You’ve been keeping tabs on your horse as you’ve gone about your barn chores, but something’s not quite right. Normally, he’s never far from his buddy, and he’d be ranging around his paddock looking for the best bites of grass. Today, however, he’s spent most of his time hanging in the shady corner by himself. He seems normal enough when you bring him in, but as you’re grooming, you get out the thermometer. That’s when you really start to wonder what’s up: His temperature is just topping 102 degrees Fahrenheit.
You know that’s a little high—you’ve been in the habit of checking your horse’s temperature once or twice a month, and it’s always been about 100 degrees—but what do a couple more degrees really mean?
“There are several reasons why horses can have an increased body temperature that would not be a fever,” says Rose Nolen-Walston, DVM, DACVIM, of the University of Pennsylvania. “So the first question to ask when you take a horse’s rectal temperature and it is high is, ‘Is this a fever or not?’”
A “normal” body temperature for individual horses can vary, from about 98 to 101 degrees Fahrenheit, with 100 being average. But it’s also normal for a horse’s body temperature to fluctuate during the day. It may be somewhat higher in the evenings than in the mornings, for example, and it is likely to rise naturally on hotter days or after exercise. A mare’s temperature may rise and fall during different stages of estrus. All of these fluctuations are temporary.
“If you ride your horse and work him hard on a hot day, his temperature rises, but this is called hyperthermia rather than a fever,” says Nolen-Walston. “The main causes of hyperthermia include exercise, extreme heat and humidity, and anhidrosis [an inability to sweat].” Allowing him to rest and drink—and perhaps hosing him down with cool water—ought to bring his temperature down to normal within a half hour or so.
If, however, your horse’s temperature remains elevated with no obvious cause, then it’s time to investigate the reasons why. “Most of the time, if a resting horse has an increased rectal temperature it’s because he has a fever,” says Nolen-Walston.
Rise in body temperature is one of the first and most easily recognized signs of many illnesses, and it is part of the immune system’s defense against infection. “Fever is a response by the body—along with inflammatory processes—to try to combat pathogens by stimulating molecules to speed up healing processes,” says Katherine Wilson, DVM, DACVIM, of the Virginia–Maryland Regional College of Veterinary Medicine.
The best course of action when a horse has a fever can vary. How high his temperature is, and how long it lasts, can help you decide whether it’s best to let a fever run its course—or to call in a veterinarian right away. Here’s a look at how fevers work and how veterinarians suggest you handle them.
How fever works in horses
Fever is related to the body’s internal temperature regulation system, which is controlled by the hypothalamus. A small structure at the base of the brain, the hypothalamus receives sensory input from sensors in the central nervous system that monitor the heat of the blood as it circulates through the brain, as well as from nerves that detect temperatures near the surface of the skin. This gives the hypothalamus information about both internal and external temperatures.
“The hypothalamus determines the body’s temperature set point,” explains Nolen-Walston. That is, the hypothalamus determines the horse’s “normal” body temperature and acts to maintain a consistent internal temperature despite fluctuations in the external world. When the body’s internal temperature deviates too far from normal, the hypothalamus triggers a cascade of involuntary actions to “adjust the thermostat.”
If the horse starts getting too cold, smooth muscles in the skin contract to raise the hairs on his body, trapping an insulating layer of warm air against the skin; muscle contraction also produces vasoconstriction, a narrowing of the blood vessels in the skin, to cut down on the heat escaping into the air. If he remains cold too long, he will begin shivering to generate heat. The hypothalamus might also stimulate the release of adrenaline and other hormones that increase metabolism, effectively causing tissues and organs throughout the horse’s body to “burn hotter,” and prompts behavior changes: The horse seeks shelter. Conversely, if the horse gets too hot, the hypothalamus initiates activities to reduce body temperature. The muscles supporting each hair will relax so his coat lies flat, and the blood vessels widen to facilitate radiation of heat away from the skin. If that’s not enough to cool him down, he will begin sweating.
The process that produces a fever begins when the immune system encounters a pathogen, such as a bacterium or virus. Among the first responders are lymphocytes, which initiate a cascade of biological events. To help neutralize the effects of the pathogens and eliminate them from the body, these cells release large number of cytokines, blood-borne protein messengers that affect the behaviors of other cells. Many of these cytokines have a pro-inflammatory effect—they stimulate all of the familiar signs of inflammation: localized heat, pain, swelling and redness. One type of cytokine, called a pyrogen, circulates in the blood and is detected by the hypothalamus, which responds by raising the body’s “set point” to a higher temperature. “Fever is one aspect of inflammation,” says Wilson. “We think of inflammation as redness, heat, pain and swelling—and fever is often a part of that.”
The raising of the body’s temperature set point is what distinguishes a true fever from other forms of overheating. “If there is something wrong in the body, like an infection, the body produces chemicals that change that temperature set point and make it higher for a while, and this is a fever,” says Nolen-Walston. “In other situations the body simply becomes hotter but the brain set point hasn’t changed.”
When the set point is raised, the hypothalamus stimulates the body to heat itself just as it would if it were in a cold environment. Vasoconstriction traps heat in the interior of the body, while the metabolic rate goes up. Eventually, the horse might start to shiver to generate more internal heat, even on a warm day.
If a fever starts getting too high, the hypothalamus may abruptly switch to cooling mode: “The second stage of fever involves sweating and panting, and dilation of blood vessels at the skin surface to route more blood to the skin for cooling—making the skin feel hot,” says Wilson. “The horse is breathing hard to try to get rid of the extra heat via the respiratory system.”
How a rise in body temperature helps fight off infection isn’t entirely understood. “There is a lot of debate in human and veterinary medicine regarding the benefits of fever,” Wilson says. “It may improve healing by speeding up chemical reactions in the body and improving inflammatory reactions to foreign invaders.” The extra heat may also inhibit the activities of temperature-sensitive viruses and bacteria. “We think the higher temperature increases the horse’s metabolism and thus the ability to fight off infections,” says Nolen-Walston.
What we do know is that, as the infection wanes, the immune response eases, the levels of pyrogens in the bloodstream drop, and the body’s temperature set point will return to normal.
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How to handle a mild fever in horses
You might suspect something is wrong if your horse acts a bit dull and goes off his feed. But the only way to be certain that he has a fever is to take his temperature. You also need to know your horse’s normal temperature to interpret the results. A thermometer reading of 100 might be normal for most horses, but if your horse’s temperature is usually closer to 98, then 100 might be a mild fever.
A slightly elevated temperature—just two or three degrees higher than normal—that lasts only a day or two does no harm and is not usually a cause for concern. Your horse may simply be fighting off some mild infection you might never have noticed. If he was vaccinated recently, a slight fever might be just a side effect of building his immunity. If all you notice is a fever of less than two or three degrees and a slight dullness, you might just let your horse rest and check his temperature periodically for the next day or two. Because fever is an active part of the immune system’s function, you might actually prolong the illness if you give the horse medication to bring it down. Consider calling your veterinarian, however, if the fever persists for several days or if the horse begins showing other signs of illness.
“Most of the time we get called out for some other reason, rather than a fever. There are usually other important signs of disease that are noticed first, such as the horse has stopped eating or is breathing hard, rather than the fact that the horse has an elevated temperature,” says Wilson. “Some people, however, do take their horse’s temperature every day and may notice the fever before the horse is showing other signs of illness. I recommend doing this, because the horse’s temperature is good information to tell the veterinarian before he/she comes out to look at the horse.”
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When faced with a horse with a mild fever but few if any other signs of illness, a veterinarian will first try to identify the cause. “A good history of the horse through the past day or days can be helpful. Was the horse coughing, or was there a change of diet or any evidence of diarrhea? Was there exposure to other horses that may have been sick? Did the horse have some kind of injury or serious wounds? All of these things might direct us to a diagnosis and the cause of the fever,” says Wilson.
“Then we usually try to determine which body system might have an infection, causing the fever. We listen to the lungs, check for diarrhea, look at the gums, etc.,” she adds. “Probably the biggest thing that helps us in diagnosis, however, is to run bloodwork on the horse. A complete blood count will help us know the degree of inflammation. Changes in white blood cell counts usually indicate an active infection, depending on which types of cells are elevated in number. This may help us know whether the infection is viral or bacterial.”
If the general examination yields some clues, the veterinarian can pursue more specific tests. “The ultimate way to diagnose an infectious disease is to test for that specific disease, usually by running some kind of bloodwork,” Wilson says. “The problem, however, is that there is no general screening test; you have to make an educated guess as to what it might be and then test for that particular disease.”
Often, however, the cause of a mild fever is elusive. “If we can identify a specific cause such as a virus or bacteria, we will try to target that disease process with the appropriate treatment,” says Wilson. “Unfortunately, even if we test for all the common things it might be, sometimes the tests all come back negative. The horse still has a fever, and we are scratching our heads as to why.”
If the horse seems generally well apart from an unexplained mild fever, the veterinarian might opt not to treat it. “Fever in itself is usually not a problem in horses,” says Nolen-Walston. “We almost never see brain damage from fever in horses. The important thing for horse owners to remember is that there is usually nothing particularly dangerous about the fever itself.”
The decision to treat the fever will depend on the horse’s general attitude. “Most of the time we treat a fever because the horse feels miserable and won’t eat or drink. Every horse is different regarding whether and when he might not feel good,” says Nolen-Walston. “If your horse’s temperature is 102 or 103 and he is happy—eating and drinking—there is no need to specifically treat the fever.”
How to handle a high fever in horses
A high fever—elevated by three or more degrees—is a more serious warning sign. In addition to dullness, you might see chills/shivering, sweating, increased respiration and pulse rate, fluctuations in skin temperature or reddening of the gums. An acute fever tends to spike high but come down quickly. A persistent high fever could indicate a serious illness. Either way, it’s a good idea to call your veterinarian.
“A few infections tend to cause very high fevers,” says Wilson. “Whenever I see a horse with a fever of 105 or higher, my first thoughts for possible causes would include strangles, anaplasmosis and Potomac horse fever and some of the viruses, such as equine influenza. Often a viral infection will induce a higher fever than a bacterial infection, but this alone is not a good way to try to diagnose what is wrong with your horse.”
Another cause of high fevers is endotoxemia—a systemic inflammatory condition that develops when toxins released by certain bacteria as they die get into the bloodstream. “Horses are uniquely sensitive to endotoxins that are produced by a molecule that is part of the cell wall of gram-negative bacteria,” says Wilson. “There are a lot of these bacteria inside the horse’s intestine as normal inhabitants. They live and die there and go through their life cycle in the colon. When a horse has colitis, some of the endotoxin from the bacteria’s dead cell walls may leak through the colon lining into the bloodstream. This causes a very dramatic cytokine response—and fever.” Endotoxemia can also occur if tissues of the lungs or uterus are inflamed.
Usually, a horse with a high fever will show other obvious signs of illness that point toward a specific cause. “If there are swollen lymph nodes under the jaw or thick nasal discharge, this would make us suspect strangles. If the horse has a cough or abnormal lung sounds, we will suspect a virus or pneumonia. With Potomac horse fever, we would probably see diarrhea or signs of laminitis,” says Nolen-Walston. “If the horse has a colic in which the intestine is twisted, we may see endotoxemia and high fever along with severe colic pain. Horses with anaplasmosis may have a high fever with no other signs except maybe mild swelling of the legs.”
With appropriate testing to confirm the diagnosis, a veterinarian will begin treatment for the disease as a whole, which will also ultimately address the fever as well.
When a fever is a too high to tolerate
Extremely high fevers—above 106 degrees—or any fever that goes on for too long can eventually take a physiological toll on a horse. The body uses calories and water to maintain the higher temperature, which over time can lead to weight loss and dehydration. Prolonged high temperatures may change the chemical structures of heat-sensitive enzymes, which can affect metabolic functions throughout the horse’s body. What’s more, too high a fever may make a horse’s immune response less effective.
That said, in practice, a veterinarian’s main concern is likely to be the effects a very high fever has on a horse’s willingness to eat and drink. “Rarely do temperatures get high enough for long enough time to actually damage tissues that are crucial for the animal to function,” says Wilson. “The biggest reason we end up treating fever most of the time is because a fever makes the horse feel bad. If the horse feels miserable he won’t eat or drink, and this can lead to secondary problems.”
For that reason, your veterinarian is likely to administer medications specifically to attempt to bring down a very high fever in addition to other treatments for the underlying disease. “The first thing we’d use to treat a fever is a nonsteroidal anti-inflammatory drug [NSAID] like flunixin meglumine [Banamine] or phenylbutazone [bute],” says Nolen-Walston. “These will often bring down a fever.”
These drugs do have to be administered with care, as directed, however. “The important thing for horse owners to know is that these drugs do not work any better if given at higher doses than recommended by the veterinarian, and they will actually be harmful,” says Nolen-Walston. She has treated horses who were hospitalized after their owners administered additional medication when the prescribed doses failed to curb the fever. “The owners told me they didn’t have any choice because the fever didn’t come down. But if the fever doesn’t come down with the proper dose, giving more will be toxic,” she says. “I have seen horses die from too much Banamine or bute.”
If your horse has been prescribed one of these medications, and his fever does not come down as expected, says Nolen-Walston, “consult your veterinarian to see what the highest safe level is. The important thing to remember is that these drugs are much more toxic when the horse is not eating or drinking. If the horse is feeling miserable and you are giving NSAIDs and he is not getting any better, don’t give these drugs for more than a day without having your veterinarian take a look and give you some more advice.”
If medications alone are not enough to reduce your horse’s fever, your veterinarian might suggest alternate methods of cooling him down. “Often we try to cool the body in some other way, by using fans or cold hosing, to help increase evaporation over the entire body,” says Nolen-Walston. “If the horse is really overheated, we can give cool intravenous fluids. You don’t have to cool the fluid very much, because even at room temperature it will be lower than body temperature.”
Cold hosing and fans can also be used to cool a horse at home, but remember that fever is only one symptom of a bigger issue that needs to be addressed. “If you are trying to bring down a horse’s temperature and cold water hosing isn’t doing the trick, call your veterinarian,” says Nolen-Walston. “Unless he/she tells you to do something else, most of the time you can wait for the veterinarian to arrive. It would be unusual that the horse would be in critical shape just from fever, but you could work at reducing the high temperature.”
As your horse recovers, it’s a good idea to keep tabs on his temperature at least once daily for another week or two. “There are certain specific diseases that cause fever for a day or so and then the temperature will drop back to normal,” says Nolen-Walston. “Then in three or four days the horse will have another fever. You can’t assume that just because the fever went down for one reading that you’re out of the woods.”
A mild fever may leave your horse feeling sluggish for a time, so it’s best to let him have some rest while he recovers. Most of the time though, a fever is just a sign that his immune system is keeping things under control, and your horse will be back to his old self in no time.
This article first appeared in EQUUS issue #448, January 2015.