The rectum of the horse constitutes the last portion of the gastrointestinal system. It extends 30 to 40 inches from the anus going toward the mouth. There are two different parts of the rectum – the peritoneal part, and the retroperitoneal portion. The peritoneal portion lies closer to the mouth, and the retroperitoneal portion lies closer to the anus. The peritoneal cavity contains the internal organs of the abdomen within a closely fitting sac-like structure. The retroperitoneum lies outside of the peritoneal cavity and does not communicate with it. The mucosa – the portion that lines the inside of the rectum The muscularis – a thin muscular portion The submucosa – the portion that lies between the muscularis and the mucosa The serosa – the portion that comes into contact with the peritoneal cavity and is the outermost layer
A deep tear that involves the peritoneal portion is likely to cause contamination of the peritoneum, and consequent peritonitis, or infection of the peritoneum. Rapid surgical management is necessary in order to save the horse, and even with the best of treatment, the horse may die. A tear, even a deep one, that involves the retroperitoneal portion, may sometimes be successfully managed medically, and may not need surgery.
The rectum has four layers:
Classification of Rectal Tears
A Grade I rectal tear is superficial and causes disruption of the mucosal layer only.
A grade II rectal tear involves only the muscularis. This is often an incidental finding, and feels like a small, smooth divot in the rectal wall. The mucosal layer is intact. This is thought to be the consequence of a healed rectal tear during the early years of a horse's life.
A grade III tear involves all layers except the serosa. However, with only the serosa intact, there is now only a very thin layer between the rectal contents (manure) and the peritoneal cavity. Even if the serosa stays intact, the horse will develop peritonitis because bacteria and their products can cross this very thin layer into the peritoneal cavity.
A grade IV tear is one that involves all layers of the rectum. This means that there is now no barrier between the rectum and the peritoneum. Manure with all its bacteria and other contaminants now spills directly into the peritoneal cavity. Peritonitis quickly ensues.
Causes of Rectal Tears
Studies show that rectal tears have little if anything to do with the capabilities or experience of the person performing the rectal examination. The rectal tear is thought to be due not to the practitioner 'poking a hole' in the rectum, but due to the horse tightening and contracting the rectum around the veterinarian's hand. The rectum then essentially splits around the hand, rather than the hand forcibly causing the tear.
Horses that are poorly restrained during rectal examinations are more likely to develop rectal tears – so if your veterinarian wants to sedate or twitch your horse during an examination, you should not protest.
In one study, researchers tried to create rectal tears in order to evaluate special instruments for repairing the tears. Much to their surprise, no matter how hard they tried, they couldn't cause tears. This indicates that horses that suffer rectal tears probably have some underlying defect in the tissue of the rectum that predisposes them to develop rectal tears.
It's true that horses that have small pelvic girdles are more likely to get rectal tears. This would include young horses of either sex, stallions and geldings. Horses that are nervous and anxious are also more likely to develop rectal tears because they are more likely to contract the rectum around the veterinarian's hand.
Anything that makes the rectal tissue weaker and less able to heal will make it more likely that a horse will develop a rectal tear. One disease that older horses are likely to have is called Equine Cushing's Disease. One of the consequences of this disease is excessive production of corticosteroids by the horse's body, which results in poor tissue healing and weak tissue structure. Even if an older horse does not have Equine Cushing's Disease, they still have poorer tissue healing, and weaker tissue than a younger horse. Malnourished horses or chronically ill horses may also have weaker tissue and poorer tissue healing than healthy horses.
Occasionally, a rectal tear occurs as a breeding accident, with the rectal tear as a result of intromission attempted in the rectum.
In a few cases, rectal tears have occurred as a result of a foreign body – either something that the horse ingested or backed into, or as a result of cruelty by humans. In a few rare cases, horses with Cushing's disease have developed rectal tears secondary to small colon and rectal impactions. The rectal tissue is so weak that as the horse strains and pushes against the impaction the rectum splits around the impaction, creating a rectal tear.