Fever in Horses

Proper steps in assessing and treating fever in horses

According to the American Horse Council, there are more than 7 million horses in the United States, with data indicating about 1 million of those horses experiencing a fever in the course of a year. While it may cause concern for horse owners, fever is fairly common in horses. But how do veterinarians properly assess fever and when – or if – it needs to be treated?

What is fever?

Fever or pyrexia is defined as body temperature higher than normal. The hypothalamus at the base of the brain determines the body’s temperature setpoint, similar to that of your house’s thermostat. The body reacts to stimuli to cool or heat the body to maintain that set point. If there is something wrong in the body due to infection or illness, chemicals change the setpoint and make it higher for a while. This contrasts another condition, hyperthermia, which is a raising of the body temperature due to exercise, extreme heat and humidity, or anhidrosis, a condition in some horses that interferes with their ability to sweat to cool themselves.

“It’s important to realize fever is a clinical sign, not a disease in and of itself,” said Dr. Dan Dreyfuss of KindredBio. Dreyfuss was a practicing veterinarian for 31 years before joining the biomedical company. “It’s not Potomac Horse Fever, it’s just fever, which indicates an underlining disease. It can be an immune-mediated disease or an infectious disease. In horses, the most common reason for a horse to run a fever is a respiratory disease.”

How do you tell a horse is running a fever?

“Most often, horses are not eating and drinking normally or their behavior changes. These are usually the signs horse owners might notice first,” Dreyfuss said. “Horses that separate from the herd, display lethargy or depression should have their temperature taken.”

What is a horse’s normal temperature?

“Normal temperature varies in horses, just like in people,” Dreyfuss explained. “An adult’s normal temperature will range from 99 degrees to 101 degrees. Once you get over 101, for most horses, that would be a low-grade fever. For clinical studies, we often define fever as greater than 102 degrees. Foals will run a little higher than adult horses for the first several months of life.”

When is hot too hot?

“Whether a fever is too hot really depends on the underlying cause of the fever and the horse,” Dreyfuss said, who worked extensively with racehorses during his years of veterinary practice. “For instance, bringing the younger horses to the track was kind of like kids in kindergarten. They all pooled together and shared their little diseases and colds from where they came from. A lot of them were viral or respiratory.”

Dreyfuss said it was not unusual to have a 2-year-old horse come to the track and someone takes their temperature only because it was a normal, routine part of their care, and the horse is 104, but they are eating and drinking.

“They look totally normal,” he added. “No one would know they had a fever if they hadn’t taken the horse’s temperature. So that’s 104 in horses not looking sick.”

On the other hand, he said you could have an adult horse with pneumonia running a temperature of 102.5 or 102.6, and they look sick.

“What’s considered a high fever – yes, you get to 104, 105 is a high fever – but the clinical significance of it really depends on the underlying cause of the fever,” Dreyfuss said.

Importance of routine care

Part of routine care is establishing a normal temperature for an individual horse, Dreyfuss recommended. That way horse owners know the baseline for each horse. Getting comfortable taking a horse’s temperature and taking it every day for a few days will help establish a normal range for the horse.

The horse has a fever. Now what?

The first thing to do after determining the horse has a fever is a thorough physical exam.

“Listen to the chest, heart, lungs and belly,” Dreyfuss said. “My goal always was to localize a reason why the horse has a fever. Determine if there is a respiratory or G.I. tract issue or do they have a wound on a leg that’s swollen and infected, which could be leading to the symptoms.”

A next step would be some diagnostic testing. Most veterinarians will do a minimum data base, which is a complete blood count and serum chemistry.

“A test that’s fairly common in horses now is called SAA or serum amyloid A. It gives a measure of inflammation in the body. That would be a second step,” according to Dreyfuss.

Beyond that, if needed, veterinarians can pursue tests or diagnostic procedures that are specific to the system they think is involved.

“Fever could lead to laminitis when you are dealing with diseases that have endotoxemia,” Dreyfuss explained. “Those are your more serious infectious diseases like pleuropneumonia or any diseases that cause colitis, like Potomac Horse Fever. There are exceptions to every rule, so never say never and never say always, but generally, if you are dealing with fever and laminitis, then you are dealing with a serious infectious disease.”

Fever treatments

The most common step to bring down fever has been to administer NSAIDs, or nonsteroidal anti-inflammatory drugs like banamine and bute. A new option available is Zimeta, the only FDA-approved product to treat pyrexia in horses.

Dreyfuss pointed out not every horse that has a fever needs to be treated with an anti-pyretic.

“Fever exists for a reason,” he explained. “It helps the body to fight off the infectious process. In my experience, horses that run fevers are very quick to stop eating and drinking. That lack of eating and drinking can be as important or sometimes more important than the actual cause that is leading to fever as a clinical sign.

“They may have a mild respiratory disease, but if they aren’t eating properly, they don’t have the calories they need to fight off that infection, and if they aren’t drinking, they are prone to obstructions in their G.I. tract, which can lead to pain or colic. That can be more serious than the respiratory disease that caused them to run a fever,”
he added.

Take home message

It’s important for horse owners to:

a. Recognize the signs a horse has a fever

b. Be comfortable taking a horse’s temperature

c. If fever persists more than 24 hours or increases rapidly, or if a horse is really depressed, it’s crucial to contact a veterinarian.

“(The) most important thing you want is a veterinarian to determine the underlying cause of the symptom of fever,” Dreyfuss said. “It dictates the treatment of that individual horse.”

For DSRs discussing fever with veterinarians, it’s important to recognize fever is a clinical sign. It’s not a disease in and of itself. Not all fevers need to be treated or controlled with an anti-pyretic, but there is a product available for this if needed.

“It’s very important when you do make that decision for the horse especially if they have gone off feed and water, you want to use something that has demonstrated both efficacy and safety in horses that are sick,” Dreyfuss concluded.

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