Research continues into the cause, diagnosis, and treatment of colonic ulcersAlthough gastric ulcers get a lot of attention, colonic ulcers and right dorsal colitis have gained recognition as important equine health concerns. | iStockGastric ulcers get a lot of attention, but ulcers can also form further down the digestive tractin 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 Universitys 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 ConsiderationsTo understand colonic ulcersthe more correct term for hindgut ulcersit 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 digestapartially digested feed from the small intestinein 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 6.37.5 (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 cecums mucosal lining contains goblet cells, which produce protective mucus. Prostaglandin E2 (PGE2), a natural chemical messenger in the horses 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. Histologythe study of structure, arrangement, and function of cells within tissue under a microscopeshows 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.Colonic Ulcers Risk Factors & SignsSigns of colonic ulcers in horses include inappetence, depression, lethargy, colic, weight loss, and intermittent diarrhea. | The Horse/Alexandra BeckstettColonic ulcers can form in any section of the colon, says Aja Harvey, DVM, Dipl. ACVIM, associate veterinarian and internal medicine specialist at B.W. Furlong and Associates, in Oldwick, New Jersey. Researchers do not know the exact cause of colonic ulcers, but the risk factors for development are quite similar to those we know predispose horses to develop ulcerations within the squamous gastric epithelium, she says, describing the upper part of the stomach. These include:Stress of any kind for the horse (i.e., training/competition, transport, management, reproduction)Dehydration from situations such as inadequate water consumption, working in hot environments with failure to replace lost fluids, long trailer rides with inadequate water consumptionAcidic environment in the large colon (hindgut acidosis) because of grain- heavy dietsLack of or reduced access to pasture grassAdministration of non-steroidal anti-inflammatory drugs (NSAIDs), most commonly phenylbutazone (Bute) and/or flunixin meglumine (Banamine)Andrews notes that signs of colonic ulcers are subtle and often vague but can include:InappetenceSigns of depression (The horse stands motionless, weight shifted forward, neck low and level, eyes open, and ears still. He seems shut off from his surroundings.)Other changes in behaviorIntermittent diarrhea (manure might look normal between diarrhea episodes)Colic (abdominal pain)LethargyWeight lossExercise intoleranceIncreased heart rate and respiratory rateIn later stages, ventral edema (fluid swelling along the underside of the belly)Subtle changes might not seem significant, but do not ignore them.Ulcer Location & Probable CausesJonathan Yardley, DVM, associate professor of clinical equine field services at The Ohio State Universitys College of Veterinary Medicine, in Columbus, agrees ulcers can form in any section of the colon; but for reasons unknown, they more often occur in the right dorsal colon. The predilection for lesions in this section may be attributed to higher susceptibility to ischemia (inadequate blood supply), slower (digesta) transit time, and reduced luminal diameter, he says.Andrews cites several studies in which researchers have focused on the right dorsal colon in determining risk factors; the development of RDC; and the development of colonic ulcers with high doses of specific and stacked NSAIDs (concurrent use of more than one of these drugs). Results from several of them appear below.In a crossover observational study conducted at Louisiana State University, McConnico et al. (2008) examined the physiological effects of Bute on the right dorsal colon of cannulated horses. Prolonged oral Bute administration (4.4 milligrams/kilogram every 12 hours for 21 days) caused a significant decrease in plasma albumin concentrations from Day 3 to Day 21. The decrease in plasma albumin (hypoalbuminemia) indicates intestinal mucosal barrier dysfunction. Low blood protein, primarily albumin, should be a component in the diagnosis of colonic ulcers, says Andrews.The researchers suggested plasma albumin concentration could be one of the most sensitive hematologic variables to evaluate when monitoring a horse for chronic NSAID-associated colitismore sensitive than measuring total protein concentration alone. Right dorsal colon mucosal biopsies did not reveal any significant differences in edema or tissue necrosis (death) between treatments. In addition, scientists detected no significant differences in PGE2 concentration between treatment groups. Using an ultrasonic flow probe, researchers found arterial blood flow in the right dorsal colon increased only on Days 16 and 21 of Bute treatment compared with the control, with a gradual overall decrease in blood flow during both treatments. Finally, concentrations of the volatile fatty acid acetic acid dropped significantly after two weeks of Bute treatment and continued to decline over time.Another study (Flood et al., 2023) involved a retrospective look at 35 horses diagnosed with right dorsal colitis at seven Australian equine hospitals. The researchers set out to identify NSAID usage, clinical pathology (apparent disease or damage), treatments, and outcomes. In this study 84% of the horses with known NSAID dose and duration histories had received an overdose before developing RDC. Researchers defined overdose as >8.8 mg/kg Bute in a single day, >4.4 mg/kg daily for more than four days, or >2.2 mg/kg daily for more than eight consecutive days. For flunixin, overdose meant >2.2 mg/kg in a single day. Three out of the 35 cases involved stacking, which can predispose horses to RDC.Still, 16% of horses developed RDC after recommended dosages, suggesting NSAID overdose is not a prerequisite for developing the disease. The most common signs were colic and diarrhea, while the most frequent abnormal clinicopathological findings were hypoalbuminemia, hyperlactatemia (elevated blood lactate concentration), and elevated serum amyloid A (SAA) concentrations. The researchers saw hypoproteinemia less often than previously reported. They performed abdominal ultrasonography in 31 cases and found 77% had subjective thickening of the right dorsal colon walls.In the final study (Bauck et al., 2022) researchers evaluated mechanisms of bicarbonate secretion in the equine large colon to determine any differences between the right dorsal and ventral colon. They collected tissue samples from these structures in 10 horses undergoing surgery and measured bicarbonate secretion and changes in pH levels. They evaluated the effects of phenylbutazone on four of the samples and compared right dorsal and ventral colon baseline bicarbonate secretion. They saw greater bicarbonate secretion in the right dorsal colon than the right ventral colon, and phenylbutazone decreased bicarbonate secretion in the right dorsal colon. These study results might supply another clue to the development of RDC in the horse.Finally Andrews explains the potential connection between prostaglandins and the right dorsal colon: Under normal circumstances prostaglandin, particularly PGE2, functions to protect (via stimulation of mucous secretion) the large intestine lining and dilate blood vessels to enhance blood flow to the mucosa. Colonic ulcers might be caused by a depletion or blocking (through stress or NSAID use) of prostaglandins, which help protect the colon, he says.Definitive Diagnoses ElusiveOutside of a postmortem examination, colonic ulcers are notoriously hard to diagnose in most horses, says Harvey. Transabdominal ultrasound, in conjunction with peripheral blood work, often serves as (one of) the most sensitive diagnostic tools to evaluate the different segments of the colon to see if there are any regions of inflammation in the form of wall-thickening and to assess if there is a reduction in the protein or white blood cell count.In addition, Andrews suggests gastroscopy (endoscopic examination of the stomach) to rule out gastric ulcers. He does point out that gastric ulcers can occur because of the stress of colonic ulcers.Meanwhile, Yardley recommends a fecal PCR test to rule out serious pathogens such as Clostridioides, Salmonella, Neorickettsia risticii (Potomac horse fever), or equine coronavirus.Andrews says that in addition to low albumin, other blood work indicators include low blood calcium, high blood lactic acid, low red blood cell counts due to colonic bleeding, and high fibrinogen and SAA values. He mentions other helpful tests such as measuring fecal pH and using a fecal occult blood test (FOBT) to show the presence of blood components (hemoglobin and albumin) in the manure, which indicates bleeding in the colon. He points out that he uses an FOBT to see if the selected treatment is workingit turns negative with effective treatment.Current Hindgut Ulcer TreatmentsCurrent treatment is tied closely with management, says Harvey. Veterinarians seek to reduce inflammation and increase healing of the colon lining. Our sources suggest the following treatments:Increase turnout time to reduce stressEnsure adequate water intakeDiscontinue any NSAID usePrescribe medications such as:Sucralfatecoats and decreases acid in the ulcer bedMisoprostolsynthetic analog of PGE1 and very similar to PGE2, thought to protect gastrointestinal mucosa by increasing mucous and bicarbonate secretion and enhancing mucosal blood flow. Side effects include abdominal cramping. Monitor the horse and lower the dose or stop the drug if needed.Put the horse on a low-residue diet (less bulk through the compromised colon)Decrease or in some cases eliminate long-stem haysFeed an alfalfa-based pelleted complete horse feedAdd psylliumpsyllium mucilloids lubricate the colon and shorten transit time. They might help decrease inflammation in the colon by increasing production of short-chained fatty acids.Omega-3 fatty acids (EPA and DHA) have been shown to help reduce inflammation and reduce recurrence of gastric ulcers, so veterinarians have suggested using it in horses with colonic ulcers. Cold-water fish oil and algae serve as excellent sources of EPA and DHA; flaxseed oil is a good plant source.Probioticsscientists believe these beneficial bacteria in the hindgut support a healthy mucous liningTake-Home MessageColonic ulcers and RDC occur less frequently than stomach ulcers but could cause more severe clinical signs, says Andrews. Be cautious when interpreting older studies that only focus on a single population without reported clinical signs, notes Yardley.Although research continues regarding cause and diagnosis of a wide variety of hindgut disorders, including colonic ulcers, the difficulty lies in identifying horses that just have colonic ulcers or colonic ulcers in conjunction with gastric ulcers and no other comorbidity, says Harvey. She adds that it would take time to gather enough cases of clinically affected horses to provide the most accurate research statistics.This article is from the Research Roundup 2025 issue ofThe Horse: Your Guide to Equine Health Care. We at The Horse work to provide you with the latest and most reliable news and information on equine health, care, management, and welfare through our magazine and TheHorse.com. Your subscription helps The Horse continue to offer this vital resource to horse owners of all breeds, disciplines, and experience levels. 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