My Horse Has a Fever: Now What?

Learn to recognize the signs of various temperature-raising conditions in horses.

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My Horse Has a Fever: Now What?
If your horse is battling any sort of internal issue, chances are he'll spike a fever. | Photo: Photo: Alexandra Beckstett, The Horse Managing Editor.

As a hands-on horse owner, you know your animals better than anyone. So when one of your steeds is a bit slow to the paddock gate or not as excited to see his feed bucket as usual, you can sense that something’s wrong. And, because you are also an educated horse owner, you know the first thing you should do is check your horse’s vital signs, which includes taking a rectal temperature.  

A horse’s normal rectal temperature can range anywhere from 99.5° F to 101.5° F, but most horses have their own narrow spectrum, so it’s very important to know what’s normal for your horse. If you take his temperature routinely, and it is always between 99.7° F and 100.1° F, then a temperature of 101.3° F is probably too high for him.  

“Monitoring your horse’s temperature is especially important for horses that travel or horses that are exposed to other horses that may travel (e.g., at a boarding barn),” says Kelly Carlson, DVM, Dipl. ACVIM, of Heartland Equine Hospital, in Tonganoxie, Kansas. “An elevated temperature is often the first sign that something is ‘off’ with your horse, and early recognition may allow diagnostics or treatment before more serious problems arise.”  

Is It Really a Fever?

So, let’s say your horse’s temperature is clearly higher than his norm. Now what?

The next step is determining if it is a true fever. Horses can develop an elevated temperature for one of two reasons: hyperthermia (high body temperature) or pyrexia (fever). In a normal horse, neuronal controls in the brain try to maintain body temperature at a “set point.” (Think of selecting a temperature on your home’s thermostat that controls your heating and cooling system.) Temperature sensors throughout the body recognize changes in internal and ambient temperatures and trigger feedback mechanisms, such as sweating, to preserve the set point.  

With hyperthermia, the body temperature rises but the set point does not, meaning the horse is simply overheating. Hyperthermia can be caused by exercise, anhidrosis (the partial or total inability to produce sweat), and reactions to certain toxins or drugs. Severe hyperthermia is known as heat stroke.

Prolonged high-intensity exercise causes muscles to produce more heat, and core body temperature increases when the heat produced exceeds the body’s ability to dissipate it. In these cases the temperature should return to normal once the horse has been properly cooled out. Anhidrosis inhibits this ability to cool down during and after exercise and can lead to hyperthermia, though inability to sweat can fuel hyperthermia even without exercise if ambient temperatures are high enough. Heat stroke occurs when core temperature rises above a critical level, leading to multisystemic problems that can range from central nervous system dysfunction to respiratory issues. Signs might develop in horses with rectal temperatures greater than 107° F (41.5° C). At this temperature the body’s ability to maintain homeostasis (internal equilibrium) fails, leading to circulatory shock and even death.  

In the case of pyrexia, a true fever, the set point increases and the established feedback mechanisms then work to maintain this new set point. In other words, some abnormal process in the body has convinced the brain that it should be warmer. (Essentially, something has dialed up the number on the thermostat without your permission.) Molecules produced by infection, inflammation, or a generalized immunologic response are to blame; they can act as pyrogens (fever-starters), changing the set point. Though fever is most commonly associated with infectious disease processes, veterinarians must not overlook inflammatory, neoplastic (tumor), or immune-mediated causes.

“It is important to remember that true fevers happen for a reason: The body is responding to a source of infection or inflammation,” says Carlson. “True fevers will not completely resolve until the underlying stimulus has resolved.”

Determining whether your horse has hyperthermia or a fever will help guide your immediate treatment efforts. Cool hyperthermic horses with cold water baths, and keep them in the shade on a hot day. In the case of a fever, remember that the fever itself is not a disease but merely a symptom. Many horse owners have access to anti-inflammatory medications such as phenylbutazone (Bute) or flunixin meglumine (Banamine), and while these products will lower a true fever, they will not treat the underlying disease. Therefore, consult your veterinarian before giving any medications that could potentially mask a worsening problem.  

“Fevers of unknown origin can be very frustrating cases, and as veterinarians we must take a systematic approach to determine the exact cause,” says Andrew Van Eps, BVSc, PhD, MACVSc, Dipl. ACVIM, senior lecturer and specialist in equine medicine at The University of Queensland Equine Hospital, in Gatton, Australia.

The first step in helping your veterinarian diagnose a fever of unknown origin is to document a persistent fever. Monitor and record the temperature over several hours and at least twice daily. Combine this information with historical findings to try to identify a fever cycle’s pattern.  

My Horse Has a Fever: Now What?
Monitoring and recording your horse's temperature at least twice a day will help your veterinarian diagnose the problem and monitor treatment progress. | Photo: Photo: Stephanie L. Church/The Horse

“It is important to discuss any recent events with your veterinarian, including recent travel, changes in housing, changes in pasture or stall mates, medications, and any recent episodes of illness, including choke, coughing or nasal discharge, colic, or diarrhea,” says Carlson.

Fevers can be classified as intermittent, with periods of normal temperature between elevations; remittent, where there is daily variation that remains above the horse’s normal range; and biphasic, where there are two isolated temperature spikes over the course of a few days. You should review vaccination and deworming history, recent travel, known exposure to sick animals, and any illness or weight loss.  

A thorough physical exam is paramount and can help the veterinarian identify the affected body system. He or she should observe the horse’s body condition and demeanor, examine the head and neck, palpate the sinuses and the submandibular lymph nodes that lie beneath the jaw, listen to the chest and gastrointestinal tract with a stethoscope, and perform an oral exam, noting any foul odor from the mouth or nostrils (which might indicate an infection or necrotic—dead or dying—tissue somewhere in the head, throat, or lungs).  

Now that your veterinarian has stepped in and determined that your horse does, in fact, have a true fever, let’s look at some common causes.

Thoracic (Chest-Related)  Causes of Fever

Respiratory causes of fever include infection with both viral and bacterial pathogens. Viral diseases at play could be equine influenza, equine herpesvirus (EHV-1 and -4), equine viral arteritis, and equine rhinitis. Clinical signs of these diseases can be very similar and include fever, clear nasal discharge, and cough. Viral infections usually run their course and simply require symptomatic treatment with anti-inflammatories. Secondary bacterial infections can develop in the lungs after a viral infection as well as after shipping long distances. With these horses, veterinarians see more purulent (pus-containing) nasal discharge, cough, and increased respiratory rate in addition to a fever, though it is difficult to differentiate viral from bacterial infections by clinical signs alone. Your veterinarian can submit blood samples or a nasal swab to test for respiratory viruses, and most commercial labs offer a set panel of these tests.

Strangles is a bacterial infection caused by Streptococcus equi subsp equi that mainly affects horses’ upper respiratory tracts. Infected animals typically have a fever, snotty nose, poor appetite, and swelling of the submandibular lymph nodes. Abscesses form in the lymph nodes and eventually open and drain thick white material, but before that they sometimes become so large that they compress the airway, causing difficulty breathing—hence the name “strangles.” This bacterium is highly infectious and spreads easily through a barn via nose-to-nose contact or contact with contaminated feed/water buckets, fences, and even human hands and clothes. To diagnose strangles, your veterinarian will submit a nasal swab or pharyngeal wash to the lab.  

While strangles typically only affects the upper respiratory tract, other bacteria can invade lower. If your veterinarian suspects your horse’s fever is due to bacterial pneumonia, he or she should evaluate the thorax via a rebreathing exam, during which a large bag placed over the horse’s nose causes him to take slow, deep breaths and allows for a better listen to lung sounds. He or she can also perform a transtracheal wash with cytologic evaluation and fluid culture to yield a diagnosis or use endoscopy (a small camera inserted through the nose and down into the airways) to evaluate the upper respiratory tract (nasal passages and guttural pouches) as well as the trachea. A chest ultrasound can help identify pneumonia, as well.   

Although quite rare, if the practitioner notes a heart murmur during a febrile episode with no other sources of fever identified, he or she should suspect bacterial endocarditis (inflammation of the heart’s lining caused by bacterial infection of one or more heart valves). An echocardiogram at a referral hospital is necessary to diagnose this condition.  

Abdominal Causes of Fever

Gastrointestinal disease that can lead to fever includes colitis, which is colon inflammation caused by bacteria such as Salmonella and Clostridium perfringens or difficile. A veterinarian can use fecal culture to detect Salmonella, and he or she can also test feces for clostridial toxins. Horses with colitis typically have loose manure or diarrhea in addition to a fever. Neorickettsia risticii, which causes Potomac horse fever, or PHF, is another bacterial cause of colitis that can induce high fevers as well as the hoof disease laminitis. Your veterinarian can test blood samples for antibodies against PHF.

Coronavirus has only recently been identified as a viral cause of GI disease; it can lead to poor appetite and, potentially, colic signs (abdominal discomfort). “This virus may be much more prevalent than we know because we have only recently begun testing for it,” says Van Eps. Veterinarians should submit fecal samples for testing if they suspect coronavirus.  

While GI parasites do not normally cause a fever, severe cases of cyathostomiasis (small strongyle infection) can cause colitis similar to the other infectious causes of diarrhea. Cyathostomiasis can be difficult to diagnose because the larvae that cause the most damage do not produce eggs visible on fecal flotation.  

Less common causes of fever originating from the abdomen include hepatic (liver) disease, renal (kidney) disease, or an intra-abdominal abscess. With these conditions, mild intermittent signs of colic often accompany the fever. Inflammatory bowel disease and intestinal neoplasia (cancer), though rare, can also cause fever, along with weight loss and mild colic. Running a complete blood count and serum biochemistry can help your veterinarian identify infection or dysfunction of an organ system. A more advanced way to investigate the abdomen is abdominal ultrasound, by which your veterinarian can evaluate the appearance of the liver, spleen, and kidneys, measure GI wall thickness, and assess the peritoneal fluid (the normal fluid surrounding the intestines that can increase or change consistency with inflammation or infection).  

My Horse Has a Fever: Now What?
If a fever's cause is unknown, your veterinarian can take blood samples to test for various diseases. | Photo: Stephanie L. Church/The Horse

Blood-Borne Pathogens

Several viral and bacterial pathogens can infect a horse’s bloodstream and cause fever and a variety of other signs. Equine infectious anemia (EIA) virus infects the red and white blood cells and can be transmitted from horse to horse via insects. The disease’s initial signs include fever, anemia (when the blood doesn’t have enough healthy red blood cells), low platelet count, and edema (fluid swelling) in the limbs. Severe cases of EIA can present quickly and lead to sudden death due to a massive destruction of red blood cells. There is no effective treatment or vaccine for this disease, which is why veterinarians employ the Coggins test to identify carriers and quarantine them to prevent disease spread.

Another bacterial disease is anaplasmosis, which is transmitted by ticks and commonly seen in the late summer and early fall. Infection with the causative pathogen, Anaplasma phagocytophilum, causes high fevers (>103$deg;F), inappetance, low platelet counts, and stiffness and edema of all four limbs. Your veterinarian can diagnose anaplasmosis by identifying the bacteria within the red blood cells or by polymerase chain reaction testing.

Equine piroplasmosis is another tick-borne disease caused by one of two parasites (Babesia caballi or Theileria equi) that live within the red blood cells. Clinical signs generally become apparent after red blood cell damage and include fever, dullness, lethargy, and pale or yellow mucous membranes. Your veterinarian can obtain blood samples to look for serum antibody titers against the parasites.

Fever can also accompany some neurologic diseases, such as EHV-1, Eastern/Western equine encephalitis (EEE/WEE), and West Nile virus (WNV). So if your horse has a fever, pay close attention to his mental status and coordination while walking. EHV-1 is sometimes, but not always, preceded by respiratory signs. Eastern equine encephalitis, WEE, and WNV are transmitted by mosquitoes and are usually seen in warmer, wetter weather. Your veterinarian can take blood samples to test for these diseases, and, in the case of EEE, WEE, and WNV, vaccination is highly effective for preventing infection.

Take-Home Message

Even if your veterinarian can examine the horse quickly and perform appropriate diagnostics, not every high-temperature mystery gets solved. Many times a fever abates and does not return. We often assume a mild viral infection and are happy the horse is back to normal.

Fevers have a multitude of causes. Your veterinarian can narrow the list of potential problems with a thorough physical exam and more specific testing. Treatment with anti-inflammatories will help control the fever and help your horse feel better (though remember that not all fevers lead to noticeable signs or require immediate treatment). To ensure the fever does not return, be sure you’re treating the underlying cause, as well.


Written by:

Joan Norton, VMD, Dipl. ACVIM, founded Norton Veterinary Consulting and Education Resources to promote equine veterinary education to horse owners, professionals, and veterinarians. She is the author of Equine First Aid Handbook.

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