Normal adult equine body temperature runs higher than our human version of 98.6° Fahrenheit, although anything above 101.5° is considered a fever for horses. What do you do when your horse is running a temperature? Let’s take a closer look, including possible causes and how you can help.
What Causes a Fever?
The medical term for fever is pyrexia (pyro from the Greek word for fire). A molecule that causes a fever is called a pyrogen.
Think of any warm-blooded animal’s normal body temperature as a thermostat set by a part of the brain called the hypothalamus. A pyrogen changes this internal set-point within the brain and turns the dial up, resulting in a fever.
Pyrogens can come from both outside and inside the body. Some types of bacteria contain specific chemicals that will directly cause a fever. Other bacteria, viruses, and infectious organisms stimulate the body’s immune system to produce pyrogens that travel through the bloodstream into the brain and tell the hypothalamus to turn up the heat.
Typically, a feverish horse will be lethargic and off-feed. He may have droopy ears and seclude himself from the herd or won’t come to the fence or stall door for a treat. If severe enough, he may show signs of dehydration, such as an extended skin tent and/or dry and tacky gums.
Taking a Horse’s Temperature
Ideally, you should train your horse to accept having his temperature taken before he gets sick. Temperatures should be taken rectally on horses; temporal thermometers designed for use on your forehead will not work through a horse’s hair coat.
Digital thermometers are relatively inexpensive; price differences typically indicate the speed at which they give a reading, and are not a reflection of their accuracy.
Have your horse tied or have someone else hold him while you take his temperature. Stand hip to hip and face his tail. Use the hand closest to his body to lift and hold his tail out of the way so that you can insert the thermometer with your other hand.
Completely insert the thermometer into the horse’s anus; inaccurate readings result from not inserting it deep enough. It helps to put some petroleum jelly (Vaseline) on the thermometer first.
Causes of Fever in Horses
The most common cause of fever is an infection somewhere in the body, typically bacterial or viral. Rarer causes include some cancers and autonomic immunologic reactions (meaning self-induced and not associated with a foreign material).
For more common causes, fevers can be divided into the general body system associated with the infection:
◆ Respiratory: The influenza virus, equine herpesvirus (EHV-1 and EHV-4), and the bacterial disease called strangles are all common infections of the horse’s respiratory system, infecting the airways, lymph nodes along the throat, and sometimes the lungs.
◆ Digestive: Colitis caused by bacteria such as Salmonella and Clostridium can result in diarrhea and fever. Potomac Horse Fever is also a common gut-related cause of fever.
◆ Circulatory: Vector-borne diseases from insect bites can cause blood-borne illness, such as anaplasmosis and piroplasmosis. Fever is one of the first signs of these diseases.
◆ Neurologic: Mosquito-vectored viruses such as Eastern or Western Equine Encephalitis (EEE/WEE) and West Nile Virus will have a fever component to their neurologic syndrome.
It’s important to point out that vaccines are available that can prevent the majority of the infectious causes listed above. Some of these diseases can be severe and life-threatening. Keeping your horse up to date on his annual vaccines goes a long way in preventing many instances of fever.
Fever of Unknown Origin
On occasion, the cause of a fever is challenging to pin down. The medical term for this is the mysterious-sounding fever of unknown origin, or FUO; this is meant as a temporary term. There’s always a trigger for the body to respond with a fever—some triggers are just more elusive than others.
◆ To make things more challenging, sometimes a fever can be “undulating,” meaning it is intermittent or will bounce around in severity. One thing you can do is measure your horse’s temperature at least twice daily at the same time of day to see if there’s a pattern. This information can be very helpful for your veterinarian.
◆ Be patient. Your vet may take additional blood (or other) samples for further, more specific diagnostics, and results can take a while to come back. Sometimes, consultations with internal medicine specialists can help to brainstorm possible causes and solve the mystery in order to help your horse.
How to Treat a Horse’s Fever
After you discover that your horse has a fever and call your vet, he or she will conduct a complete physical exam, including palpation of lymph nodes and listening to the chest and gut. Make sure your vet knows your horse’s history, including recent travel, exposure to new horses, changes in housing or feed, recent illness, current medications, and vaccine status.
Your vet may then decide to take a blood sample to evaluate dehydration, major organ function, and blood cell counts. Blood cell numbers can tell your vet a lot about the severity and sometimes the general type of infectious process that may be going on.
Occasionally, further diagnostics may be needed (see sidebar, right). These can include ultrasound, fecal samples, more blood tests, or other samples, such as nasal swabs for specific pathogens.
Treatment will vary depending on the fever’s cause (such as antibiotics versus other therapeutic drugs). However, management of the fever itself is usually routine, involving a non-steroidal anti-inflammatory like flunixin meglumine (Banamine) or phenylbutazone (bute). A relatively new drug available in the U.S. called dipyrone (Zimeta) can also be used specifically to reduce fever in horses.
If your horse is dehydrated and not eating, IV fluid therapy may be necessary. Offering mashes and other means can help entice your horse to eat. Supportive care while waiting for the cause of the fever to be pinned down and hopefully treated is key while managing a fever.
Additionally, if an infectious cause is suspected or confirmed and there are other horses on the property, informing others and keeping your horse separate is important to prevent the spread of a pathogen.
Key Takeaway
Fevers are one way the body tells us something is wrong, so we should make sure we’re listening. Knowing how to identify when your horse has a fever and working with your veterinarian are the best ways to properly care for your horse.
This article about how to treat a horse’s fever appeared in the January/February 2024 issue of Young Rider magazine. Click here to subscribe!
