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.