When you or I feel nauseous and our stomach tells us we need to get rid of whatever is making us ill, a complex chain of reactions takes place so that our stomach can eject whatever is causing the problem. When the problem is ejected via the mouth (as opposed to the other end!) we call it vomiting.
For horses however, the story is quite different. Horses, along with rodents and rabbits, are the only mammals that can’t bring up their stomach contents but why?
The science of not vomiting
A lot of it boils down to the anatomy of the horse and its physiology.
In most mammals, including humans, peristaltic waves (the muscle contractions that move food) can go both directions, allowing for regurgitation. Horses are unique because their peristaltic waves only move in one direction: down.
The Swiss tie
The horse also has a very strong sphincter called the cardiac sphincter or Swiss tie, which is so strong it won’t allow stomach contents to go back up the esophagus and is in effect, a one way valve. In humans, the valve between the esophagus and the stomach is relatively flexible and can open and shut like a door. That is why humans often suffer from indigestion or acid reflux.
The esophagus and stomach angle
Another reason horses can’t vomit is that the esophagus joins its stomach at quite a steep angle in comparison to most mammals. When the stomach is distended, it puts even more pressure on the valve, closing it even tighter.
The stomach position
In addition to this, the horse’s stomach is deep within the rib cage and it can’t be squeezed by the strong abdominal muscles that play a part in vomiting.
Finally, the brain
Last but not least, while other mammals have well developed neural pathways that control the vomiting reflex, a horse has a very poor vomiting reflex and very few neural pathways, if any.
A horse's esophagus and stomach.
Vomiting vs choking
While people have documented what looked to be a horse vomiting, it is likely that they have witnessed regurgitation from a choke, where a bolus of food or something like a piece of carrot gets caught in the esophagus. When choking the horse may extend its neck and find it difficult to swallow. Regurgitated material may come out of the nostrils and mouth and this can be accompanied by foam and mucus but this is not vomit as the food never made it as far as the stomach.
The risks of not being able to vomit
Horses are prone to colic, a severe digestive issue that can often be fatal.
Their inability to vomit makes horses more susceptible to digestive issues like colic. Colic in horses is often fatal as the stomach can easily twist, causing a gastric torsion. There is also a risk that the hind gut, where most of the digestion takes place can become impacted. An old fashioned trick when a horse is suffering from colic, is to pour beer down its throat and this can actually help to clear impactions. However, there are many causes of colic and you need to talk with a vet asap if your horse is showing any symptoms.
Because the horse’s stomach can’t empty, it is at risk of rupturing if it gets overfilled with gas or liquid. As every horse owner will know, colic can be a life threatening situation. Knowing how the horse's physiology and anatomy works gives us a greater understanding of what our equine friends are going through when they succumb to a colic episode. Colic usually requires urgent veterinary intervention.
Why did horses evolve with their peculiar digestive system?
We don’t really know the reasons why horses have evolved with an inability to vomit, but there are some theories.
The need for speed
One speculation is that when a horse gallops the intestines shunt backwards and forwards like a piston against the stomach. This shunting keeps the Swiss tie closed. In other mammals, this constant thumping against the stomach would cause them to vomit. Perhaps the horse evolved so that when fleeing predators, it wasn’t slowed down by the vomiting reflex.
The need for feed
Another suggestion is that as horses have to graze little and often - sometimes up to 20 out of 24 hours a day, the whole passage of food is a constant and only goes one way - from front to back. They are also selective grazers and are usually very good at avoiding toxic plants or weeds. Because they generally don’t binge feed, their stomach is not put under sudden pressure by a large amount of food being ingested in one go.
It’s not just vomit!
Horses can’t burp either! Not only are horses unable to vomit, they also can’t belch. Once again, thanks to that one way valve, their whole system for digestion is designed to go one way, including gas!
So, if a horse can’t burp, how does it release gas? Fortunately, they are able to release gas at the other end and anyone who owns horses will tell you they can produce prodigious farts, especially when tearing around in their paddocks - this often seems to be the most common time to get rid of gas.
Did you know that horses can’t breathe through their mouths?
Another thing a horse can’t do is breathe through its mouth. Unlike most animals that pant when stressed or have been running too hard, horses don’t have this option.
This is because most of the time the soft palate, which separates the mouth from the nasal passages, blocks off the pharynx (windpipe) from the mouth. This means the horse can only breathe through its nostrils. It’s designed this way to stop them inhaling food. The only exception is when the horse swallows: at this point the soft palate opens, allowing food to go down the esophagus. A horse is therefore known as an obligate nasal breather!
A complex, if amazing, animal
In summary, the horse has complex digestive and respiratory systems that don't allow vomiting or burping and prevent the horse from breathing through its mouth.
Knowing how this all works can help horse owners better understand the need for horses to have access to food 24 hours a day. Whether it is in the form of grazing, hay or a mix of both, the horse needs a constant supply of forage to allow the constant passage of food moving through the animal in order to keep it healthy.