# Why can't horses vomit?



## corriehorse (23 March 2010)

Whilst at the yard this evening one of the other liveries asked me why horses can't vomit. I explained that it is because of the cardiac sphincter muscle at the top of the stomach acting as a one way valve. He then asked why this is. This is where I'm stumped. Why have horses evolved with a one way valve preventing them from vomiting?


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## JoG (23 March 2010)

I *think* its because they are trickle feeders and so that muscle is stronger than in other mammals so this promotes continual movement of digested food from the stomach through the rest of the digestive system....

Rats can't vomit either!!


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## LMuirEDT (23 March 2010)

Hmmm... good point, never thought of that before but now u have me thinking...


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## HumBugsey (23 March 2010)

Can any herbivore/ruminants vomit?


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## somethingorother (23 March 2010)

I am far too interested in this thread to be healthy. Watching...


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## Serenity087 (23 March 2010)

Ruminents don't vomit par se, but they regurgitate their cud all the time!!

Horses are hind gut fermenters, so not ruminents at all! Hence why they don't chew cud!

However, why it's a one way street, I don't know.  I can, however, ask a bunch of horse science type people!


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## TarrSteps (24 March 2010)

If I remember correctly, it's basically so they don't regurgitate when they gallop at speed.  A horse has to move to function properly - gut movement and circulation depend on it (part of the function of the frog is to drive blood back up the leg) - which is why it's so difficult and potentially dangerous to immobilise horses for long periods.  The flip side of this is when they "flee" various symptoms "synch up", including the diaphragm, which opens and closes like a bellows and operates the lungs without separate muscular effort.  So presumably this would affect the gastrointestinal tract too . . . messy if the horse could throw up!  I would imagine fast ruminents have other adaptations to prevent such a problem.

Horses can suffer from "reverse peristalsis", where the gut effectively reverses its push, in extreme circumstances.  I saw it once in a horse that had been poisoned and I never, ever, want to see it again.


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## Kallibear (24 March 2010)

Horse can 'vomit' but not as we know it. yes, they do have a very strong sphincter at the enterance of the gut but they can and do have reflux at times (unusual and never a good sign!) as someone said it's mainly because they are flight animals and at a gallop the last thing they need is food coming back up!

However the real reason they can't vomit (as in - out the mouth) is because they have an extra long soft palate which acts like a one way value. It stops food going down into the trachea and air getting into the oesophagus. however when food come back up the wrong way it only allows it out through the nose, not into the mouth. Which is why you occasionally see horses with food coming out their nose (grass sickness for example)


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## corriehorse (24 March 2010)

Thank you everyone for your replys. It really stumped me when he asked why they were designed this way and just couldn't give him an answer, but thanks to the HHO'ers, I can now!


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## Lucy_Nottingham (24 March 2010)

Hi,

Ok vomiting is where food content from the stomach passes up the oesophagus into the mouth and is expelled (in dogs and humans etc generally at a suprising force!)
Horses cannot vomit as they a) have the strong sphincter as described by others and b) because of the shape of their stomach as well, it is significantly more "bent" round compared to animals stomachs which can vomit.

Grass sickness animals do not "vomit" they cannot swallow food due to the nervous system which controls the muscle contractions down the oesophagus being effected by the toxins that cause the disease. This isn't vomiting this is......... **thinks** regurgitation I think..... but basically the food doesnt reach the stomach, it sits in the oesophagus, and with the lowered head position of GS horses, the food content just trickles back up the oesophagus and as you say, out the nose.

If horses get back up of gut/intestinal content to the point where it backs up into the stomach a horses stomach is more likely to rupture than they vomit as that is just how well designed horses are NOT to vomit! 

However, why they are designed like this........ Im not sure! although the above suggestion of prevention of vomiting etc when running away from predators etc is a very reasonable one!


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