Systemic Inflammatory Response Syndrome (SIRS) in Horses: Signs, Causes & Treatment

Systemic inflammatory response syndrome (SIRS) in horses occurs when the body’s immune response spreads beyond the site of an illness or injury and begins disrupting normal tissue and organ function. Rather than remaining localized, inflammation affects the entire body and can interfere with circulation and oxygen delivery.

SIRS is not a disease on its own, but a serious complication that may arise in horses with conditions such as severe gastrointestinal disease, sepsis, trauma, or other underlying triggers.

Early signs can be difficult to distinguish from those of the underlying condition, but affected horses may worsen quickly and require emergency veterinary care.

Keep reading to learn what can trigger SIRS, how to recognize signs of deterioration, and what to expect from veterinary diagnosis, treatment, and supportive care.

Systemic Inflammatory Response Syndrome (SIRS) in Horses

Systemic inflammatory response syndrome (SIRS) is a life-threatening complication of many serious illnesses and injuries in horses. It most commonly develops in horses with severe colic, colitis, infection, or major trauma, as well as after surgery. However, any condition that triggers a severe inflammatory response can lead to SIRS. ^[1][2]

Inflammation is an essential part of the body’s normal defense system, helping to fight infection and repair damaged tissues. However, when this response becomes widespread and uncontrolled, it can damage healthy tissues instead of protecting them.

As SIRS progresses, blood flow to vital organs may be impaired, increasing the risk of laminitis, shock, and multiple organ dysfunction. Prompt veterinary treatment is critical for horses with SIRS.

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Pathophysiology: How SIRS Develops

Inflammation is a normal process that helps the body fight infection, remove damaged tissue, and begin the healing process. In healthy horses, this response remains confined to the affected area and gradually subsides as healing occurs.

Under certain circumstances, the inflammatory response becomes excessive and spreads throughout the body. Instead of protecting the horse, the inflammatory reaction spreads beyond the original injury or illness and damages healthy blood vessels and organs throughout the body. ^[1][3][4]

As blood vessels widen and become more permeable, fluid leaks into surrounding tissues. This leakage reduces circulating blood volume and makes it harder to maintain normal blood pressure and organ perfusion. At the same time, oxygen delivery to vital organs becomes less efficient. ^[4]

The body’s clotting system also gets activated. Tiny blood clots can form within small blood vessels, further reducing circulation and increasing the risk of tissue damage. ^[4]

Without prompt treatment, these widespread changes can impair the function of organs such as the lungs, kidneys, liver, and gastrointestinal tract. As organ damage worsens, horses may progress to shock, multiple organ dysfunction, and, in severe cases, death. ^[4]

Risk Factors

Any serious illness or injury that triggers widespread inflammation can increase a horse’s risk of developing SIRS. The highest-risk situations are those that involve severe infection, extensive tissue damage, impaired circulation, or leakage of bacteria and inflammatory compounds into the bloodstream. ^[1][2][3]

Risk factors for SIRS in horses include: ^{[1][2][3][5][6][7][8][9][10]}

• Severe gastrointestinal disease: Strangulating colic, colitis, intestinal ischemia, intestinal rupture, and endotoxemia are important risk factors in adult horses because they can damage the intestinal barrier and trigger widespread inflammation
• Sepsis or severe infection: Septicemia, pleuropneumonia, peritonitis, metritis, septic arthritis, and severe pneumonia can cause the inflammatory response to spread beyond the original site of infection
• Neonatal septicemia: Foals are particularly at risk of SIRS when bacteria enter the bloodstream and trigger sepsis
• Major trauma or surgery: Extensive wounds, fractures, burns, colic surgery, and postoperative complications can activate inflammatory pathways even when infection is not present
• Retained placenta or toxic metritis: Mares with retained fetal membranes can develop uterine infection, endotoxemia, laminitis, and systemic inflammation if treatment is delayed
• Heatstroke: Severe overheating can cause widespread tissue injury and may trigger systemic inflammation, especially during strenuous exercise in hot, humid conditions
• Severe snake envenomation: Venom can damage tissues, disrupt clotting, and provoke intense inflammation that may become systemic in severely affected horses
• Delayed veterinary treatment: Any severe illness or injury may become more dangerous if diagnosis and treatment are delayed, allowing inflammation, circulatory compromise, or organ dysfunction to progress

These risk factors do not guarantee that a horse will develop SIRS, but they indicate situations where close monitoring and prompt veterinary care are especially important.

Severity

SIRS can affect nearly every organ system in the horse’s body. As circulation becomes impaired and tissues receive less oxygen, vital organs may no longer function normally. The risk of complications increases when inflammation is severe or treatment is delayed. ^[3]

Common complications in affected horses include: ^[3][7]

• Laminitis: This condition is one of the most serious complications of SIRS. Widespread inflammation and changes in blood flow can damage the sensitive tissues that suspend the coffin bone within the hoof capsule. Because horses with SIRS have a higher risk of developing laminitis, veterinarians often monitor the feet closely during hospitalization. In horses considered at high risk, continuous distal limb cryotherapy may be used to reduce the risk of sepsis-associated laminitis.
• Disseminated intravascular coagulation (DIC): SIRS can overstimulate the body’s clotting system, causing numerous tiny blood clots to form within small blood vessels. As clotting factors and platelets are consumed, the horse may become prone to both abnormal clot formation and excessive bleeding. DIC is a serious complication associated with a poorer prognosis.
• Multiple organ dysfunction: As blood flow and oxygen delivery decline, vital organs may begin to fail, including the lungs, kidneys, liver, and gastrointestinal tract. This condition is known as multiple organ dysfunction and is one of the most serious consequences of uncontrolled SIRS.
• Shock: If inflammation severely disrupts circulation, the horse may develop shock. Blood pressure falls, tissues receive inadequate oxygen, and organ damage can progress rapidly. Shock is a life-threatening emergency that requires immediate veterinary treatment.

Symptoms

The signs of SIRS vary depending on the underlying illness and how severely the body is affected. Horses often show signs of both the condition that triggered SIRS and the body’s widespread inflammatory response.

Signs can overlap, and not every horse follows the same pattern of progression. ^{[2][5][6][11]}

Table 1. SIRS signs in horses by severity

Possible Early Signs

Early signs of SIRS can be nonspecific and may resemble signs of the underlying illness triggering the inflammatory response. Early warning signs in horses include: ^{[1][2][11][3][7]}

• Lethargy
• Reduced appetite
• Fever, although some severely affected horses develop a low body temperature
• Elevated heart rate (tachycardia)
• Increased respiratory rate
• Congested or dark pink mucous membranes
• Mild dehydration
• Signs associated with the underlying illness, such as colic or diarrhea

These findings warrant close monitoring, particularly when several signs develop together or worsen over time.

Signs of Poor Circulation

As inflammation disrupts circulation and oxygen delivery throughout the body, horses may develop signs of reduced tissue perfusion. This means the tissues are receiving less blood, oxygen, and nutrients than they need to function normally.

Signs include: ^[1][3][7][8]

• Weak pulse
• Cold ears and limbs
• Prolonged capillary refill time
• Increasing dehydration
• Reduced intestinal sounds
• Muscle weakness
• Sweating
• Mental dullness or decreased responsiveness

These changes may indicate that the horse’s condition is becoming more severe and requires immediate veterinary attention. ^[2][6]

Signs of Critical Illness

Severe SIRS can progress to shock and multiple organ dysfunction. Signs of critical illness include: ^{[1][2][6][3][7]}

• Very rapid heart rate
• Marked weakness or inability to stand
• Low blood pressure
• Difficulty breathing
• Laminitis
• Collapse

Because SIRS can progress rapidly, any horse showing severe or rapidly worsening signs should be evaluated by a veterinarian immediately. ^[2][6]

When to Call the Veterinarian

SIRS is a medical emergency that requires immediate veterinary attention. Contact your veterinarian right away if your horse develops signs of severe illness, particularly if the condition worsens rapidly. ^[2][3][7]

Seek veterinary care immediately if your horse has:

• Persistent or severe colic
• A high fever or abnormally low body temperature
• A persistently elevated heart rate
• Rapid or labored breathing
• Profound lethargy or weakness
• Dark, pale, or purple mucous membranes
• Cold limbs or prolonged capillary refill time
• Severe diarrhea or signs of dehydration
• Signs of shock or collapse

Prompt diagnosis and treatment offer the best chance of preventing life-threatening complications and improving the horse’s outcome.

Causes of SIRS in Horses

Systemic inflammatory response syndrome develops when a serious illness or injury triggers an excessive inflammatory response. Although many different conditions can lead to SIRS, they all have one thing in common: they cause extensive tissue damage or severe infection, or allow inflammatory compounds to enter the bloodstream. ^[3][7]

SIRS can occur at any age, but the conditions that trigger it vary between foals and adult horses. Adult horses often develop SIRS secondary to severe gastrointestinal disease, whereas neonatal foals more commonly develop it in association with septicemia. ^[2][4]

Severe Gastrointestinal Disease

Severe gastrointestinal disease is a leading cause of SIRS in adult horses. Conditions such as strangulating colic, colitis, intestinal ischemia, and intestinal rupture can damage the intestinal barrier. ^[5][8]

This damage may allow bacteria, bacterial toxins known as endotoxins, and other inflammatory compounds to enter the circulation or abdominal cavity. The resulting endotoxemia and inflammatory response can quickly affect the entire body. ^[5][8]

Common signs of gastrointestinal disease in horses include: ^[9][10]

• Changes in gut sounds
• Reduced appetite
• Diarrhea or dry manure
• Cribbing or wood chewing
• Bloating
• Hay belly

More severe signs typically present alongside signs of colic, which include: ^[10]

• Lethargy
• Sweating
• Flehmen response
• Lying down for long periods and unwillingness to rise
• Pawing
• Severely reduced appetite

Severe Infections & Sepsis

Severe or widespread infections can also lead to SIRS. Conditions such as septicemia, pleuropneumonia, severe pneumonia, septic arthritis, peritonitis, metritis following foaling, and umbilical infections in foals can all trigger an excessive inflammatory response that extends beyond the original site of infection. ^[5][7]

When SIRS develops in response to a suspected or confirmed infection, veterinarians may classify the condition as sepsis. Bacteria and the toxins they produce can trigger widespread inflammation throughout the body.

The body uses this response to control the infection, but excessive or poorly regulated inflammation can damage blood vessels, disrupt circulation, and impair oxygen delivery to tissues. ^[5][7]

As sepsis progresses, abnormalities in inflammation, blood clotting, and circulation may impair organ function. Horses with sepsis can deteriorate rapidly and develop complications such as cardiovascular instability, kidney injury, abnormal coagulation, or multiple organ dysfunction syndrome. ^[5][7]

Trauma & Surgery

Major injuries and surgical procedures can trigger SIRS even when infection is not present. Extensive wounds, fractures, burns, colic surgery, and complications following abdominal surgery can stimulate a strong systemic inflammatory response. In some horses, this response becomes excessive and affects the entire body instead of remaining confined to the injured tissues. ^[1][2][11]

Damaged cells release substances that alert and activate the immune system. These warning signals can provoke many of the same inflammatory pathways that the body uses to fight an infection, even when bacteria are not involved.

Immune cells such as neutrophils then move into the damaged area to remove injured tissue and support healing. If this response becomes poorly controlled, activated immune cells and inflammatory substances can also damage healthy tissues. ^[2][11]

Horses undergoing colic surgery may face additional inflammatory triggers. A twisted or trapped section of intestine can lose its blood supply, causing tissue damage. Restoring circulation during treatment may then produce a second wave of inflammation known as reperfusion injury.

The original intestinal disease, tissue handling during surgery, and postoperative complications can combine to increase the risk of a widespread inflammatory response. ^[1][2][11]

Less Common Causes

Although less common, SIRS can develop in response to other conditions that cause widespread tissue injury or severe inflammation. Examples include heatstroke, snake envenomation, and a retained placenta complicated by uterine infection. ^[3][12]

Although these conditions affect different body systems, each can trigger the excessive inflammatory response characteristic of SIRS.

Heatstroke can occur when a horse cannot release body heat effectively, particularly during strenuous exercise in hot, humid conditions. High humidity reduces the cooling effect of sweating, allowing body temperature to rise to dangerous levels. Signs can include: ^[13]

• A rectal temperature above 106°F (41.1°C)
• Persistently rapid heart and breathing rates
• Dehydration
• Muscle weakness
• Loss of coordination
• Collapse

Severe overheating can cause widespread tissue injury and may trigger the systemic inflammation associated with SIRS. Heatstroke requires immediate cooling and emergency veterinary care. ^[3][13]

Snake venom contains a complex mixture of enzymes, proteins, and other toxins that can damage tissues and provoke intense inflammation. Although some effects remain near the bite, severe envenomation can also disrupt blood clotting and affect the heart or nervous system. This combination of tissue injury and systemic inflammation may trigger SIRS in severely affected horses. ^[12]

After foaling, retained placental tissue can provide an environment where bacteria multiply and may lead to a serious uterine infection called toxic metritis.

Bacteria, toxins, and inflammatory substances can then enter the bloodstream, potentially causing septicemia or endotoxemia and triggering the widespread inflammatory response associated with SIRS. Affected mares also face a risk of laminitis and, in severe cases, death, making prompt veterinary treatment essential. ^[14]

Diagnosis

There is no single test that confirms SIRS. Instead, veterinarians diagnose the syndrome by considering the horse’s history, clinical signs, physical examination findings, laboratory results, and underlying illness or injury.

Historically, SIRS has been identified using changes in body temperature, heart rate, respiratory rate, and white blood cell count.

Although these criteria remain useful for assessing horses with suspected SIRS, they are not specific to the syndrome and have not been universally validated for all horses. Veterinarians therefore interpret these findings alongside the horse’s overall clinical picture. ^[2]

During the physical examination, your veterinarian will assess your horse’s heart rate, respiratory rate, body temperature, mucous membrane color, capillary refill time, hydration status, gut sounds, circulation, and overall attitude. These findings help determine how severely the horse is affected and whether organ function may already be compromised. ^[11]

Bloodwork provides additional information about inflammation, hydration status, tissue perfusion, and organ function. Common tests include: ^[3][7]

• A complete blood count (CBC)
• Serum biochemistry profile
• Blood lactate concentration
• Fibrinogen
• Serum amyloid A (SAA)
• Blood gas analysis

Additional diagnostic tests may be recommended to identify the illness or injury responsible for triggering SIRS. Depending on the suspected cause, these may include abdominal ultrasound, radiographs, blood cultures, endoscopy, or analysis of abdominal fluid. ^[3][7]

Treatment & Management

SIRS is a medical emergency requiring immediate veterinary treatment. There is no single treatment for the syndrome itself. Instead, treatment focuses on stabilizing the horse, addressing the illness or injury that triggered SIRS, controlling excessive inflammation, and supporting normal organ function during recovery. ^[8]

Identifying and treating the underlying condition is the most important step in managing SIRS.

Depending on the cause, treatment may involve: ^[3]

• Emergency colic surgery
• Medical management of colitis or enteritis
• Antimicrobial therapy
• Drainage of infected tissues
• Repair of traumatic injuries
• Treatment of complications such as a retained placenta

Many horses with SIRS require intensive supportive care in a veterinary hospital. Intravenous fluid therapy is used to restore hydration, improve circulation, maintain blood pressure, and correct electrolyte or acid-base imbalances. ^[3]

Horses with persistent low blood pressure may require medications to support cardiovascular function. Anti-inflammatory medications, such as flunixin meglumine, are commonly used to reduce pain and help control excessive inflammation. ^[3]

Horses with sepsis are also treated with appropriate broad-spectrum antimicrobial therapy. Additional supportive care may include oxygen therapy, nutritional support, plasma transfusions, gastrointestinal protectants, and careful monitoring of kidney function and blood glucose when indicated.

Horses with SIRS require frequent reassessment because their condition can change rapidly. Throughout treatment, veterinarians monitor vital signs, hydration, blood pressure, bloodwork, and signs of complications such as laminitis or organ dysfunction. Treatment is adjusted as the horse’s condition changes. ^[3]

Prognosis

The outlook for horses with SIRS depends largely on the condition that triggered the syndrome, how quickly treatment begins, and whether complications develop. ^[3][7]

Horses with mild disease that receive prompt veterinary treatment may recover completely, particularly if the underlying condition can be resolved quickly. However, horses with severe colic, septicemia, shock, or multiple organ dysfunction generally have a more guarded prognosis. ^[3][7]

Complications such as laminitis, disseminated intravascular coagulation, and organ failure can reduce the likelihood of recovery. Even after surviving the initial illness, some horses require weeks to months of rehabilitation before returning to their previous level of health. ^[3][7]

Because SIRS can progress rapidly, early recognition and prompt treatment offer the best chance of a successful outcome.

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Prevention

Owners and veterinarians cannot always prevent SIRS because it develops as a complication of another illness or injury, rather than as a disease on its own. However, prompt diagnosis and treatment of the underlying condition may limit the inflammatory response and reduce the risk of serious complications. ^[3]

Prevention is best understood as early recognition, rapid veterinary care, and careful monitoring of high-risk horses. Horse owners can help by seeking veterinary attention promptly for persistent colic, high fever, rapidly worsening illness, major injuries, severe diarrhea, or signs of infection.

Careful monitoring after surgery or foaling is also important because complications such as postoperative infection or a retained placenta can increase the risk of systemic inflammation. ^[3]

Routine management may also reduce some triggers associated with severe systemic inflammation. This includes maintaining a vaccination and parasite control program recommended by your veterinarian, seeking prompt care for wounds or infections, managing digestive disease early, preventing heat stress during hot or humid weather, and following postoperative or post-foaling monitoring instructions carefully.

Because SIRS can progress rapidly, waiting to see whether a severely ill horse improves can be risky. Contact a veterinarian immediately if a horse shows persistent or severe colic, profuse diarrhea, high or abnormally low temperature, rapid breathing, marked weakness, dark or pale gums, cold limbs, collapse, signs of shock, or sudden worsening after surgery, foaling, or injury.

Frequently Asked Questions

Here are some frequently asked questions about systemic inflammatory response syndrome in horses:

Summary

Systemic inflammatory response syndrome (SIRS) is a medical emergency in which excessive inflammation disrupts circulation and threatens organ function. Prompt veterinary treatment is essential to address the cause and prevent serious complications.

• SIRS can result from severe gastrointestinal disease, infection, trauma, or surgery.
• Early signs include lethargy, reduced appetite, abnormal temperature, rapid vital signs, and dehydration.
• Severe SIRS can cause shock, laminitis, abnormal clotting, and organ dysfunction.
• Diagnosis combines clinical findings, laboratory results, and the underlying cause.
• Treatment may include fluids, medication, surgery, supportive care, and close monitoring.
• Prognosis depends on the cause, treatment timing, and development of complications.