Why can’t a horse vomit?

Vomiting 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 oesophagus 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, controlled by distinct “vomiting centres” in the brain.

Horses are picky eaters for a reason.

The inability of horses to vomit is a unique aspect of their physiology.

They use their prehensile lips as we do our fingers, to sift through foods and pick out the most suitable morsels.

It’s important for survival to sift through potentially harmful foods and prevent them from ever getting past the mouth.

This inability is due to several anatomical and physiological factors:

1. Strong lower oesophageal sphincter: The muscle at the junction between the horse’s oesophagus and stomach, known as the lower oesophageal sphincter, is very strong and powerful in horses. This muscle is much stronger than in humans, dogs, or cats, and it acts as a one-way valve that allows food to go down but not come back up.
2. Angle and position of the oesophagus: The oesophagus enters the stomach at a steep angle in horses. This angle, combined with the positioning of the stomach itself, further hinders the possibility of material moving back up the oesophagus.
3. Stomach structure and size: The horse’s stomach is relatively small compared to the rest of the digestive system and is divided into distinct sections. The way these sections are arranged and the limited capacity of the stomach make it difficult for the stomach contents to be expelled back through the oesophagus.
4. Physiological differences: The horse’s digestive system has evolved for continuous grazing with a constant flow of small amounts of forage moving from the oesophagus to the stomach. This continuous flow, rather than large meals with long hours in between, further diminishes the likelihood of vomiting.

The inability to vomit is a critical factor in equine health.

It means that if a horse eats something toxic or suffers from certain digestive disturbances, it can’t simply expel the contents of its stomach easily.

This is one reason why colic is a serious issue in horses, as they can’t relieve pressure in their stomachs by vomiting.

It also highlights the importance of careful dietary management and monitoring of horses to prevent digestive problems.

How does not being able to vomit affect horses?

The inability of horses to vomit has several significant implications for their health and care:

1. Increased risk of colic: Since horses can’t vomit, any digestive upset can quickly escalate into colic, which is a leading cause of death in horses. Issues like gas build-up, impaction, or a twist in the intestines can’t be relieved by vomiting, which in other animals might alleviate the discomfort or remove the offending contents.
2. Sensitivity to diet changes: Horses are particularly sensitive to changes in their diet. Since they can’t vomit, they are more susceptible to problems caused by eating spoiled food, abrupt changes in feed, overeating, or ingesting toxic substances. This also makes them naturally picky about the food they will and won’t eat (and can make hiding medicines in food a good way to see your horse simply refuse to eat). Dietary changes should be made slowly over several weeks to prevent problems.
3. Importance of dental care: Proper dental care is crucial for horses. Poorly chewed food due to dental issues can lead to digestive disturbances, and without the ability to vomit, this can become a serious problem.
4. Monitoring for toxins: Horses need to be closely monitored to ensure they are not ingesting toxic plants or substances. Since they can’t vomit to expel these toxins, ingestion can lead to severe poisoning or death.
5. Risk of gastric rupture: In extreme cases, if a horse overeats or has a severe case of gas or fluid build-up in the stomach, the inability to vomit can lead to gastric rupture, a life-threatening condition that requires immediate veterinary attention.
6. Management practices: Horse owners need to be vigilant about feeding practices, ensuring consistent, high-quality feed, access to fresh water, regular exercise, and routine health checks. These practices help minimise the risk of gastrointestinal issues that could otherwise be more easily resolved in animals that can vomit.

Understanding this limitation in horses underscores the importance of preventive care and quick response to any signs of digestive distress.

Prompt veterinary intervention is often necessary to address issues that, in other animals, might be resolved through vomiting.