermis, fat, and other structures below your top layer of skin. (1)
But you might be wrong.
“Cellulitis is often overdiagnosed or misdiagnosed, and there are many conditions which can mimic it,” says Rachel Bystritsky, MD, an infectious disease specialist and assistant professor of medicine at the University of California in San Francisco. The areas of redness, swelling, and discomfort that can characterize cellulitis, in particular, are also features of a number of other maladies, not all of which are caused by infections.
Cellulitis is most often caused by the staphylococcus or streptococcus bacteria. (2) It is typically treated with oral antibiotics, such as penicillin or ampicillin (Omnipen), says Edidiong C. Kaminska, MD, a Chicago-based dermatologist.
“Usually there is a crack or break in the skin that allows bacteria to enter into it,” says Dr. Kaminska. “The most common location for cellulitis is the lower legs, but it can occur on any part of the body, including the face.”
If there is a delay in treatment, the infection could spread to the bloodstream or lymph nodes, requiring intravenous antibiotics or surgical drainage of abscesses (pockets of pus). (3)
Cellulitis is diagnosed by physical examination and taking a medical history, after which blood tests for bacterial infection may be ordered. But a paper published in 2018 on the treatment of cellulitis and soft tissue infections noted a lack of “reliable microbiologic and laboratory diagnostics for cellulitis,” which makes it hard to differentiate it from noninfectious diseases with similar symptoms. (3)
Still, there are clues that can point in the right direction. For instance, because cellulitis starts as a local infection, it usually affects only one side of the body.
“One really good way to determine if someone may or may not have cellulitis is that if the problem is in both legs, it is unlikely to be cellulitis,” says Kaminska.
In addition, if antibiotics aren’t clearing up a case of suspected cellulitis, other conditions may need to be considered.
How Does Cellulitis Differ From Erysipelas?
Cellulitis is very similar to another bacterial skin infection called erysipelas, which is also known as St. Anthony’s fire. While cellulitis generally infects the skin’s deeper layers and fat, erysipelas affects the skin’s uppermost layers, including the epidermis, as well as the lymph vessels. (3,4)
Erysipelas is usually caused by group A streptococcus bacteria, and it occurs when there is a break in the skin or problems with blood vessels or the lymph system drainage. The infection can affect the legs and arms, as well as the face or elsewhere on the body. The area of redness is usually more clearly defined than with cellulitis, and it may involve blisters or red streaking if the infection has spread along the lymph vessels. Pus is less likely to form with erysipelas. Fever and chills are more common with erysipelas than they are with cellulitis. The condition can permanently damage lymph vessels, leading to chronic swelling in a limb. (4,5)
Like cellulitis, erysipelas is treated with oral antibiotics or, if the infection is severe, intravenous antibiotics.
Venous Stasis Dermatitis vs. Cellulitis
When there is poor blood flow in the lower legs due to malfunctioning valves in the veins (venous insufficiency), the result can be another common mimic of cellulitis: venous stasis dermatitis. (6,7)
“It usually appears as redness,” says Kaminska. “The skin could be swollen, tender, and rashy on the legs.” She adds that venous stasis dermatitis is typically bilateral (affecting both legs), a telltale sign that it is not cellulitis.
With venous stasis dermatitis, fluid and blood cells can leak out of the vessels into the skin and other tissues, leading to itching, inflammation, and even open sores. (8) Swelling around the ankles, discolored skin, and varicose veins can be early signs of the condition and should be reported to your doctor before your skin gets worse. (6)
Chronic venous insufficiency can lead to a condition known as lipodermatosclerosis, or sclerosing panniculitis. (7) “It is an inflammation of the underlying fat that can cause the skin to be hard and red and also mimics cellulitis,” says Kaminska. The legs may take on a bowling pin shape. Treatments can include compression therapy, as well as pain relievers, anti-inflammatories, and blood thinners. (9)
Shingles Can Mimic Cellulitis
Herpes zoster, the viral infection also known as shingles, can be mistaken for cellulitis. Shingles is caused by the varicella-zoster virus, which also causes chickenpox. After the initial infection of chickenpox, the virus remains in your body and can reactivate decades later, causing symptoms. (10)
“Sometimes, early shingles can look just like a red rash,” says Dr. Bystritsky.
Pain from shingles, known as postherpetic neuralgia, can linger for weeks, months, or even years. If shingles has affected an eye, it can result in temporary or permanent vision loss. There is no cure for shingles, but a vaccine can prevent it or lessen the severity of symptoms. In addition, antiviral medication may help to shorten the shingles attack if you seek medical attention as soon as symptoms appear. (10)
Other early symptoms of shingles can include itching, burning sensations, and shooting pains, usually on one side of the body or face. Later, the rash may develop into small blisters that scab over in 7 to 10 days.
Sometimes symptoms that look and feel like cellulitis are the result of your skin being irritated by something that it has touched, a condition known as contact dermatitis. (7,11)
“The area tends to be itchy, also swollen,” says Kaminska. “There may be some blisters. The symptoms are usually limited to the site of contact.” The reaction may be allergic or simply a reaction to irritating substances or friction. While cellulitis typically feels warm and painful, contact dermatitis feels intensely itchy without surrounding warmth. (7)
Potential irritants are numerous and may include hair dyes; fragrances; rubber gloves; fabrics; topical antibiotics; sunscreens; acids; alkaline substances, such as soaps, detergents, fabric softeners, and solvents; or other chemicals. The reaction usually occurs 24 to 48 hours after contact. Treatments vary but could include the use of anti-itch lotions, topical steroids, or simply doing nothing other than removing the irritant or source of allergen. (11)
Differentiating between contact dermatitis and cellulitis is important because the medication used to treat one condition might impede the healing process of the other. (7)
Deep Vein Thrombosis
If a blood clot forms within a vein deep inside of a leg, arm, or the pelvis, the condition is known as deep vein thrombosis (DVT). (12)
“Deep vein thrombosis can also look a lot like cellulitis of the leg,” says Bystritsky. Both conditions involve skin that is swollen, reddened, painful, and hot to the touch. (13) But the way they are treated is very different. In the case of DVT, a person may be prescribed a blood thinner, such as warfarin (Coumadin) or heparin, to prevent blood clots. They may also wear a compression garment on the affected limb to improve blood flow and decrease the chance of complications from blood clots.
It is vital to distinguish this condition from cellulitis, because if a blood clot causing the DVT breaks off and travels to a lung, causing a pulmonary embolism, the result can be life-threatening. (12)
Lymphedema and Cellulitis
The relationship between cellulitis and lymphedema (swelling caused when damage to the lymph system prevents the fluid from draining properly) is complicated: Chronic cellulitis can damage the lymph system, leading to lymphedema in an arm or leg. (14) On the other hand, lymphedema is a risk factor for cellulitis, because it makes the skin more vulnerable to infection. (14)
The symptoms of redness, swelling, and hardening of the skin that can accompany lymphedema can also be mistaken for cellulitis. (7) And the two conditions can co-exist.
If there are no symptoms of systemic infection or warmth in the affected area, a doctor may conclude that the problem is more likely to be lymphedema than cellulitis. (7) Lymphedema can be treated with compression garments, bandages, massage therapy, and sometimes surgery. (15)
Insect Bite Hypersensitivity
“If someone has an exuberant reaction to an insect bite, where it gets really red and swollen and painful, that can mimic cellulitis,” says Kaminska. The reaction is also known as papular urticaria. Mosquito and flea bites are the most common causes, and children tend to have these reactions more often than adults because of their more sensitive immune systems.
Intense itching can occur, and reddish plaques can form, lasting one to two weeks. (8) Antihistamines may be prescribed, as well as topical steroids, to treat it. (16)