Research continues into the cause, diagnosis, and treatment of colonic ulcers

Gastric ulcers get a lot of attention, but ulcers can also form further down the digestive tract—in the hindgut. Colonic ulcers and right dorsal colitis (RDC) have gained recognition as important equine health concerns. Frank M. Andrews, DVM, MS, Dipl. ACVIM (LAIM), LVMA equine committee professor and director of equine health and sports performance at Louisiana State University’s School of Veterinary Medicine, in Baton Rouge, says awareness of these conditions has grown in recent years, along with research into how they develop.

Hindgut Digestive Anatomy and Functional Considerations

To understand colonic ulcers—the more correct term for hindgut ulcers—it helps to first look at normal equine hindgut anatomy and physiology.

The hindgut consists of the cecum, large colon, small colon, and rectum. In the cecum and large colon, microbes break down digesta—partially digested feed from the small intestine—in a process called hindgut fermentation. This process allows the horse to extract nutrients from a high-fiber diet.

The cecum contains many haustra (pouches formed due to smooth muscle bands within the cecal wall) that give the cecum its sacculated appearance. The haustra, via contractions and peristalsis, slowly mix the digesta with the microbial population to promote fermentation. In addition, the steady mixing gives microbes time to break down complex carbohydrates and fiber and to allow for water absorption.

The fermentation of fibrous plant material produces volatile fatty acids (VFAs), a major energy source for horses. These VFAs are absorbed through the cecal wall and into the bloodstream, keeping the cecal environment at an ideal pH of 6.3–7.5 (Boucher et al., 2024; Wunderlich et al., 2023), allowing fungi and anaerobic bacteria to degrade fibrous parts of the diet such as cellulose, hemicellulose, and pectins.

The cecum’s mucosal lining contains goblet cells, which produce protective mucus.  Prostaglandin E2 (PGE2), a natural chemical messenger in the horse’s body, triggers this secretion to help protect the cecal wall. In addition, the massive microbial population in the cecum also helps defend the gut by crowding out potential harmful pathogens.

Small amounts of digesta slowly pass from the cecum into the different segments of the large colon, also referred to as the large intestine. From the right ventral colon digesta passes to the left ventral colon; the ventral colon is 10-12 feet long and contains haustra similar to those in the cecum. It continues to ferment the digesta with the production and absorption of VFAs. Anatomically, a narrow transition, also called the pelvic flexure, connects the left ventral colon to the left dorsal colon. The cecum and ventral colon retain coarse particles, while the liquid and fine particles move on to the left dorsal colon and right dorsal colon. The dorsal colon contains no haustra; it mainly serves to absorb water and electrolytes from the digesta. The right dorsal colon of the horse forms the final section of the large colon and serves as a key site for fluid secretion and absorption. Its position allows microbial digestion to finish before contents move on for excretion.

The large colon also contains goblet cells that secrete mucus to help protect the large colon wall. Histology—the study of structure, arrangement, and function of cells within tissue under a microscope—shows larger, more numerous goblet cells in the large colon than in the cecum, with greater activity in the right dorsal colon. In addition, unlike the cecum, the large colon produces bicarbonate to help buffer the VFAs produced with fermentation