Diabetic Foot Infection

Diabetic foot infection is one of the most common and serious complications of diabetes, and the most frequent cause of hospitalization and nontraumatic amputation in people with diabetes. Early diagnosis and proper treatment are essential to prevent severe complications.

Table of contents

• What is Diabetic Foot Infection?
• What Causes Diabetic Foot Infection?
• Risk Factors
• Symptoms and Signs
• How is Diabetic Foot Infection Diagnosed?
• Common Bacteria Causing Infection
• Complications
• Treatment
• Prevention

What is Diabetic Foot Infection?

Diabetic foot infection is an infection of the soft tissue or bone that occurs below the ankle bones (called malleoli) in people with diabetes^[1]. It is a common but serious condition that affects many people living with diabetes. The infection occurs when harmful bacteria enter the body, most often through a wound or open sore on the foot^[4].

This condition is the most common complication of diabetes leading to hospitalization^[1]. More than half of all nontraumatic lower leg amputations (surgical removal of a limb) are related to diabetic foot infections, and 85% of all lower leg amputations in people with diabetes are preceded by an ulcer (open sore)^[1].

The estimated lifetime risk of a person with diabetes developing a foot ulcer is between 15% and 25%, with an annual rate of 3% to 10%^[1]. Around 40% of diabetic foot ulcers become infected^[9].

What Causes Diabetic Foot Infection?

Three main factors work together to cause diabetic foot infections in people with diabetes: nerve damage, poor blood flow, and a weakened immune system^[2].

Nerve damage (also called neuropathy) is a condition where high blood sugar over time damages the nerves in your feet and legs^[2]. About 75% of diabetic nerve problems affect the feet^[3]. This nerve damage causes numbness, tingling, or pain in your feet, making it hard to feel when you have a wound, cut, or blister^[2]. About 50% of people with nerve damage have no symptoms at all, which makes it difficult to know there is a problem^[9].

When you cannot feel pain in your feet, you may not notice a small pebble in your shoe, a developing blister, or a small cut. These small injuries can go unnoticed and untreated^[4].

Poor blood circulation (also called peripheral artery disease) happens when diabetes damages blood vessels and reduces blood flow to the feet^[2]. Peripheral arterial disease is present in up to 40% of patients with diabetic foot infections^[1]. Poor circulation makes it harder for injuries to heal and for the body to fight infection^[2].

People with diabetes also have a weakened immune system and increased inflammation, making them more likely to develop skin infections^[2]. Research shows that high blood sugar can impair white blood cells’ ability to reach an infection site, allowing infections to develop more quickly^[4].

Most infections occur at a site of skin trauma or ulceration^[1]. Small injuries like cuts, scuffs, and blisters can develop into diabetic ulcers, which are not infections themselves but often lead to infections when bacteria enter through these openings^[4].

Risk Factors

Several factors increase your risk of developing a diabetic foot infection^[4]:

• History of repeated foot ulcers
• Foot ulcers lasting for longer than 30 days
• Poor control over blood sugar levels
• Peripheral neuropathy (nerve damage)
• Kidney problems
• Peripheral artery disease
• Injury or trauma to the foot
• Walking barefoot frequently
• History of amputation in lower limbs

Anyone with diabetes can develop nerve damage, but these factors increase the risk^[8]:

• Blood sugar levels that are hard to manage
• Having diabetes for a long time, especially if blood sugar is often higher than target levels
• Being overweight
• Being older than 40 years
• Having high blood pressure
• Having high cholesterol

Additionally, dry skin is common in people with diabetes, which means foot skin can crack and create entry points for bacteria^[4]. Foot deformities like bunions, hammer toe, or Charcot foot (a condition where bones in the foot weaken) can result in high pressure in certain areas of the foot, increasing risk^[9].

Symptoms and Signs

Diabetic foot infection is diagnosed based on the presence of at least two classic signs of inflammation or the presence of pus^[1]. Local signs of infection include^[1]:

• Redness
• Warmth
• Swelling or hardening of tissue
• Pain or tenderness
• Discharge of pus

Other signs that suggest infection include a wound that fails to heal despite proper treatment, discharge that is not pus, bad smell, and tissue that is dead or easily breaks apart^[1].

In general, you should look out for the following symptoms^[2]:

• Any changes to your skin or toenails, such as cuts, blisters, calluses, or sores
• Frequent bleeding
• Discharge of fluid or pus
• Foul smell
• Pain
• Skin discoloration
• Swelling

Infections are classified as mild, moderate, or severe. Mild infections involve only the skin. Moderate infections spread deeper into tissues or affect a larger area. Severe infections show signs of serious illness affecting the whole body or threaten the limb^[1].

How is Diabetic Foot Infection Diagnosed?

Diabetic foot infection is a clinical diagnosis based on the presence of at least two classic findings of inflammation or pus^[1]. Your doctor will perform a thorough assessment of the wound, the limb, and your overall health^[1].

Collecting samples from the infected area helps identify the bacteria causing the infection. However, superficial wound cultures (samples taken from the surface) should be avoided because they often contain contaminating bacteria that are not causing the infection^[9]. Deep cultures obtained through sterile procedures such as incision and drainage, surgical cleaning of the wound, or bone samples help guide treatment^[9].

When osteomyelitis (bone infection) is suspected, imaging tests are important. Plain X-rays are used for initial imaging^[9]. However, magnetic resonance imaging (MRI) is the most accurate imaging study for detecting early bone infection^[1]. Computed tomography (CT) scans may also help if X-rays are inconclusive, if the extent of infection is unknown, or to help plan surgery^[9].

Blood tests may show slightly or moderately elevated white blood cell counts and erythrocyte sedimentation rate (a test that measures inflammation), but these elevations alone are not enough to diagnose infection^[6]. Initial testing in patients with suspected bone infection should include plain X-rays, a C-reactive protein test (another inflammation marker), and a probe-to-bone test (using a sterile metal probe to check if bone can be felt in the wound)^[9].

Evaluation of blood flow to the feet is critical, since peripheral arterial disease is present in up to 40% of patients with diabetic foot infections^[1].

Common Bacteria Causing Infection

Most diabetic foot infections contain multiple types of bacteria (called polymicrobial infections)^[7]. The most common bacteria are aerobic gram-positive cocci, mainly Staphylococcus species^[1]. Staphylococcus aureus and Streptococcus agalactiae are the most commonly isolated bacteria^[9].

Methicillin-resistant Staphylococcus aureus (MRSA) is present in 10% to 32% of diabetic infections and is associated with higher rates of treatment failure^[1]. Previously, MRSA infections were usually acquired in hospital settings, but recently, MRSA infections from the community are becoming more common and are linked to poor treatment outcomes^[7].

Moderate to severe infections and wounds previously treated with antibiotics often contain gram-negative bacilli. Anaerobic bacteria (bacteria that do not need oxygen) are more commonly present in wounds with dead tissue and infections of the foot with poor blood flow^[1].

Complications

Osteomyelitis (bone infection) is a serious complication of diabetic foot infection that increases the likelihood of surgical treatment^[1]. It occurs in 15% of ulcers, and 15% of those require amputation^[3].

Gangrene is a medical emergency where blood stops flowing to a specific part of your body, and tissues in that area die^[2]. A foot infection that is not treated in time can lead to gangrene. Without prompt treatment, gangrene can be fatal^[2]. Symptoms of gangrene include:

• Changes in skin color (from red to brown, and ultimately to purple or greenish black)
• Swollen skin
• Severe pain or a loss of feeling
• Skin that feels cool to the touch
• A crackling sound when you press on the skin
• Sores and blisters that release blood or foul-smelling pus
• Chills
• Fast breathing and heart rate

Amputation is the surgical removal of part or all of the foot or leg. If an infection does not improve with treatment, amputation may be necessary to prevent the infection from spreading and to save your life^[8]. Approximately 60% of patients undergoing lower leg amputation have diabetic foot ulcers as the underlying cause^[3]. Following a lower leg amputation, the five-year death rate jumps to 60%^[3]. The mortality rate after amputation is approximately 50%, exceeding the death rate of many cancers^[9].

The three-year death rate for people with diabetes increases from 13% to 28% when they have an ulcer^[3]. Once a nerve problem develops, the annual rate of ulcer formation increases from less than 1% to more than 7%^[3].

Treatment

Treatment is based on the extent and severity of the infection and other medical conditions the patient may have^[1].

Mild infections are treated with oral antibiotics, wound care, and pressure off-loading (removing pressure from the wound) in the outpatient setting^[1]. Patients can be treated at home with antibiotics that cover skin bacteria including streptococci and Staphylococcus aureus. Effective choices include cephalexin, dicloxacillin, amoxicillin-clavulanate, or clindamycin^[12]. Treatment typically lasts one to two weeks^[7].

Moderate infections may require more comprehensive treatment. Selected patients with moderate infections and all patients with severe infections should be hospitalized, given intravenous (IV) antibiotics, and evaluated for possible surgery^[1].

Severe infections require hospitalization and IV antibiotics that cover a wider range of bacteria, including gram-positive organisms, gram-negative organisms, and anaerobic bacteria^[9]. Treatment of severe infections should last approximately two to three weeks or more, depending on how extensive the infection is^[7].

For patients with bone infection (osteomyelitis), treatment requires a minimum of four to six weeks of antibiotics^[12]. The duration of treatment may be shortened in patients who undergo amputation as part of treatment^[12].

Empiric therapy (treatment started before knowing exactly which bacteria are present) for MRSA is not always necessary unless the patient has a critical infection like sepsis (a severe whole-body infection), if MRSA rates are particularly high in the local area, or if the patient had a previous MRSA infection^[7]. A negative MRSA nares culture (a test from the nose) has a high negative predictive value, meaning antibiotic coverage for MRSA may be stopped if this test is negative^[9].

Surgical cleaning (debridement) and drainage of deep tissue abscesses and infections should be performed in a timely manner^[1]. Severe or persistent infections may require surgery and specialized team-based wound care^[9].

Proper wound care and removing pressure from the infected area are essential parts of treatment. Treating diabetic foot infections is difficult because blood flow is compromised and antibiotics usually cannot reach the diseased area effectively^[3].

Prevention

All patients with diabetes should undergo a systematic foot examination at least once a year, and more frequently if risk factors for diabetic foot ulcers exist^[1]. Regular foot examinations can pick up problems before symptoms appear^[2].

Preventive measures include^[1]:

• Patient education on proper foot care
• Blood sugar control
• Blood pressure control
• Smoking cessation
• Use of prescription footwear
• Intensive care from a podiatrist (foot specialist)
• Evaluation for surgical interventions as indicated

Keep your blood sugar in your target range as much as possible. This is one of the most important things you can do to prevent nerve damage or stop it from getting worse^[8]. Other good diabetes management habits include not smoking (smoking reduces blood flow to the feet), following a healthy eating plan, getting physically active, and taking medicines as prescribed by your doctor^[8].

Daily foot care tips:

• Check your feet every day for cuts, redness, swelling, sores, blisters, corns, calluses, or other changes to the skin or nails. Use a mirror if you cannot see the bottom of your feet, or ask a family member to help^[8]
• Wash your feet every day in warm (not hot) water. Do not soak your feet. Dry your feet completely and apply lotion to the top and bottom, but not between the toes^[8]
• Cut nails carefully, straight across, and file the edges. Do not cut nails too short, as this could lead to ingrown toenails^[20]
• Never treat corns or calluses yourself. Visit your doctor for appropriate treatment^[20]
• Wear clean, dry socks and change them daily. Consider socks made specifically for patients with diabetes^[20]
• Shake out your shoes and feel the inside before wearing them^[20]
• Always wear shoes or slippers, even at home. Never walk barefoot^[20]
• Wear well-fitting, supportive footwear. Choose athletic or sneaker-style shoes with a heel counter, arch support, and ample toe space^[18]

When you check your feet every day, you can catch problems early and get them treated right away. Early treatment greatly lowers your risk of amputation^[8]. If you develop foot issues, see your healthcare provider as soon as possible. Early treatment is key to preventing serious complications^[2].