(I've always been fascinated by all things equine, including their anatomy and how their various body systems work. As a project to learn more about these amazing animals, I've decided to do a series of posts on the equine digestive system. As these come along, they'll be added to the sidebar. Please keep in mind that I am neither a vet nor an expert on these topics - if you have corrections and additions or resources to direct us to, please put that in the comments. Also, you may find it helpful to visit this article, which has excellent pictures of the digestive system and the location of the various organs.)
The second part of the equine digestive system is the esophagus, the tube that leads from the mouth to the stomach. This tube is approximately 4 feet long and terminates with the cardiac sphincter, which seals off the bottom of the esophagus from the stomach. After the horse has sufficiently chewed food and moistened it with saliva, the food is swallowed and passes down the esophagus to the stomach.
When humans choke, this usually occurs before swallowing, above the junction of the trachea (leading to the lungs) and the esophagus, leading to a blockage of air. In horses, choke almost always occurs below the junction of the trachea and esophagus, after swallowing. This is the reason that horses can continue to breathe even though they may have an obstructed esophagus.
Choke is almost always the result of rapid consumption of small particle feeds rather than forage, although it can also occur as a result of ingestion of a large object such as an unchewed apple or carrot section. It can also result from ingestion of beet pulp that has not been soaked in sufficient water for a long enough time (the one choke experience my horses have had resulted from this). Choke can also occur in senior horses with poor or absent dentition.
Choke, unless it resolves in moments, is a veterinary emergency. The horse will exhibit characteristic behaviors and will usually be in obvious distress, stretching out the head and neck and often gagging. In some cases, food and saliva will come out of the mouth and nose. Sometimes gentle massage of the esophagus - it is a somewhat delicate structure so a lot of pressure should never be used - can help. The esophagus lies along the jugular groove on the left side of the neck - if you observe closely you will be able to see food or water passing down the esophagus as the horse swallows. A signifiant risk of choke is aspiration of food and saliva into the trachea and then the lungs. This can lead to aspiration pneumonia, and all horses who have had an episode of choke should be watched carefully for signs of pneumonia, such as fever and depression, and treated accordingly. A horse with choke that has not been treated for a day or more will often be seriously dehydrated and depressed.
One reason immediate veterinary assistance is needed for choke is that the delicate structure of the esophagus can be damaged by the pressure of the obstruction, possibly resulting in scarring and even stricture, which will predispose the horse to future episodes of choke. The choke is usually resolved by the passage of a nasogastric tube and lavage with warm water (mineral oil is not used because of the risk of aspiration). If there is an obstruction that cannot be resolved with gentle pressure of a nasogastric tube and lavage, surgery may be required.
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Many people know that horses do not vomit - but why is this? There are a couple of reasons. First, the cardiac sphincter at the bottom of the esophagus, joining it to the stomach, is unusually strong, and second, the stomach of the horse is not located in the abdomen near the abdominal muscles (which in other species put pressure on the stomach when the animal vomits).
Horses are more at risk than many other species if they ingest something that is toxic or unhealthy - they cannot vomit and they also do not have a rumen, which serves a detoxifying function in many species.