Overview
The equine small intestine (intestinum tenue) is long and narrow compared with the large intestine. It digests and absorbs sugars, starch, protein and fat before the ingesta reaches the caecum. Apart from the duodenum, which is fixed, it hangs from a long mesentery whose root surrounds the cranial mesenteric artery, and moves freely among the loops of the colon.
Parts
- Duodenum: begins with a dilatation, the duodenal ampulla, and makes an S-shaped curve next to the liver. The bile duct and the pancreatic duct open together on the major duodenal papilla, and the accessory pancreatic duct opens opposite, on the minor papilla. The descending part runs caudally on the right, turns around the root of the mesentery caudally (caudal flexure, near the base of the caecum), and the ascending part crosses to the left.
- Jejunum: the longest part; its loops lie mainly in the left dorsal quadrant of the abdomen, mixed with the small colon.
- Ileum: short, with a thick muscular wall; it opens into the lesser curvature of the base of the caecum at the ileal orifice, on a papilla. The ileocaecal fold joins it to the caecum.
Clinical relevance
- Strangulating lesions: because the jejunum is so mobile, volvulus, entrapment in the epiploic foramen, pedunculated lipomas strangling the bowel in older horses and inguinal hernia in stallions are common. They cause severe colic, nasogastric reflux and distended loops on rectal palpation or ultrasound, and need emergency surgery.
- Proximal duodenitis-jejunitis: inflammation of the first segment with large volumes of nasogastric reflux; unlike obstruction, pain usually eases once the stomach is decompressed.
- Ileal impaction: has been associated with very fibrous hay and with tapeworms.
- Tapeworms: Anoplocephala perfoliata attaches at the ileocaecal junction and is associated with ileal colic and intussusception.
- Ascarids: in foals, masses of Parascaris can obstruct the small intestine, sometimes after deworming with a fast-acting drug.