The most reliable way to tell if a horse has a fever is to take its rectal temperature. A normal adult horse runs between 99.0°F and 101.0°F (37.2°C to 38.3°C), so anything consistently above 101.0°F qualifies as a fever. While behavioral cues like lethargy or loss of appetite can raise suspicion, a thermometer is the only tool that gives you a definitive answer.
What Counts as a Normal Temperature
Adult horses at rest have a body temperature between 99.0°F and 101.0°F. Newborn foals run slightly warmer, with a normal range of 100.0°F to 102.0°F. These numbers matter because a reading of 101.5°F in a foal might be perfectly fine, while the same reading in an adult horse signals a low-grade fever.
Body temperature also fluctuates naturally throughout the day. It tends to be lowest in the early morning and highest in the late afternoon, sometimes varying by about a degree. That’s why it helps to take your horse’s temperature at the same time each day for a few days when they’re healthy, so you know their personal baseline.
How Exercise and Weather Shift the Numbers
During exercise, roughly 80% of the energy produced by working muscles converts to heat rather than movement. This can push a horse’s temperature well above 101°F even when nothing is wrong. A reading taken right after a hard ride or workout isn’t reliable. Wait at least 30 to 60 minutes after exercise before checking, and make sure the horse has had a chance to cool down in shade or with airflow.
Ambient temperature plays a role too. In resting horses, surface temperatures change in direct proportion to shifts in air temperature between roughly 40°F and 77°F. On a very hot day, a horse’s core temperature may read slightly higher than usual without indicating illness. If you’re unsure whether a warm reading reflects genuine fever or environmental heat, recheck after the horse has been in a cool, shaded area for a while.
How to Take Your Horse’s Temperature
You’ll need a thermometer and lubricant (petroleum jelly or a water-based lubricant both work). Digital thermometers are the easier, safer option. They read in 15 to 30 seconds and beep when finished. Mercury or glass thermometers still work but take up to three minutes and carry a risk of shattering if the horse pushes one out. If you do use a glass thermometer, clip it to the tail hairs with a small alligator clip or string so it can’t fall and break.
To take the reading safely:
- Position yourself correctly. Stand with the left side of your body directly against the horse’s hip. This keeps you close enough that a kick would push you away rather than land with full force.
- Lift the tail gently. Shift it to one side rather than pulling it straight up, which can startle some horses.
- Insert the lubricated thermometer. Slide it into the rectum about two to three inches, angling it slightly toward the body wall rather than straight in.
- Hold it in place. Keep a hand on the thermometer until the digital readout beeps or the mercury has had time to rise fully, then remove it and read the display.
If your horse is fidgety or you’re doing this for the first time, having someone hold the horse’s head with a lead rope makes the process safer for both of you.
Behavioral and Physical Signs That Suggest Fever
A thermometer confirms a fever, but you’ll often notice something is off before you reach for one. Horses with fever commonly show depression, lethargy, and loss of appetite. They may stand quietly in a corner of the paddock instead of grazing, seem uninterested in feed, or appear dull and withdrawn.
Other signs that can accompany fever include coughing, nasal discharge, abnormal manure (either too loose or absent), colic-like discomfort, lameness, and in some cases jaundice, a yellowing visible on the gums and whites of the eyes. Not every feverish horse shows all of these. Some horses, particularly those with tick-borne infections, spike a fever with no respiratory symptoms at all, which is easy to miss if you’re only watching for a cough or runny nose.
Checking the Gums
A quick look at your horse’s gums can add useful information. Healthy gums are pink and feel slippery or “slimy” to the touch. Press a finger firmly against the gum above the front teeth, then release. The spot will blanch white and should return to pink within one to two seconds. A longer refill time can indicate dehydration or compromised circulation, both of which sometimes accompany fever. Dry, tacky gums point toward dehydration as well.
Other Vital Signs Worth Checking
If the temperature is elevated, checking heart rate and breathing rate gives you a more complete picture to share with your vet. A normal resting heart rate for an adult horse is 28 to 44 beats per minute (foals run higher, at 50 to 70). You can feel the pulse under the jaw where the facial artery crosses the lower edge of the jawbone. Count beats for 15 seconds and multiply by four.
Normal resting respiration for an adult horse is 8 to 15 breaths per minute, while foals breathe faster at 20 to 40. Watch the flank rise and fall, counting each rise as one breath. An elevated heart rate or respiratory rate alongside a fever suggests the horse’s body is working hard to fight something off, and that combination typically warrants a call to your veterinarian sooner rather than later.
When a Fever Becomes an Emergency
Most low-grade fevers in horses (101°F to 103°F) aren’t immediately dangerous, but they do signal that something is going on. A temperature above 105°F to 106°F is a serious concern and warrants immediate veterinary attention, both because of the fever itself and because whatever is driving a temperature that high needs urgent diagnosis.
Even moderate fevers become a problem when they persist. A fever lasting more than four to five days, or one that keeps coming back after appearing to resolve, is a sign your vet needs to run additional testing. Persistent or recurring fevers can point to bacterial infections, viral diseases, tick-borne illnesses, or internal abscesses that won’t clear up on their own.
Common Causes of Fever in Horses
Infections are the most frequent culprit. Respiratory viruses like equine influenza and equine herpesvirus (types 1 and 4) commonly cause fever along with coughing and nasal discharge. Bacterial infections such as strangles produce high fevers alongside swollen lymph nodes under the jaw. Tick-borne diseases, including piroplasmosis and anaplasmosis, can cause fever with depression and sometimes jaundice but little to no respiratory involvement.
Insect-transmitted viruses are another category to be aware of, especially in warmer climates. West Nile virus, for example, can cause fever alongside neurological signs. Horses in areas with heavy midge or mosquito populations face exposure to multiple viral diseases simultaneously, so a fever in bug season is worth investigating even if your horse seems only mildly off.
Non-infectious causes exist too. Reactions to vaccines commonly produce a mild, short-lived fever within 24 to 48 hours of injection. Heat stress, wounds with developing infections, and certain inflammatory conditions can all raise body temperature. The key is to not assume a cause. A thermometer tells you the temperature is elevated, but only a proper workup identifies why.