Equine gastric ulcers are extremely common, especially in performance horses. But ulcers can also occur in the horse’s hindgut and have negative consequences for digestive health.
Hindgut ulcers are also known as colonic ulcers. Veterinarians often refer to the condition as right dorsal colitis (RDC) since most hindgut ulcers occur in this region of the large intestine.
Horses affected by hindgut ulcers may experience decreased performance, weight loss, reduced appetite, diarrhea, recurrent colic or a rough coat.
Colonic ulcers can be caused by stress, hind gut acidosis, use of non-steroidal anti-inflammatory drugs (NSAIDs), parasites and other disturbances to the gut microbiome. Dietary management plays a key role in preventing hindgut ulcers from recurring and in supporting the healing of the intestinal lining.
The Importance of the Horse’s Hindgut
To understand the impact hindgut ulcers can have on the horse, it’s important to start with the basic anatomy and physiology (or function) of the hindgut. The hindgut is composed of the cecum, large colon, and small colon. In horses, the large colon is quite large, and can be subdivided into several regions- left ventral colon, right ventral colon, right dorsal colon, and left dorsal colon.
Horses evolved to have such large, specialized hindguts because they are herbivores that digest their food via fermentation in the hindgut. Therefore, when the function of the hindgut is impaired, it can have wide-ranging impacts on the health and well-being of your horse.
When feed moves through the horse’s digestive system, the stomach and small intestine produce enzymes that start to break down the feed. Simple sugars and amino acids are mostly absorbed in the small intestine.
But fibre makes up a huge portion of the horse’s diet and it does not get digested in the small intestine. Horses cannot break down fibre without the help of microbes in the hindgut.
Bacteria, yeast and other microorganisms digest fibre through a process known as fibre fermentation. This process provides the horse with energy, volatile fatty acids, vitamins, minerals, and amino acids necessary for good health.
These nutrients are then assimilated through the intestinal wall for utilization in the horse’s body. A healthy intestinal wall provides a protective barrier that allows nutrients to be absorbed, but doesn’t allow toxins and microbes to enter the bloodstream.
If this barrier becomes damaged by ulcers or compromised by leaky gut syndrome, harmful substances can cross into the bloodstream and can lead to infection and disease.
When hindgut health is compromised, a horse may also have trouble absorbing important nutrients. This can result in poor coat and hoof condition, reduced immune function, and a change in behaviour.
Equine Hindgut Ulcers
Hindgut ulcers in horses occur when there is erosion of the epithelial cells that line the intestinal wall. An open sore or ulceration can form, accompanied by inflammation and thickening of the intestinal mucosa.
Ulceration may occur due to a decrease in the pH level of the colon caused by excess levels of lactic acid. The more acidic environment may result in damage to the mucosal lining, resulting in compromised intestinal barrier function.
This may be accompanied by changes in the equine microbiome including a proliferation of pathogenic bacteria, a decrease in the population of beneficial probiotic microorganisms, and reduced defenses against toxins in the gastrointestinal tract.
Definitive diagnosis of hindgut ulcers is difficult in horses because a gastroscope (used to diagnose gastric ulcers) will not reach the colon. Instead, a presumptive diagnosis will usually be made based on bloodwork and observation of symptoms such as weight loss, intermittent diarrhea or colic.
While they may not be as common as gastric ulcers, hindgut ulcers are believed to affect a large number of horses. In one study that examined 500 horses, researchers concluded that almost all performance horses have some kind of ulcer and that at least 60% had hindgut ulcers. 
Hindgut Ulcers vs. Gastric Ulcers
The primary difference between gastric ulcers (EGUS) and hindgut ulcers is their location in the horse’s digestive tract. Gastric ulcers occur in the stomach, while hindgut ulcers occur in the large colon.
Additionally, these two types of ulcers often have different causes. The main causes of gastric ulcers are intense training programs or long periods of forage restriction.
Hindgut ulcers are most commonly associated with the overuse of non-steroidal anti-inflammatories (NSAIDs), such as Bute® and Banamine®.
Causes of Hindgut Ulcers
1) Overuse of NSAIDs
Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly prescribed by veterinarians and used by horse owners for a variety of conditions including musculoskeletal pain, lameness, and colic.
The most common equine NSAIDs linked with toxicity and hindgut ulcers (in order of severity) include:
- Phenylbutazone (Bute®)
- Flunixin meglumine (Banamine®)
- Firocoxib (Equioxx® or Previcox®)
NSAIDs work by blocking two specific enzymes that are involved in the arachidonic acid pathway- a key pathway involved in inflammation. These enzymes are cyclooxygenase-1 and -2 (or COX-1 and COX-2). The anti-inflammatory and pain-mediating results of NSAIDs come from blocking COX-2.
However, both COX-1 and COX-2 are targeted by traditional NSAIDs. COX-1 plays a bigger role in maintaining a healthy gastrointestinal mucosal lining and platelet function. Prolonged inhibition of COX-1 can lead to GI ulceration due to disruption of this mucosal barrier and interfere with blood clotting.
As a result, it is not recommended to use NSAIDs for longer than 5-7 consecutive days, unless directed by your veterinarian. 
The exception to this rule would be the use of COX-2 selective NSAIDs, such as firocoxib, which are safer for long-term use. Firocoxib selectively targets COX-2, reducing the incidence of side effects like ulcers.
Administering NSAIDs at high doses, for longer than recommended, or administering NSAIDs concurrently puts your horse at high risk for developing right dorsal colitis. Studies show that there is a relatively low safety margin with NSAID use.
Foals and ponies appear to be more likely to develop complications from these medications. Horses that are given NSAIDS and do not have adequate access to water may be even more prone to developing hindgut ulceration.  
2) Excess Grain Intake
If a horse consumes too much grain in a short amount of time, the resulting buildup of lactic acid can kill off the “good” bacteria and promote the growth of harmful bacteria. This is known as hindgut acidosis.
Hindgut acidosis or excess acidity in the colon is most commonly caused by starch overload in the horse’s diet. Excessive grain intake and insufficient forage can result in a significant drop on the pH level of the hindgut.
Typically, starches will be broken down and digested by enzymes in the small intestine. However, when large amounts of grain are fed in a short period of time, not all of the starch will be digested in the small intestine and will instead be transported to the large intestine.
When starch is fermented in the large intestine, this increases the production of lactic acid and changes the microbial population of the hindgut.
Horses with hindgut acidosis often have frequent low-grade colic that is never well explained. To prevent this, grain should always be fed in small amounts and stored in a location that horses cannot gain access to free-choice. Grain should also never be purposefully fed in large amounts.
You can learn more about non-grain feed options for your horse in this article.
Horses that experience any form of ongoing stress are at greater risk of both gastric and colonic ulcers. When a horse is under stress, the hormone cortisol is produced by the adrenal glands.
In the short-term, cortisol can have a positive effect helping horses better respond to stressors in the environment. However, chronically elevated levels of cortisol can have negative consequences. Cortisol inhibits the synthesis of prostaglandins (PG), which are hormone-like fat compounds.
Prostaglandins have a range of important roles in the body, including for mucous production. Prostaglandins are necessary to promote mucous secretions to form the mucosal barrier which protects the digestive system from stomach acid.
Horses under chronic stress from overtraining, frequent transport, or unnatural management conditions, such as routine stalling, are at higher risk for developing both gastric and hindgut ulcers.
Parasites including tapeworm, small strongyles and others may also cause ulcers at the site where they attach to the intestinal wall.
Parasitic colitis occurs when a becomes heavily infected with Cyathostomins, or small strongyles, which become encysted in the intestinal lining as part of their natural lifecycle. The emergence of these parasites can result in inflammation and lesions in the gut barrier.
To avoid an overload of internal parasites, talk to your veterinarian about an appropriate deworming program for your horse.
Various bacteria, viruses, protozoan, and toxins may also lead to hindgut ulcers in some instances. 
Signs and Symptoms of Hindgut Ulcers
Horses experiencing hindgut ulcers can have varying signs and symptoms. Some of these overlap with common signs of gastric ulcers. It is important to consult with your veterinarian to evaluate which type of ulcer your horse has or whether they have both.
Early signs often include mild, intermittent or recurring colic, lethargy, and/or loss of appetite. As the condition progresses, more symptoms may appear. These symptoms may include:
- Sudden girthiness
- Sensitivity in the flank area
- Difficulty bending, collecting, and extending
- Chewing wood (cribbing)
A horse experiencing hindgut ulcers may experience acute (more severe and sudden) symptoms or they may experience chronic (milder, more intermittent) symptoms.
In acute cases, a horse may have fever, depression, loss of appetite, colic, and/or watery diarrhea. They may also experience free fecal water syndrome (FFW). The horse may be dehydrated and have deterioration of the mucous membranes (loss of pink color in gums). 
Horses experiencing chronic right dorsal colitis may exhibit weight loss, recurring low-grade colic, swelling (edema) along the central midline of the belly, and/or loose manure. 
Blood work for these horses may show protein loss (hypoproteinaemia) or low albumin levels (hypoalbuminaemia). In one study, four horses with symptoms of hindgut ulcers were examined and displayed both of these markers. 
Other changes in blood work may include high levels of white blood cells and/or low calcium levels.
Diagnosing Hindgut Ulcers
Unfortunately, hindgut ulcers are more difficult to diagnose than gastric ulcers. Gastroscopy, which uses a scope to directly look for ulcers in the stomach and proximal small intestine, can’t be used in the hindgut.
Instead, veterinarians often rely on observation of symptoms to make a presumptive diagnosis. Your veterinarian will likely start with a physical exam and investigate the horse’s health history by asking questions.
Fecal occult blood tests are used in some cases, but their accuracy isn’t proven, and a diagnosis of hindgut ulcers should never be made on this test result alone.
Peritoneal (belly) fluid analysis might show a small increase in white blood cell count and an increase in total protein concentration.
Transabdominal ultrasound is the only definitive method of diagnosing hindgut ulcers. This type of ultrasound takes specialized equipment and skill on the part of the veterinarian. The procedure involves an ultrasound of the abdomen, focusing on the right side to image the right dorsal colon.
If the ultrasound shows a thickened colonic wall, hindgut ulcers are likely to be diagnosed. 
Prevention and Treatment of Hindgut Ulcers
Veterinarians recommend that horses with hindgut ulcers be managed medically or surgically.
Surgery is often used as a last resort if medical management has failed, and it often involves a resection of the right dorsal colon with bypass. Though some horses can recover from this type of surgery, the prognosis for survival is often described as poor. 
Medical management of hindgut ulcers is usually more successful. In one study evaluating treatment of horses with right dorsal colitis, three out of five horses fully recovered. 
As with many equine health conditions, early diagnosis leads to a better prognosis. Therefore, it’s important to have your horse evaluated by a veterinarian as soon as symptoms of hindgut ulcers first appear.
Medical treatment of hindgut ulcers is often a four-pronged approach: 
- Avoid further use of NSAIDs
- Avoid situations that might cause stress for your horse
- Modify the horse’s diet
- Use medication or supplements targeted toward treating hindgut ulcers
Dietary Management of Colonic Ulcers
A primary goal for horses with hindgut ulceration is to reduce the amount of work the colon has to do by limiting long fiber hay consumption. Hay should be replaced with short fiber hay cubes, pellets, or chopped hay.
In some cases, a complete feed is suitable. Small, frequent meals should be offered whenever possible to support gut health and improve nutrient absorption.
Psyllium is often recommended for these horses as it will help to lubricate and shorten transit time for feed and roughage and increase water content in the intestines. Psyllium also increases fatty acid concentration in the colon and reduces inflammation.
Probiotics and prebiotics can also be helpful for increasing the number of “good” bacteria and restoring gut health in horses with hindgut ulcers. Your veterinarian may also prescribe medications such as sucralfate or misoprostol.
Minimizing stress is an important part of recovery from right dorsal colitis. This might include reduction of strenuous exercise or training, more turnout time, and minimal transport.
Once a treatment plan has been implemented, it generally takes one to two weeks to see a reduction in symptoms. However, it can take two to three months for ulcers to fully heal.
Optimum Digestive Health for Hindgut Ulcers
Mad Barn’s Optimum Digestive Health supplement can help to maintain hindgut health in horses diagnosed with hindgut ulcers or for those at risk of developing the condition.
Optimum Digestive Health is a natural dietary supplement containing prebiotics, probiotics, digestive enzymes, yeast and toxin binders. It can be used in conjunction with medications and dietary changes to support the digestive system and immune function in horses.
Optimum Digestive Health was designed with five main goals in mind:
- To restore balance to the microflora within the gastrointestinal tract
- Improve feed efficiency and support nutrient assimilation
- Supports hindgut function and the fermentation of fibre
- Combats toxins by inhibiting their absorption from the gut
- Supports the immune system by increasing immunoglobulin activity
The ingredients in Optimum Digestive Health have been clinically studied and shown to increase the population of beneficial bacteria in the hindgut while reducing harmful pathogenic bacteria from the Streptococci Sp genus.
By bringing the hindgut microbiome into a more favourable balance, Optimum Digestive Health can support digestion and immune function.
Optimum Digestive Health can also help to prevent the causes of hindgut acidosis by limiting the passage of starch into the hindgut. It is a source of digestive enzymes that have been shown to assist in the absorption of nutrients within the small intestine.
The main ingredients in Optimum Digestive Health include 20 billion CFU’s of Mad Barn’s 5-strain blend of probiotics, Yea-Sacc 1026™ yeast culture, the prebiotic Integral A+™, Kelp Meal, Allzyme® SSF and Bio-Mos™.
Allzyme® SSF is a digestive enzyme complex which contains a fungus known as Aspergillus niger that produces a combination of enzymes. These enzymes work directly in the gastrointestinal tract to help break down feed components.
In equine studies, enzyme-supplemented diets increased digestibility of all nutrients, especially fibre, and also improved fecal gas production. Allzyme® SSF may be especially beneficial for horses that have trouble maintaining their weight, are prone to having digestive upsets, or for lactating mares needing extra nutrients to support milk production.
Bio-Mos™ is a proprietary ingredient made from a specific strain of Saccharomyces cerevisiae yeast. This ingredient is designed to nourish beneficial microorganisms in the gut that promote athletic performance and feed efficiency.
It has been clinically studied and used for over twenty years to promote gastrointestinal health and integrity in a variety of agricultural animals, including horses.
Bio-Mos™ works by reinforcing the normal function of the digestive system and stimulates the body’s natural defenses. It helps to normalize the gut microflora, supporting a healthy balance between good and bad bacteria and contributing to the development of the immune system.
Optimum Digestive Health is a highly palatable, pelleted supplement that provides complete GI tract coverage and can help to maintain the integrity of the hindgut. If you suspect your horse has hindgut ulcers, talk to your veterinarian about feeding Optimum Digestive Health.
Signs of hindgut ulcers should always be taken seriously. If you suspect that your horse might be affected by hindgut ulcers, have him examined by your veterinarian as soon as possible.
In some cases, hindgut ulcers might be unavoidable, but by limiting NSAID use, minimizing stress whenever possible, and managing and feeding your horse appropriately, you can greatly reduce his chances for developing this debilitating condition.
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