Fever (Pyrexia) in Horses: Causes, Associated Signs & Treatment for High Body Temperature

Fever in horses is a common symptom of infection and inflammation and can range from a mild, temporary increase in body temperature to a warning sign of serious illness. Since a horse’s body temperature also rises after exercise or during hot weather, it’s important to distinguish a true fever from transient hyperthermia.

The significance of a fever depends on the horse’s health context. The duration of fever, accompanying symptoms, history of recent travel or disease exposure, and the horse’s overall condition all provide important clues about the underlying cause and the need for veterinary attention.

Because fever can appear before more specific signs of illness, consistent monitoring of body temperature aids in early detection of disease. Read on to learn how to recognize fever in horses, when to seek veterinary care, and how biosecurity can help reduce the spread of infectious disease.

What Is a Fever in Horses?

Fever, also called pyrexia, refers to a sustained increase in body temperature caused by the body’s immune response to disease. Inflammatory signals cause the brain to raise the body’s temperature set point, triggering heat production and conservation until the horse reaches this higher target temperature. ^[1]

This increase in temperature is part of the body’s defense response and can support immune activity while making conditions less favorable for some infectious organisms.

Normal rectal temperature in an adult horse is generally about 99 – 101.5°F (37.2 – 38.6°C). ^[1][2] A resting temperature above 101.5°F (38.6°C) should prompt closer monitoring for fever, particularly if the horse also shows other signs of illness.

Temperatures in the 101.5 – 103°F (38.6 – 39.5°C) range warrant increased vigilance, while temperatures above 105°F (40.5°C) require urgent veterinary attention.

Thermoregulation & the Fever Response

Body temperature in horses is regulated by the hypothalamus, a region of the brain.

The hypothalamus controls several involuntary processes in the body, including: ^[1]

• Thermoregulation
• Hunger and thirst
• Sleep
• Blood pressure

The hypothalamus receives information about internal and environmental temperature and coordinates responses that adjust heat production and heat loss to keep body temperature near its normal set point. ^[1]

During infection or inflammation, substances from infectious organisms and damaged tissues stimulate immune cells to release inflammatory signaling molecules called cytokines. These inflammatory signals promote production of prostaglandin E2 (PGE2) in the hypothalamus.

PGE2 raises the body’s temperature set point, so the horse’s normal body temperature is temporarily perceived as too low. The body’s thermoregulatory responses then work to increase and conserve heat until the new set point is reached. ^[1]

When the inflammatory stimulus resolves and the hypothalamic set point is reset to normal, the body shifts back toward heat loss. This regulated change in the set point is what distinguishes fever from an uncontrolled rise in temperature caused by excess heat production or impaired heat dissipation.

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Monitoring Equine Body Temperature

It’s helpful for owners and caretakers to perform routine basic health checks for the horses in their herd. By keeping track of each horse’s base measurements, you can detect subtle changes that may require veterinary intervention before they progress.

To measure a horse’s rectal temperature, use a digital thermometer with a small amount of lubricant on the tip. Stand beside the horse’s hindquarters rather than directly behind them, and position yourself close to the hind leg with your body facing away from the horse’s head. This reduces the risk of injury if the horse kicks.

Gently move the tail to one side with one hand. Some horses may clamp their tail down, so use steady, gentle pressure rather than forcing it aside. With your free hand, insert the thermometer into the rectum.

Angle the thermometer slightly toward the side of the rectum rather than directing it straight into the center. This helps keep the tip against the rectal wall and reduces the chance of measuring the temperature of fecal material instead of the horse’s body temperature.

Rectal Temperature Check
Photo: Dr. Madison Ricard, DVM, PhD, DACVP, PAS

Keep the thermometer in place until the reading is complete. Digital thermometers typically beep when they have finished measuring. Traditional mercury thermometers require approximately one minute to take an accurate reading, but they are not recommended because they can break during use. ^[3]

If a mercury thermometer must be used, it should be securely attached to a line so it can be retrieved if it slips further into the rectum. Digital thermometers are generally safer and easier to use for routine temperature monitoring.

Table 1. Guide to equine body temperature monitoring ^[4]

What to Do When a Horse Has a Fever

Practical steps when a horse develops an unexplained fever include: ^[1][5][6]

• Recording and monitoring the horse’s temperature
• Monitoring appetite, drinking, manure, and other symptoms
• Providing rest and fresh water
• Avoiding exercise
• Contacting a veterinarian when appropriate
• Avoiding medication unless directed by a veterinarian

Importance of Biosecurity

Because infectious disease is a common cause of fever, an unexplained fever should be treated as potentially contagious until the cause is known. Some infections that cause fever can spread rapidly between horses, leading to serious consequences in shared housing environments.

Biosecurity measures should begin as soon as fever is detected. Move the affected horse away from other horses when possible and avoid sharing buckets, grooming supplies, tack, thermometers, or other equipment. People handling the horse should wash or sanitize their hands and clean or change contaminated footwear and clothing before working with other horses.

Monitor the temperatures of other potentially exposed horses and watch for other symptoms. Limit unnecessary movement of horses on and off the property until a veterinarian can assess the situation and advise whether additional isolation, testing, or movement restrictions are needed.

When to Call the Veterinarian

Fever requires prompt veterinary attention when it is high, persists, returns, or occurs with symptoms suggesting serious illness. The horse’s overall condition and accompanying symptoms are generally more important than relying on a temperature reading alone. ^[1][7]

Veterinary attention is particularly important when fever occurs with: ^[1][5]

• Severe lethargy or weakness
• Refusal to eat or drink
• Difficulty breathing
• Colic or significant diarrhea
• Neurologic symptoms
• Dehydration
• Difficulty standing

Fever in foals warrants greater concern, as does fever affecting several horses in the same barn. Multiple affected horses can indicate a contagious disease and should prompt immediate biosecurity measures. ^[5][6]

Fever vs. Hyperthermia

An elevated body temperature does not always mean the horse has a fever. Exercise or hot and humid weather can temporarily raise a horse’s body temperature, which is known as hyperthermia. ^[1][2][8]

In cases of fever, the body raises its temperature set point as part of its defense response to infection and inflammation. Higher body temperature facilitates immune function and creates less favorable conditions for some infectious organisms. ^[1]

Hyperthermia occurs when the horse produces or absorbs more heat than it can dissipate. Exercise can substantially increase body temperature, particularly in hot and humid conditions that interfere with heat loss.

Rectal temperature may also remain elevated or continue rising briefly after exercise, so a high temperature measured immediately after work does not necessarily indicate fever. ^[1][2][8]

Table 2. Fever vs. hyperthermia in horses

Symptoms Associated with Fever in Horses

Some feverish horses show few other symptoms, particularly during the earliest phases of infection. When additional symptoms occur, they vary depending on the underlying cause and severity of the condition. ^[1][5][9]

Common symptoms associated with fever in horses include: ^{[1][5][9][7][10]}

• Lethargy
• Reduced appetite
• Shivering or sweating
• Increased heart rate
• Increased respiratory rate
• Dehydration
• Weight loss in cases of prolonged illness

Other symptoms may provide clues to the underlying cause. Different patterns can point toward different body systems, such as: ^{[5][9][7][11]}

• Respiratory signs: Signs involving the airways and lungs, such as coughing, nasal discharge, increased respiratory rate, or difficulty breathing
• Gastrointestinal signs: Signs involving the digestive tract, such as diarrhea, changes in manure, reduced appetite, or colic
• Musculoskeletal signs: Signs involving the muscles, joints, or limbs, including swelling, stiffness, lameness, or an abnormal gait
• Neurologic signs: Signs involving the brain, spinal cord, or nerves, such as incoordination, stumbling, weakness, changes in behavior, or changes in vision

The severity of accompanying symptoms is just as important as the temperature itself when determining how urgently veterinary assessment is needed.

What Causes Fever in Horses?

Fever in horses can result from infectious or noninfectious disease. Infection is a common cause, but inflammation, immune-mediated disease, and some cancers can also cause an increase in body temperature. ^[1][7]

Infectious Causes

Bacterial and viral infections are common causes of fever in horses. In some cases, fever develops before more specific symptoms appear, making the underlying infection difficult to identify initially. ^[11]

Infectious diseases that may lead to fever in horses include: ^{[5][9][10][11]}

• Equine influenza
• Equine herpesvirus
• Strangles
• Equine coronavirus
• Anaplasmosis
• Potomac horse fever
• Bacterial pneumonia

Equine Influenza Virus

Equine influenza is a contagious viral respiratory infection and a common cause of fever in horses. Fever may appear with other respiratory signs, but early in the illness, the horse may simply appear dull or unwell. ^[5][11][9]

Influenza typically causes an abrupt onset of high fever, which can reach 106°F (41.1°C), along with depression, reduced appetite, weakness, and a dry, harsh cough. In uncomplicated cases, the fever and other acute signs often resolve within several days, although coughing can persist for weeks. ^[12]

Equine Herpesvirus

Equine herpesvirus, especially EHV-1 and EHV-4, can cause fever in horses. These viruses are usually under consideration during investigation of fever in a horse because they can present with fever before other signs are obvious. ^[5][11]

EHV-1 often produces a biphasic fever, with an initial temperature spike early in infection followed by a second rise several days later. The first fever can precede respiratory signs, while the second often occurs once the virus is detectable on bloodwork. ^[13]

EHV testing may require both a nasopharyngeal swab and a blood sample because the timing of nasal shedding and blood involvement can differ during infection. This matters when fever appears early, before the full pattern of illness is clear. ^[11]

Strangles

Strangles is caused by Streptococcus equi equi. It is a contagious bacterial respiratory disease that can cause fever in horses. ^[5][11]

Fever is often the first sign of strangles and can reach 103 – 106°F (39.4 – 41.1°C). Nasal discharge, depression, and swelling of the lymph nodes under the jaw often develop within 24 – 48 hours after the initial fever.

Because pyrexia can occur before obvious respiratory signs or bacterial shedding, temperature monitoring can help identify affected horses early in an outbreak. ^[14]

Equine Coronavirus (ECoV)

Equine coronavirus is a contagious viral infection of the intestinal tract that primarily affects adult horses. Even though the infection is localized to the digestive system, fever, lethargy, and reduced appetite are the key initial signs of ECoV. Some horses later develop gastrointestinal signs, including: ^[5][11]

• Diarrhea
• Colic
• Changes in manure

Fever is one of the most common signs of equine coronavirus infection and reach up to 105°F (40.5°C). Fever is much more common than diarrhea, so the absence of loose manure does not rule out coronavirus during diagnosis. ^[15]

Anaplasmosis

Anaplasmosis, or equine granulocytic anaplasmosis, is a seasonal tick-borne bacterial infection caused by Anaplasma phagocytophilum. It is transmitted by Ixodes ticks and is not considered contagious between horses. ^[5][9]

A horse with anaplasmosis may also present with: ^[5][11]

• Stiffness
• Limb edema (lower leg swelling)
• Lethargy
• Behavior changes
• Loss of appetite

Fever is a prominent sign and is typically around 103 – 104°F (39.5 – 40°C), although temperatures can occasionally rise above 107°F (41.7°C). Fever may persist for 6 – 12 days and can occur in a biphasic pattern. ^[16]

Because the disease is linked to tick exposure, the horse’s region, season, and recent environment can help guide the veterinarian’s diagnosis in suspected cases. ^[5][11]

Potomac Horse Fever

Potomac horse fever is associated with Neorickettsia infection. It can cause fever before gastrointestinal signs, such as diarrhea or colic, become obvious.

Risk varies by region and season, so veterinarians consider location and timing when deciding whether Potomac horse fever belongs on the list of possible causes. ^[5][11]

Bacterial Pneumonia

Bacterial pneumonia can cause fever when infection and inflammation develop in the lower airways or lungs. Affected horses may also show: ^[5][10]

• Coughing
• Nasal discharge
• Increased respiratory effort
• Reduced appetite
• Poor performance

Blood tests can help veterinarians track inflammation in horses with bacterial pneumonia. Certain markers in the blood may rise when the illness is active and decrease as the horse improves, helping the veterinarian monitor how the horse is responding to treatment. ^[10]

Noninfectious Causes

Noninfectious fever can occur with inflammatory or immune-mediated conditions and some forms of cancer. Tissue inflammation or injury can trigger an inflammatory response that raises body temperature.

Some horses may develop a temporary fever following vaccination or as a reaction to certain medications. ^[1][7][17]

Inflammatory Conditions

Tissue inflammation or injury can trigger an immune response that raises body temperature. In these cases, fever reflects the body’s inflammatory response rather than a contagious disease. ^[1][17]

Blood tests can help veterinarians look for signs of active inflammation. These tests do not show the exact cause of fever in noninfectious cases, but they help show the severity of inflammation and disease progression. ^[17]

Immune-Mediated Disease

Immune-mediated disease can cause fever when the immune system mounts an inflammatory response when no active infection or injury is present. These cases may be difficult to identify due to vague early signs and the response of multiple body systems. ^[1][7]

Cases of immune-mediated disease in horses often require further diagnostic testing because the fever may persist or recur without a clear infectious cause. ^[7]

Cancer

Some cancers can cause persistent or recurrent fever in horses by triggering ongoing inflammation or broader illness throughout the body. ^[1][7]

Although cancer is not a common cause of fever in horses, veterinarians may consider it when fever continues without an obvious infectious, inflammatory, or environmental explanation. ^[7]

Vaccination Response

Some horses may develop a temporary fever after vaccination. This side effect is usually short-lived and reflects the intended immune response to the vaccine. ^[1]

A mild, brief temperature increase after vaccination is not the same as fever caused by an active infectious disease. Owners should still monitor the horse and contact a veterinarian if fever post-vaccination is high, prolonged, or accompanied by concerning symptoms. ^[1]

Medication Reactions

Certain medications can cause fever or fever-like reactions in horses. Providing your horse’s recent medication history can help the veterinarian decide whether to consider a drug reaction when diagnosing the underlying cause of a fever. ^[1]

Medication reactions can resemble other causes of fever because the horse may simply appear unwell. Owners should not stop or change prescribed medications without veterinary guidance, but they should report any new fever that develops after treatment begins. ^[1]

Diagnosis of Fever in Horses

Once fever is confirmed, diagnostic testing focuses on identifying the underlying cause. The horse’s symptoms, history, recent travel or exposure, and pattern of fever help determine which tests are appropriate. ^[1][5][11]

Diagnostic tests for the underlying causes of fever include: ^{[1][11][7][17]}

• Complete blood count
• Serum biochemistry
• Fibrinogen
• Serum amyloid A
• PCR testing
• Bacterial culture
• Blood culture
• Urinalysis
• Fecal testing
• Ultrasounds
• X-rays
• Endoscopy

The diagnostic workup helps determine whether the fever is caused by infection, inflammation, immune-mediated disease, or another underlying condition. Identifying that cause is important because treatment is directed at the disease process rather than the elevated temperature alone.

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Treatment of Fever in Horses

Treatment for fever in horses depends on the cause and may include: ^[1][5]

• Antibiotics for bacterial infections
• Anti-inflammatory medications
• Stall rest or controlled exercise
• Pain management
• Supportive care

Persistent or recurrent fever may require additional or repeated testing when the cause is not initially apparent. Recording a horse’s temperature over time and monitoring for new symptoms can help identify patterns and narrow the possible causes. ^[1][11][7]

Medications that lower body temperature can help manage fever and related signs, but reducing a fever does not treat its underlying cause. Continued monitoring and appropriate treatment of the primary disease remain essential.

Preventing Illnesses That Cause Fever

Fever itself is not contagious, but many infectious diseases that cause fever can spread between horses. When an infectious cause is possible, biosecurity measures are essential for reducing the risk of disease transmission, especially in shared living environments. ^[5][6]

Recommended practices for limiting disease spread on horse farms include: ^[5][6]

• Keeping recommended vaccinations up to date
• Isolating horses that develop fever or other symptoms of infectious disease
• Monitoring temperatures and other symptoms in potentially exposed horses
• Using dedicated buckets, grooming supplies, tack, and other equipment for isolated horses
• Cleaning and disinfecting equipment and contaminated areas appropriately
• Washing or sanitizing hands between horses
• Changing or disinfecting footwear and clothing after handling isolated horses
• Limiting unnecessary movement of horses and people during a suspected outbreak
• Caring for healthy horses before handling sick or isolated horses

Prompt isolation is particularly important when a horse develops an unexplained fever, as the symptom may be an early indicator of contagious disease. Equipment used for an isolated horse should remain in the isolation area to keep infectious organisms from spreading indirectly on equipment, clothing, and footwear. ^[6][11]

General preventive health care also helps reduce the risk of illnesses that cause fever. Horses should receive regular veterinary examinations and follow a veterinarian-guided vaccination schedule based on age, location, travel, exposure risk, and intended use.

Annual wellness exams give veterinarians an opportunity to review vaccination needs, identify emerging health concerns, and adjust preventive care before illness develops.

Frequently Asked Questions

Here are some frequently asked questions about fever in horses:

Summary

Fever in horses is a clinical sign of infection or inflammation that must be distinguished from exercise- or heat-related hyperthermia. Monitoring symptoms, seeking veterinary guidance, and using appropriate biosecurity can help protect the affected horse and others in the barn.

• Normal rectal temperature in an adult horse is generally 99 - 101.5°F (37.2 - 38.6°C)
• Fever may result from infectious disease, inflammatory or immune-mediated conditions, cancer, vaccination, or medication reactions
• Respiratory, gastrointestinal, systemic, and neurologic symptoms can help identify the possible underlying cause
• Diagnostic testing may include bloodwork, PCR testing, cultures, urinalysis, fecal testing, X-rays, ultrasound, or endoscopy
• High, persistent, or recurrent fever and serious accompanying symptoms require prompt veterinary attention
• Horses with unexplained fever should be isolated and managed with dedicated equipment until infectious disease is ruled out