Why can’t horses vomit?

EQUUS - - Contents - Joe Ber­tone, DVM, MS, DACVIM EQUUS Med­i­cal Edi­tor

Q: Most horsepeo­ple know that horses can’t throw up. But when my young daugh­ter asked me why not, I didn’t have an an­swer. So, why can other species like cats, dogs and peo­ple vomit but not horses?

Name with­held by re­quest A: This ques­tion ac­tu­ally breaks down into two parts: Why is it phys­i­cally im­pos­si­ble (or at least very dif­fi­cult) for horses to vomit? And why should that be?

Vom­it­ing (eme­sis) is a com­plex phys­i­o­log­i­cal event that re­quires a closely co­or­di­nated se­quence of re­flex­ive mo­tions. When you are go­ing to throw up, you draw a deep breath, your vo­cal cords close, your lar­ynx rises, and the soft palate shifts to close off your air­ways. Then your di­aphragm con­tracts down­ward, which loosens pres­sure on the lower esoph­a­gus and the sphinc­ter where it en­ters the stom­ach. Next, the mus­cles of the ab­dom­i­nal wall con­tract spas­mod­i­cally, which puts sud­den pres­sure on the stom­ach. With the up­ward “doors” open, the con­tents have a clear exit path­way. All of th­ese sep­a­rate ac­tions hap­pen in­vol­un­tar­ily, of course, con­trolled by dis­tinct “vom­it­ing cen­ters” in the brain.

Horses, how­ever, have a num­ber of key phys­i­o­log­i­cal dif­fer­ences to en­sure that any food they in­gest takes only a one-way trip. For ex­am­ple, the mus­cles of the equine lower esophageal sphinc- ter are much stronger than in other an­i­mals, mak­ing it nearly im­pos­si­ble to open that valve un­der back­ward pres­sure from the stom­ach. Also, the equine esoph­a­gus joins the stom­ach at a much lower an­gle than in many an­i­mals, so when the stom­ach is dis­tended, as with gas, it presses against the valve in such a way that holds it even more tightly closed. And, lo­cated deep within the rib cage, the equine stom­ach can­not be read­ily squeezed by the ab­dom­i­nal mus­cles. Fi­nally, horses have a weak vom­it­ing re­flex---in other words, the neu­ral pathways that con­trol that ac­tiv­ity in other an­i­mals are poorly de­vel­oped in horses, if they ex­ist at all.

All that said, how­ever, vom­it­ing in horses has oc­ca­sion­ally been re­ported. But it’s pos­si­ble that some of th­ese cases may ac­tu­ally have been choke--the “vom­ited ma­te­rial” may have been ejected from a block­age in the esoph­a­gus, not from the stom­ach. It’s also pos­si­ble that un­der cer­tain cir­cum­stances, a se­ri­ously ill horse could re­gur­gi­tate, which is dif­fer­ent than vom­it­ing. Vom­it­ing is a re­flex­ive, mus­cu­lar ac­tion that ex­pels ma­te­rial un­der great pres­sure. Re­gur­gi­ta­tion is pas­sive. If the esophageal mus­cles go flac­cid, in­gested food may ooze from the nose and mouth.

Nearly ev­ery ver­te­brate we know of vom­its. Vom­it­ing has been ob­served in fish, amphibians, rep­tiles and birds as well as most mam­mals. Horses are a no­table ex­cep­tion---as are rats, mice, rab­bits and most other ro­dents. Usu-

Horses have a num­ber of key phys­i­o­log­i­cal char­ac­ter­is­tics that en­sure that any food they in­gest takes

only a one-way trip.

ally, vom­it­ing is a de­fen­sive ac­tion, to re­move in­gested toxic sub­stances from the body, for ex­am­ple, but some an­i­mals have more spe­cial­ized rea­sons for bring­ing food back out of their stom­achs. Ru­mi­nants, like cows, re­gur­gi­tate food to chew the cud. Wolves and other wild ca­nine species may swallow food in large chunks, then carry it back to their dens to be vom­ited up to feed their pups.

So why did horses evolve this way? We can only spec­u­late. At some point, the need to re­tain food in the stom­ach must have been a more im­por­tant sur­vival mech­a­nism than the need to eject tox­ins. Be­cause horses are built to graze---to take in very small por­tions at a time as they feed through­out the day---and be­cause they are fairly fussy about the plants they browse, it’s pos­si­ble that they never needed to vomit be­cause they would con­sume toxic doses only rarely.

An­other clue may come from how horses run. When a horse gal­lops, his in­testines shift for­ward and back like a pis­ton, which ham­mers the stom­ach. In any other species, that would pro­duce vom­it­ing. Per­haps the horse evolved such a pow­er­ful lower esophageal sphinc­ter to pre­vent him from vom­it­ing as he eluded preda­tors.

We may never re­ally know.

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