Right dorsal colitis (RDC) is a specific type of ulcerative disease in horses localized to the right, upper region of the large intestine. RDC is sometimes referred to as hindgut ulcers.

Horses with RDC can show signs of weight loss, diarrhea, lethargy and reoccurring episodes of colic.

In many cases, horses affected by hindgut ulcers have received a non-steroidal anti-inflammatory drug (NSAID) in the recent past. Prolonged NSAID use or overuse can cause RDC in horses.

Hindgut dysfunction can also develop as a result of excessive grain intake, acidosis, stress, or parasites.

In order to treat this disease, a combination of medical, behavioural, dietary and sometimes even surgical intervention is required. If you suspect RDC in your horse, or your horse is experiencing any of the symptoms mentioned, consult your veterinarian.

This article will focus on the signs, causes and treatment of RDC in horses.

What is Right Dorsal Colitis?

Right dorsal colitis is a type of inflammatory bowel disorder in the horse. It is characterized by ulcer development and inflammation in the upper region of the large intestine on the right side of the horse. [2]

Although it is largely unknown why this disease is localized to the right dorsal colon, there are some hypotheses. First, anatomically, the right dorsal colon is short and narrow, suggesting that maybe NSAIDs spend more time in contact with the intestinal wall in this region. [1]

Other hypotheses suggest that the right dorsal colon is more prone to alterations in volatile fatty acid production and ischemia, or restriction of blood supply, after NSAID use. [1]

Contrary to the classification of this condition as an inflammatory disorder, the cause of RDC has actually been linked to the use of non-steroidal anti-inflammatory drugs, or NSAIDs.

Three different NSAIDs have been linked to the development of RDC; flunixin meglumine (banamine), meloxicam and most commonly, phenylbutazone (bute). [1]

You may recognize or have given one of these NSAIDs to your horse. However, not all horses who are given NSAIDs develop RDC.

In fact, if you take a look at one of the NSAIDs in your medicine cabinet at home, such as ibuprofen, you may notice a warning on the bottle: NSAIDs used for humans can cause stomach ulcers in some cases as well.

There are a number of underlying predispositions that can increase the risk of hindgut ulcers due to NSAID use, such as: [1]

  • Dehydration
  • Being off feed
  • Lesions in the large intestine
  • Underlying gastrointestinal disorders
  • Enterotoxaemia – a gastrointestinal disease caused by toxins released by bacteria in the gut

Common Symptoms

Like other gastrointestinal disorders, the symptoms of RDC are general and are common symptoms for many disorders.

Therefore, diagnosis cannot be made on the observation of symptoms alone. Further diagnostic testing is required.

Common symptoms include:

  • Girthiness or irritability under saddle
  • Diarrhea or loose manure
  • Severe colic
  • Blood in the feces
  • Chewing wood, cribbing or other stereotypic behaviours
  • Dehydration
  • Weight loss
  • Lethargy
  • Sensitivity in the flank area
  • Reluctance to move/recumbency
  • Accumulation of fluid in the lower abdomen

Diarrhea and colic are the most common clinical symptoms of RDC.

This disease is reported to be more prevalent in young horses (under 15 years). Younger horses are more likely to be worked and therefore more likely to experience lameness or injury, requiring NSAID therapy. [1]

How do NSAIDs Cause RDC?

Non-steroidal anti-inflammatory drugs work by blocking the cyclooxygenase enzyme (COX). There are two forms of this enzyme: COX-1, which is involved in the protection of the gastrointestinal tract; and COX-2, which is involved in inflammation.

Both the COX-1 and COX-2 enzymes are involved in the synthesis of prostaglandins which are fat-based compounds with hormone-like effects. Only COX-1 is involved in prostaglandin production in the gut.

Within the gastrointestinal tract, prostaglandins are found at high levels in stomach juices and in the gastric mucosa (a layer of the lining of the stomach). Prostaglandins are involved in mucus and bicarbonate secretion, helping to protect against agents that can cause damage to the intestinal lining.

NSAIDs that target COX-1 may interfere with the natural protective mechanisms of the gastrointestinal tract, allowing for the development of ulcers in the gut lining. [1]

Certain non-selective NSAIDs, like phenylbutazone, block COX-1 to a greater extent than they block COX-2.

Types of NSAIDs

Phenylbutazone

Phenylbutazone (“bute”, PBZ) is one of the most widely used NSAIDs in horses. It is frequently given to horses showing signs of pain or discomfort following an injury.

While effective as an anti-inflammatory and analgesic (pain-relieving drug), its use is associated with side effects, particularly if administered for long periods of time or at high dosages.

Flunixin Meglumine

Flunixin meglumine (Banamine®) is an injectable NSAID prescribed for horses with inflammation and pain associated with musculoskeletal injuries.

Flunixin meglumine is also used in veterinary medicine to alleviate pain associated with colic episodes.

This drug is a non-selective NSAID, meaning it inhibits both COX-1 and COX-2 enzymes at a similar rate.

Meloxicam

Meloxicam is administered to horses in the form of an oral paste for pain relief and to control inflammation.

Phenylbutazone is an older non-selective drug, meaning it targets both COX-1 and COX-2 equally. In comparison, meloxicam is a newer and more potent NSAID that preferentially inhibits the COX-2 enzyme.

Chances are if you own a horse or any other companion animal for that matter, your vet has likely prescribed meloxicam at least once. [3]

Research suggests that meloxicam may be safer than phenylbutazone when used appropriately. However, overuse of this drug has also been shown to cause Right Dorsal Colitis.

Research Review of NSAIDs and RDC

Phenylbutazone vs. Meloxicam

One study in horses investigated the safety of various doses and lengths of administration of meloxicam compared to the most commonly associated NSAID with RDC, phenylbutazone.

Thirty-three light horses aged 3 or older were given one of the following treatments:

  • Meloxicam at the recommended dose for 6 weeks (three times the normal treatment period)
  • Meloxicam at 3 times the recommended dose for 2 weeks
  • Meloxicam at 5 times the recommended dose for 2 weeks
  • Phenylbutazone at the recommended dose for 2 weeks
  • A placebo for 6 weeks

The researchers reported that in the horses receiving 3 times the dose of meloxicam for two weeks, three horses developed gastrointestinal symptoms like loose stool, two had poor appetites and one developed hypoproteinemia (low protein in the blood) and hypoalbuminemia (low albumin in the blood).

In the 5 times recommended dose group, two horses developed a thickened right dorsal colon indicative of hindgut ulcers and two other horses developed diarrhea.

One horse in the phenylbutazone group became lethargic, demonstrated an episode of colic, had hypoproteinemia and hypoalbuminemia and a thickened right dorsal colon, indicative of RDC.

The horses receiving the recommended dose of meloxicam for three times the length of recommended treatment displayed no adverse side effects.

Overall, the results of this small study show that administering meloxicam at doses above the recommended dosage caused symptoms of RDC in some horses, even when given for the normal treatment period.

The study also showed that normal phenylbutazone use can contribute to RDC.

Stacking Phenylbutazone and Flunixin Meglumine

Researchers have also identified a significant risk of RDC when stacking multiple NSAIDs together. Stacking is sometimes done by horse owners to achieve a greater analgesic (pain-relieving) effect.

This practice was once common among racehorses but has now been banned by racing federations around the world due to the dangers it poses to the horse.

In a clinical trial, 13 adult horses were treated with either phenylbutazone alone or phenylbutazone and flunixin meglumine for 5 days. Bloodwork was conducted on day 0 and day 5.

Horses treated with both NSAIDs had lower total protein and albumin levels in their blood on day 5 compared to both control horses and horses treated with only phenylbutazone. This is indicative of protein loss through the intestinal wall.

Horses treated with just phenylbutazone also had lower total protein and albumin on day 5 compared to control horses. [4]

One horse on this trial that was receiving the combination treatment developed RDC. Four other horses given the combination treatment showed evidence of ulcers and one horse developed ulcers after treatment with phenylbutazone alone.

This study highlights the risk of adverse side effects when using a combination of two NSAIDs.

Together, the two trials outlined above suggest that there is an increased risk of developing harmful side effects when NSAIDs are used in combination and/or at higher than recommended doses.

Meloxicam vs. flunixin meglumine

In an experimental trial, horses that were receiving NSAIDs for reasons other than intestinal disease were admitted to a teaching hospital in Copenhagen and compared to control teaching horses that were not receiving NSAIDs.

Horses treated with NSAIDs received either flunixin meglumine, meloxicam or both depending on their individual treatment plans.

Four of the ten treated horses developed early signs of RDC confirmed by increased right dorsal intestinal wall thickness on ultrasound. However, clinical symptoms were mild and only one horse showed decreased protein and albumin in the blood.

This suggests a high incidence of RDC in NSAID-treated horses but with low severity.

Furthermore, it highlights the importance of monitoring horses being treated with NSAIDs, as the cases of RDC were confirmed via ultrasound, not clinical symptoms.

Any horses receiving NSAIDs should be monitored closely to watch for side effects and minimize the risk of hindgut problems.

Diagnosis

Usually, a diagnosis is made based on the recent use of NSAIDs along with the presence of some or all of the symptoms outlined above.

The gold standard to confirm a diagnosis of RDC is bloodwork and ultrasound.

The most common findings in bloodwork are:

  • Anemia: low red blood cell count
  • Hypoproteinemia: low protein in the blood
  • Hypoalbuminemia: low albumin, a liver protein, in the blood
  • Hypocalcaemia: low calcium in the blood

These deficiencies are likely due to intestinal damage, meaning horses are not able to absorb these nutrients effectively. intestinal damage can also cause the nutrients to leak into the intestine causing their levels in blood to drop.

Ultrasound is the only definitive way to diagnose RDC as it allows visualization of the right dorsal colon.

The right upper portion of the large intestine can be visualized via ultrasound on the right side of the horse near the lower ribs. Thickening of the large intestine, as well as fluid accumulation, can confirm RDC diagnosis. [1]

Treatment

Consult with your veterinarian to determine the appropriate treatment plan for your horse if they are affected by RDC.

There are many changes that can be made to the exercise and dietary regimens of a horse with RDC, in combination with medical therapy. [1] [2]

Medical:

  • Avoid or minimize the use of NSAIDs. If drugs are necessary to manage your horse’s pain, use an NSAID that targets the COX-2 enzyme more than the COX-1 enzyme, such as ketoprofen (ketofen). Other pain medications may be indicated instead.
  • Sucralfate, a drug used to treat ulcers, may be prescribed in many cases of RDC.
  • Intravenous fluid and/or plasma therapy may be used to treat dehydration and hypoproteinemia.
  • Bloodwork should be done continuously during treatment in order to monitor changes in protein and albumin concentrations as they return to normal.

Exercise:

  • Decrease work/exercise where possible in performance and working horses. This will reduce dehydration and stress during recovery.

Dietary:

  • Right dorsal colitis targets the large intestine, the site of fibre fermentation. A diet lower in fibre or the feeding of several small meals per day can decrease the workload of the large intestine.
  • Replace long fibre hays with short fibre hay cubes, pellets, or chopped hay.
  • Consider adding oils to the diet to provide calories and fat that are easy to digest. This may be particularly beneficial for horses that have lost weight.
  • The addition of psyllium mucilloid – a soluble fibre – to the diet can aid in production of short-chain fatty acids in the large intestine. Psyllium can be fed at a rate of 100 g per day for 3-6 months.
  • Dietary changes are usually recommended for 3-6 months. When protein and albumin concentrations in the blood return to normal, roughage and other normal dietary regimens can be slowly reintroduced.
  • Submit your horse’s diet for evaluation online and our equine nutritionists can help you come up with a feeding plan to support your horse’s recovery.

Consider the use of a digestive health supplement to better support your horse’s hindgut function. There are several ingredients such as probiotics, prebiotics, yeast and digestive enzymes, that can help to maintain the normal function of your horse’s gastrointestinal tract and reduce the risk of digestive upsets.

Optimum Digestive Health Equine Supplement
  • Prebiotics, probiotics & enzymes
  • Support hindgut development
  • Combats harmful toxins in feed
  • Complete GI tract coverage

Surgical Interventions

In severe cases, surgery may be required. In a case study of one 14-year-old Warmblood gelding, medical and dietary treatment did not lead to any signs of improvement. Experimental surgery involving resection of the intestines was conducted. [6]

The affected area of the colon was resected and bypassed. The surgery was successful, other than the development of an abdominal abscess infection at the surgical site. The horse was treated with antibiotics for several months in order to resolve the abscess.

At the 9-month and 2.5-year owner check-up, the horse was confirmed to have returned to normal and was being ridden. Despite complications, the horse made a full recovery and the authors suggest surgical resection as a possible alternative treatment for severe cases of RDC.

Prognosis

The mortality rate for Right Dorsal Colitis is estimated to be between 20 to 40%.[1]

The key to successful treatment of RDC is early diagnosis. This includes monitoring protein and albumin levels in the blood of horses that are receiving NSAIDs for extended periods of time.

Long-term use of NSAIDs should be avoided. Never administer more than what your vet has recommended and never combine NSAIDs unless under the supervision of a veterinarian.

Careful selection of NSAIDs should also be considered before prescription, especially in horses with underlying gastrointestinal conditions which may have a higher risk of developing RDC.

Cyclooxygenase-2–selective NSAIDs present a lower risk of hindgut ulcers, but can also contribute to RDC if misused.

Consider natural alternatives for improving comfort and reducing inflammation in your horse. You can support your horse’s hindgut health by feeding them a forage-rich diet and adopting management practices that support the gastrointestinal tract.

Is Your Horse's Diet Missing Anything?

Identify gaps in your horse's nutrition program to optimize their well-being.