It’s feeding time, and your horse usually nickers and eagerly dives into his feed bucket, but today he stands quietly and appears disinterested. You check his temperature, and the thermometer reading rapidly climbs to 103.7 F. Your horse has a fever, which most likely caused the change in his behavior. So, now what? Let’s address the basics of fever in horses with questions veterinarians frequently receive from horse owners.

What is a normal temperature for my horse?

The standard temperature range for horses is 99.5-101.5 F, but regularly monitor and establish your individual horse’s baseline because each horse varies. Most horses tolerate temperature taking with a rectal thermometer and should be trained to safely allow you to perform this simple procedure. Record your horse’s temperature for several consecutive days to determine his normal. A simple drug-store digital thermometer provides an accurate temperature in seconds. “Monitoring temperatures can be helpful in determining each individual’s normal values and allows large equine facilities to determine problems early,” says Alison LaCarrubba, DVM, Dipl. AVBP, associate professor at the University of Missouri College of Veterinary Medicine, in Columbia. Routinely monitoring temperatures can play a critical role in maintaining herd health and preventing disease spread because it allows you to isolate horses before infectious disease spreads far and wide, she adds.

What does it mean when my horse has a fever?

Fever, or pyrexia, happens when the body’s temperature set point has increased, typically due to an infection or inflammatory process. However not all elevated temperatures are truly fevers. Pyrexia is caused by the part of the brain called the hypothalamus. Alternatively, hyperthermia, or increased body temperature without a change in the brain, can be caused by exercise, exposure to heat, anhidrosis (inability to sweat), or a reaction to a drug or toxin. In cases of hyperthermia, sweating and other cooling measures such as moving to a shaded area and applying cool water to the body often quickly bring the temperature to a normal range.

What causes fevers in horses?

Most fevers in horses are caused by infections—frequently affecting the respiratory or gastrointestinal systems or developing from a wound or laceration—cause most fevers. Less common causes include autoimmune or neoplastic (abnormal tissue growth) conditions. Sometimes a horse will develop a fever, and it will resolve without a diagnosis, which is a sign the immune system is performing well and fighting off an unknown pathogen by raising the body’s temperature. If your horse’s fever persists beyond one or two days, or he has significant clinical signs, call your veterinarian, who can perform a physical exam and appropriate diagnostic tests based on exam findings.

“Exam findings such as a cough, nasal discharge, swollen lymph nodes, diarrhea, visible abscesses or wounds, along with bloodwork including a CBC (complete blood count) and chemistry as well as SAA (serum amyloid A) can often help us identify infection or organ systems involved,” says Carol Clark, DVM, Dipl. ACVIM, of Peterson Smith Equine Hospital + Complete Care, in Ocala, Florida. If the fever persists with an unknown cause, your veterinarian might refer your horse to an internal medicine specialist for more complex testing. “We (internal medicine specialists) will look more in-depth by using additional imaging including ultrasound or radiographs of thorax and abdomen, as well as sampling procedures such as bronchoalveolar lavage and transtracheal wash, which can lead to submission of samples for testing for specific pathogens or diseases,” says Clark. Identifying the underlying disease process causing the fever will help veterinarians effectively treat your horse.

How do veterinarians treat fevers in horses?

“A fever is the body’s way of reacting to inflammation and infection,” says LaCarrubba. “The fever itself is a symptom of the primary problem but not actually the primary problem.” Veterinarians might give your horse medication to reduce the fever so the horse does not develop secondary conditions due to inappetence or dehydration, she adds, but this should not replace creating a treatment plan for the root cause of the fever.

General cooling techniques, including spraying cool water on the horse and putting fans in his stall, along with administering medications—most commonly non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine (Banamine), phenylbutazone (Bute), or dipyrone, will help make a febrile horse feel better, but these supportive treatments only manage the clinical sign of fever. Never use more than the recommended dosage or frequency of anti-inflammatory medications—more is not better and giving excess medication can lead to other potentially life-threatening conditions (i.e., liver problems). Follow your veterinarian’s direction to keep your horse safe.

“Good biosecurity practices such as washing hands, using appropriate PPE (personal protective equipment), and cleaning shared equipment, along with isolating a febrile horse from direct contact with herd-mates is vital in stopping the spread of infectious diseases,” says Clark.

Addressing the underlying disease process helps the horse feel better long-term and eliminates the cause of the fever. In the case of some viral infections, the above supportive care is appropriate. But for diseases caused by bacterial infections, such as pneumonia, enteritis, and colitis, your veterinarian needs to carefully and appropriately select antibiotics to treat your horse.

When is my horse’s fever considered dangerous?

While it is rare for a high fever alone to cause permanent organ damage in horses, a temperature over 105-106 F (and the underlying cause of this fever) is cause for serious concern and warrants immediate medical care. Persistent (more than four to five days) low to moderate fevers, or recurrent fevers, are also indicators your veterinarian needs to perform additional testing to identify the cause.

Take-Home Message

Veterinarians frequently manage fevers. While a fever itself is not necessarily dangerous, the underlying disease process as well as the secondary problems, such as a horse not eating or drinking, can be devastating for the animal’s health. Regularly monitoring your horse’s temperature can help detect illness early, stop the spread of infection, and enable timely veterinary diagnosis and treatment.