What fever means

When your horse’s tem­per­a­ture climbs, it may be best to sim­ply let the process run its course. But some­times you’ll want to get a vet­eri­nar­ian in right away.

EQUUS - - Contents - By Heather Smith Thomas

When your horse’s tem­per­a­ture climbs, it may be best to sim­ply let the process run its course. But some­times you’ll want to get a vet­eri­nar­ian in right away. Here’s what you need to know.

Here’s what you need to know.

ou’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 o won­der what’s up: His tem­per­a­ture ature 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 0 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 tot in­ves­ti­gate the reas rea­sons why. “Most oof of the time, if a restinn ing horse has an innc in­creased rec­tal teem tem­per­a­ture it’s beeca be­cause he has a fever,”fe says NolenWal­ston.W 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.


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.


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 (see “How to Take a Horse’s Tem­per­a­ture,” page 29). 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

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.

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.”

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 NolenWal­ston. “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.”



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 stran­gles0, anaplas­mo­sis0 and Po­tomac0 horse fever and some of the viruses, such as equine0 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 col­i­tis0, 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 lymph0 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.


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

An acute fever tends to spike high but come down quickly. A high fever that per­sists could in­di­cate a se­ri­ous ill­ness. Ei­ther way, it’s a good idea to call your vet­eri­nar­ian.

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 non­s­teroidal0 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 own- ers ad­min­is­tered ad­di­tional m med­i­ca­tion when the pressc scribed doses failed to curb the fee fever. “The own­ers told me they didn’tdiid have any choice be­cause thethh fever didn’t come down. ButBuu if the fever doesn’t come down withih 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.”

Amild 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.

SICK LEAVE: A horse with a mild fever may need noth­ing more than a few days of rest to make a full re­cov­ery.

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