Clostridial Myositis in Horses: Signs, Causes & Treatment

Clostridial myositis is a rare but life-threatening bacterial infection that causes rapid damage to a horse’s muscle tissue. Also known as clostridial myonecrosis, the condition is sometimes called gas gangrene because some infections produce gas within the affected tissues. ^[1][2]

The infection can develop after an intramuscular injection, wound, or other muscle injury. However, in some cases, there is no obvious wound or identifiable point of entry. ^[1][2]

Early signs may resemble a routine injection-site reaction, with localized swelling, heat, and discomfort. Clostridial myositis can progress rapidly, and affected horses may become severely ill within hours. ^[3]

Clostridial myositis is a veterinary emergency. Contact your veterinarian immediately if your horse develops rapidly increasing, painful swelling, particularly near a recent injection or wound. Do not wait to see whether the swelling improves on its own. ^[1][2][3]

Keep reading to learn how to spot the signs of clostridial myositis so you know when to call the veterinarian for prompt evaluation.

Clostridial Myositis in Horses

Clostridial myositis is a serious bacterial infection that causes rapid damage to a horse’s muscle tissue. It is caused by bacteria from the Clostridium species, which can produce powerful tissue-damaging toxins that injure muscle cells, small blood vessels, and surrounding tissues. ^[4][5]

As the infection progresses, damage to small blood vessels reduces blood flow and oxygen delivery to the affected muscle.

Clostridium bacteria thrive in these low-oxygen conditions, allowing them to multiply quickly and cause further tissue damage. The infection can then spread through the muscle and into nearby tissues. ^[2][3][5]

Some Clostridium species also produce gas as they multiply. This gas can collect within the muscle or under the skin and may cause a crackling sensation when the area is touched. Known as crepitus, this finding can be an important clue that gas-producing bacteria are present. ^[2][5][6]

The effects of clostridial myositis are not limited to the affected muscle. Bacterial toxins and substances released during tissue damage can enter the bloodstream and affect the entire body. Severely affected horses can become critically ill and may rapidly develop shock and other life-threatening complications. ^[1][2]

Veterinarians may also use the term clostridial myonecrosis to describe this condition. Myonecrosis, or death of muscle tissue, refers to the severe muscle damage that can occur as the infection progresses. ^[2][3]

Table 1. Clostridial myositis in horses at-a-glance

Clostridium Bacteria

Clostridium bacteria are widely present in the environment. Their hardy spores can survive in soil, manure, and decaying organic material. Some species can also be found in the digestive tract of healthy animals without causing disease. ^[4][5]

Several Clostridium species are associated with clostridial myositis in horses, including: ^[5][6]

• Clostridium perfringens
• Clostridium septicum
• Clostridium sordellii
• Clostridium novyi

The species involved can influence the toxins produced and the severity of the resulting disease. ^[5][6]

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Pathogenesis of Colstridial Myositis: What Happens in the Muscle?

Once clostridial bacteria begin multiplying within muscle tissue, they release powerful toxins and enzymes that damage muscle cells and small blood vessels. This damage reduces blood flow to the affected area, limiting the amount of oxygen that reaches the tissue. ^[5]

Clostridial bacteria thrive in low-oxygen environments, so these changes create conditions that allow the infection to progress rapidly. A destructive cycle can develop: ^[3]

1. Bacteria multiply within the affected muscle
2. Toxins damage muscle tissue and blood vessels, causing inflammation and reducing blood flow
3. Muscle tissue begins to die as damage increases and oxygen delivery decreases
4. The infection spreads into nearby muscle and surrounding tissues
5. Gas and fluid may build up within the damaged tissues, contributing to swelling
6. Toxins and inflammatory substances enter the bloodstream, potentially causing serious illness throughout the body

As more tissue is damaged, conditions become increasingly favorable for bacterial growth, allowing the cycle to continue. This rapid progression explains why a horse with clostridial myositis can become seriously ill within a short period and requires immediate veterinary treatment. ^[1][2][3]

Symptoms of Clostridial Myositis

Signs of clostridial myositis can develop suddenly and worsen rapidly. A horse with a relatively small area of swelling may become seriously ill within several hours. Signs vary depending on where the infection develops, how much muscle is affected, and whether bacterial toxins are affecting the rest of the body. ^[2]

Early clostridial myositis can resemble a routine injection-site reaction or minor muscle injury. Signs may include: ^[2]

• Localized swelling around an injection site, wound, or affected muscle
• Heat and marked pain in the affected area
• Stiffness or reluctance to move
• Fever
• Reduced appetite
• Lethargy or unusual quietness

One of the most important warning signs is how quickly the affected area changes. Swelling that continues to expand, becomes increasingly painful, or is accompanied by signs of illness requires prompt veterinary assessment. ^[1][2]

As the infection progresses, swelling can spread through the affected muscle and surrounding tissues. The area may initially feel warm, firm, and extremely painful. As blood flow decreases and tissue begins to die, the skin over the affected area may become cool or discolored. ^[2]

Some clostridial bacteria produce gas as they multiply. Gas trapped within the tissues can cause a crackling or popping sensation beneath the skin when the area is touched. This is known as crepitus. Owners should not repeatedly press or manipulate a painful swelling to check for this sign. ^[5][6]

Signs that the infection is becoming more severe can include: ^[3]

• Rapidly expanding swelling
• Increasing or severe pain
• Skin that becomes cool, dark, or discolored
• A crackling sensation beneath the skin
• Weakness or difficulty standing
• A rapid heart rate
• Changes in gum color
• Dehydration or poor circulation
• Signs of shock

Not every horse with clostridial myositis develops noticeable gas beneath the skin, and the absence of crepitus does not rule out the condition. Any rapidly worsening, painful swelling, particularly after an injection or wound, should be treated as an emergency. ^[5][6]

As tissue damage progresses, bacterial toxins and inflammatory substances can enter the bloodstream and affect the entire body. Horses can deteriorate quickly once this occurs, becoming increasingly weak and seriously ill. ^[1][2][5][6]

Table 2. Clostridial myositis warning signs in horses

When to Call the Veterinarian

Clostridial myositis is a veterinary emergency. Contact your veterinarian immediately if your horse develops rapidly increasing, painful swelling, especially near a recent injection site, wound, or area of muscle injury.

Early signs can look like a routine injection-site reaction, but clostridial myositis can worsen within hours. ^[1][2][3]

Call your veterinarian urgently if you notice:

• Swelling that expands quickly or becomes increasingly painful
• Heat, marked tenderness, stiffness, or reluctance to move
• Fever, lethargy, reduced appetite, weakness, or difficulty standing
• Skin over the swelling that becomes cool, dark, tight, or discolored
• A crackling or popping sensation beneath the skin
• Rapid heart rate, abnormal gum color, dehydration, poor circulation, or signs of shock

Do not repeatedly press or manipulate the swelling to check for gas beneath the skin. Do not wait to see whether the swelling improves on its own. Prompt veterinary treatment is critical to limit tissue damage, control infection, and improve the horse’s chance of recovery. ^{[1][2][3][5][6]}

Causes & Triggers

Clostridial myositis develops when disease-causing Clostridium bacteria multiply within muscle tissue. These bacteria thrive in environments with little oxygen. Spores of Clostridium bacteria can be found in normal equine muscle in a dormant (resting) state. Injections, wounds, and traumatic injuries can activate these spores or damage tissue in ways that create favorable conditions for bacterial growth. ^[6]

Different Clostridium species produce different toxins, which can influence how the infection develops, the extent of tissue damage, and the horse’s prognosis. ^[5] Not every horse with clostridial myositis has a recent injection or visible wound. In some cases, the precise source or trigger cannot be identified. ^[1][5]

Intramuscular Injections

Clostridial myositis is a recognized but uncommon complication of intramuscular injections. When it occurs, pain and swelling typically develop around the injection site and become progressively worse. ^[2][4]

Infection development after an injection does not necessarily mean the injection was administered incorrectly. Clostridial spores are widespread in the environment and may be found dormant within normal equine muscle. Tissue damage and reduced oxygen levels at an injection site can create conditions that allow clostridial bacteria to multiply. ^[7]

Rapidly increasing swelling or pain following an intramuscular injection requires prompt veterinary attention, particularly when accompanied by: ^[3][5]

• Fever
• Poor appetite
• Lethargy
• A crackling sensation beneath the skin

Wounds & Trauma

Clostridial bacteria can also enter muscle through deep punctures, contaminated wounds, or other injuries. In some cases, a wound that appears minor at the skin’s surface may extend deeper into the underlying tissues. ^[2]

Trauma can damage tissue and reduce local blood flow and oxygen delivery, creating low-oxygen conditions that favor bacterial growth. ^[3][5]

Diagnosis

Because clostridial myositis can worsen rapidly, veterinarians often begin both diagnosis and treatment together. Veterinarians may strongly suspect the condition based on the horse’s signs, particularly when rapidly increasing pain and swelling develop near a recent injection, wound, or area of muscle injury. ^[2]

The veterinarian will examine the affected area and assess the horse’s overall condition. They may look for expanding swelling, severe pain, changes in skin temperature or color, and gas beneath the skin. However, not every clostridial infection produces noticeable gas, so the absence of crepitus cannot rule out the condition. ^[1][2][3]

Diagnostic testing may include: ^[2][3][8]

• Bloodwork: Can help assess the severity of inflammation, muscle damage, dehydration, and other changes associated with systemic illness.
• Ultrasound: Can help identify fluid or gas within affected tissues and determine how extensively the muscle is involved.
• Fluid or tissue samples: Samples from the affected area may be examined under a microscope and submitted for bacterial culture or other laboratory testing to identify the bacteria involved.

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Treatment & Management

Because clostridial myositis is a veterinary emergency, veterinarians start treatment immediately when the condition is suspected. The immediate priorities are: ^[1][2][3]

• Controlling bacterial growth
• Removing severely damaged tissue
• Supporting the horse before further complications develop

Clostridial myositis requires aggressive veterinary treatment. Many affected horses need hospitalization because the infection can spread rapidly and bacterial toxins can cause severe whole-body illness and shock. Treatment generally combines: ^[2]

• Antibiotics
• Surgical management of the affected tissue
• Pain control
• Wound care
• Intensive supportive care

Antibiotics

Antibiotics are used to control clostridial bacteria and limit further spread of the infection. Treatment is usually started as soon as clostridial myositis is suspected rather than waiting for laboratory confirmation. ^[1]

The veterinarian selects antibiotics based on the suspected bacteria, the severity of the infection, and the horse’s individual condition. They may later adjust treatment if laboratory testing provides additional information. ^[2][3]

Surgical Treatment

Antibiotics alone may not be sufficient because severely damaged muscle has poor blood flow. This can make it difficult for adequate concentrations of medication and oxygen to reach infected tissue. ^[2][3][5]

Surgical treatment commonly involves making openings in the affected area to expose infected tissue to oxygen and allow fluid and gas to drain. Dead or severely damaged tissue may also need surgical removal, a procedure known as debridement. ^[2]

Some horses require repeated procedures as the full extent of tissue damage becomes apparent. The resulting wounds can be large and may require ongoing cleaning, bandaging, drainage, and monitoring as they heal. ^[2][3]

Supportive Care

Clostridial myositis can affect much more than the infected muscle. Bacterial toxins and inflammatory substances can enter the bloodstream, and severely affected horses may develop: ^[6]

• Dehydration
• Poor circulation
• Weakness
• Shock

Depending on the horse’s condition, supportive treatment may include: ^[8]

• Intravenous fluids to maintain hydration and circulation
• Pain-relieving and anti-inflammatory medications
• Careful monitoring of heart rate, circulation, hydration, and other vital signs
• Wound cleaning, drainage, and bandaging
• Additional treatment for shock or other complications

Treatment can be intensive, and recovery may take time. Even after the infection is controlled, horses with extensive muscle damage may require prolonged wound care and rehabilitation. ^[2][3]

Prognosis

Clostridial myositis is a life-threatening condition, but recovery is possible. Prognosis varies between horses and depends on factors such as: ^[2]

• The Clostridium species involved
• The extent of muscle damage
• The severity of illness
• How quickly treatment begins

In one retrospective study of 37 horses with clostridial myonecrosis, 73% survived to discharge. Horses were hospitalized for a median of 12 days, and all required continued treatment after leaving the hospital. ^[2][9]

Early recognition and aggressive treatment provide the best chance of survival. Horses treated before extensive muscle damage or shock develops generally have a better outlook than horses that are already critically ill when treatment begins. ^[1][2][3]

Horses that survive may require ongoing wound care while damaged tissues heal. The length of recovery depends on the location and extent of the infection and the amount of tissue affected. Exercise and rehabilitation should be introduced according to veterinary guidance and the individual horse’s recovery. ^[2][3][5]

Severe cases can be fatal despite prompt and intensive veterinary care. For this reason, rapidly expanding, painful swelling, especially following an intramuscular injection or wound, should always be treated as an emergency. ^[2]

Prevention

Clostridial myositis cannot always be prevented because Clostridium spores are common in the environment and some cases occur without an obvious wound or injection-site problem. ^[5][10] However, careful injection practices, prompt wound care, and early recognition of abnormal swelling can help reduce risk and support faster intervention if infection develops. ^{[3][9][11][12]}

Safe Injection Practices

Intramuscular injections should only be given when appropriate and according to veterinary instructions. ^[13] Avoid giving unnecessary intramuscular injections, especially when an oral option is more appropriate for the medication and situation. ^[14]

Before administering an injection, use clean equipment and handle needles carefully. Do not reuse needles, and do not allow the needle to contact dirty surfaces before injection. ^[11][15] Choose an appropriate injection site with adequate muscle mass, and avoid injecting into areas that are dirty, swollen, painful, scarred, or previously irritated. ^[13]

Monitor injection sites after treatment. Mild soreness can occur after some injections, but swelling that expands, becomes increasingly painful, feels unusually hot or cold, or is accompanied by fever, dullness, poor appetite, stiffness, or reluctance to move requires prompt veterinary attention. ^[5][9][13]

Wound & Trauma Management

Good wound care can also help reduce the risk of serious bacterial infection. Deep puncture wounds, contaminated wounds, crushing injuries, and wounds that involve muscle should be assessed by a veterinarian because the skin opening may appear small while deeper tissues are damaged. ^[3][12][16]

Do not assume a wound is minor based only on its surface appearance. Clean visible contamination when it is safe to do so, protect the area from further dirt or trauma, and contact a veterinarian for wounds that are deep, painful, swollen, draining, near a joint, or associated with lameness or systemic illness. ^[12]

Early Recognition

The most important preventive step is recognizing when a normal reaction is becoming an emergency. ^[13][17] Clostridial myositis can initially resemble a routine injection-site reaction or muscle injury, but it progresses much more rapidly. ^[9]

Contact your veterinarian immediately if a horse develops: ^[5][9][17]

• Rapidly increasing swelling near an injection site, wound, or injured muscle
• Marked or worsening pain in the affected area
• Fever, lethargy, poor appetite, or weakness
• Skin that becomes cool, dark, tight, or discolored
• A crackling sensation beneath the skin
• Rapid heart rate, abnormal gum color, dehydration, or signs of shock

Do not wait to see whether these signs improve on their own. Early veterinary treatment offers the best chance of limiting tissue damage and improving the horse’s outcome. ^[2][3][9]

Frequently Asked Questions

Here are some frequently asked questions about clostridial myositis in horses:

Summary

Clostridial myositis is a rapidly progressive bacterial infection that causes severe muscle damage and potentially fatal systemic illness in horses. Rapidly expanding, painful swelling after an injection, wound, or muscle injury requires immediate veterinary care.

• Clostridial toxins damage muscle and blood vessels, creating low-oxygen conditions that support bacterial growth
• Signs include rapidly expanding swelling, severe pain, fever, discolored skin, crepitus, weakness, and shock
• Diagnosis may involve clinical findings, bloodwork, ultrasound, and testing of fluid or tissue samples
• Emergency treatment includes antibiotics, surgical drainage and debridement, pain control, wound care, and supportive care
• Prognosis depends on the bacterial species, extent of muscle damage, illness severity, and speed of treatment