equine colic

Many equine veterinarians will agree that horse gastrointestinal tracts could be considered an evolutionary equivalent to a “face-palm” emoji. Ingested food must twist and turn upside down and backwards through multiple tubes of various sizes, shapes, and functions, much like a game of marble run. In this light, the intestinal tract’s convoluted nature leaves little room for speculation about why colic remains a predominant cause of both morbidity and mortality in horse.

Ileocecal Junction Anatomy

While either the small or large intestines are usually the focus of colic discussions, the junction of the small intestine and cecum at the start of the large intestine—the ileocecal junction—also deserves mention.

Partially broken-down food travels approximately 60 feet from the stomach to the far reaches of the small intestine called the ileum, a muscular tube that propels the ingesta in a forward (toward the rectum) direction. There, the bolus of ingesta advances through the ileocecal junction into the cecum. A key feature of the junction—which, in reality, serves as a valve or sphincter that projects slight into the cecum—is that once material enters the cecum it can’t re-enter the ileum.

Once in the cecum, the food bolus begins fermenting, a process by which intestinal microbes (bacteria, fungi, etc.) break down structural carbohydrates to produce energy for horses. After spending approximately seven hours fermenting in the cecum, ingesta passes into the large intestine for additional fermentation prior to excretion.

When Things Go Wrong

While the structure and function of the ileum, cecum, and ileocecal junction appear relatively straight forward, all plumbing systems—which is essentially all the equine gastrointestinal tract really is—can clog.

Ileal impactions pose a major issue, preventing ingesta to pass through the ileum into the cecum. In fact, one study¹ indicates that impactions of the small intestine occur most frequently in the ileum. Considering the ileum has a smaller diameter than the preceding region of the intestinal tract, the ileum, and a narrow sphincter through which ingesta must pass to reach the cecum, this fact isn’t surprising (again, think of plumbing getting blocked).

Risk factors for ileal impactions include:

  • Geography (ileal impactions occur most commonly in the southeastern United States);
  • Ingestion of fine, stemmy, high-roughage forages such as Coastal Bermuda;
  • Decreased water consumption contributing to the production of a dry mass of ingesta in the ileum;
  • Weather changes;
  • Equine tapeworm infection; and
  • Less commonly, ileal hypertrophy (enlargement), mesenteric thrombotic disease (blocked bloodflow to the ileum), and roundworm (ascarid) impaction.

Tapeworm Blockages

Blockage of the ileocecal junction with equine tapeworms remains an important cause of disease in horses². These tapeworms, primarily Anoplocephala perfoliata, can grow up approximately 3 inches long by .5 inch wide. A. perfoliata prefer to colonize the ileocecal junction and cecal wall, causing not only physical obstruction but also damage to the muscle wall impeding its function. Diagnosing equine tapeworm infections remains challenging and new methods, such as a saliva-based test, are in various stages of development.

In lieu of testing, many owners elect to simply deworm their horses once or twice annually with either praziquantel or pyrantel pamoate. Although resistance to these chemical dewormers has not yet been reported in tapeworms, increasingly frequent reports of resistance to various dewormers in other equine parasites exist, making rote treatment an unfavorable method of managing equine tapeworms.

For optimal gastrointestinal health or concerns regarding ileal impactions, dietary changes, and tapeworm infections, consult your veterinarian and have your horse examined at least once a year.


References:

  1. Fleming K, Mueller EPO. Ileal impaction in 245 horses: 1995–2007. Can Vet J. 2011;52(7):759–63.
  2. Lightbody KL, Davis PJ, Austin CJ. Validation of a novel saliva-based ELISA test for diagnosing tapeworm burden in horses. Vet Clin Pathol. 2016;45(2):335-46.