Diarrhea in Horses – Causes & Support for G.I. Health

Updated April 12, 2024 | By: Andris J. Kaneps, DVM, PhD, DACVS, DACVSR

Diarrhea is defined as an increase in the frequency, volume, or fluid content of stools. For reference, horses normally pass manure about 8-12 times per day. Frequent, loose stool in horses can range from mild, chronic diarrhea that is a nuisance but does not affect a horse’s overall health to severe, acute diarrhea that is a medical emergency.

Read more about the signs and symptoms of diarrhea in horses, possible causes, veterinary diagnosis, and methods for treatment and management.

Classifying the Condition as Acute or Chronic

Diarrhea can be classified as either acute or chronic depending on how long a horse has been having diarrhea and its progression.

• Acute refers to a condition that comes on suddenly, lasts a short time, or rapidly progresses. Acute diarrhea in horses can quickly become extremely serious (even life-threatening), so it is important owners contact their veterinarian right away.
• Chronic refers to a condition that persists over a long period of time. Some experts consider frequent, loose stool that lasts 7 days to be chronic diarrhea, for others the cutoff is 2 weeks, and some use 1 month as the threshold.

In some cases, chronic diarrhea can develop after a bout of acute diarrhea.

Depending on the cause, some horses with chronic diarrhea may seem bright and healthy with a good appetite and hydration level. The prolonged watery, soft stool may not affect the horse’s weight, energy, or overall health, and could mostly cause the hind legs, tail, and environment to be constantly soiled.

However, other causes of chronic diarrhea can result in a sick horse that gets progressively worse the longer the condition continues. So, it’s important your vet is involved as soon as possible.

Signs, Symptoms, & When to Call the Vet

Since diarrhea in horses is an increase in the frequency, volume, or fluid content of feces, a mild case can be seen as just a few additional manure piles in a day than normal that are more “cow plop” in consistency than the regular, formed fecal balls. If your horse is showing no other signs of illness, this might be a case of “wait and see.”

However, your horse should be examined by a vet as soon as possible if the diarrhea is:

• Profuse and watery
• Explosive or pipestream
• Accompanied by other signs of illness (like colic, lack of energy, little to no appetite, fever, or purple to red gums instead of the normal pink color)

The Effects of Diarrhea on a Horse’s Body

While messy and unpleasant for owners, diarrhea in horses poses a more significant threat of dehydration. Frequent watery stools lead to rapid water loss, causing weakness, lethargy, and electrolyte imbalances.

Beyond dehydration, diarrhea hinders nutrient absorption, leading to deficiencies, protein loss, and reduced appetite. This translates to poor performance and low energy levels in your horse.

In severe, unmanaged cases, particularly with foals or those with infectious diarrhea, complications can be fatal. Early diagnosis and treatment of the underlying cause are crucial.

Causes of Diarrhea in Horses

Diarrhea can develop in horses from a wide variety of both infectious and non-infectious causes, including:

Infectious Causes

• Bacteria (Salmonella, Clostridia, Rhodococcus, Lawsonia, Potomac Horse Fever [Neorickettsia])
• Viruses (Equine Coronavirus, Rotavirus)
• Parasites (worms)

Non-infectious Causes

• Changes in the diet (whether it be the type of feed, water or frequency given)
• Antibiotics
• NSAIDs (non-steroidal anti-inflammatory medications) like phenylbutazone (Bute)
• Inflammatory Bowel Disease (IBD)
• Eating sand
• Stress (such as from trailering or competing)
• Gastric ulcers
• Toxins (poisonous plants)
• Secondary to another disease or condition (like peritonitis)
• Altered organ function (such as chronic liver disease or heart failure)
• Cancer

Diarrhea may be caused by inflammation or infection of the small intestine, where it is referred to by the medical term enteritis, or in the large intestine, where it is referred to as colitis. When both parts of the intestine are involved, the medical term used is enterocolitis.

Increased Risk for Diarrhea in Foals

Diarrhea in foals is very common [1]. Foals have low reserves of body fluids and electrolytes, as well as not completely developed immune systems. Prompt veterinary diagnostics and treatment are essential for your foal’s well-being.

Common Causes of Diarrhea in Foals

• Foal heat diarrhea: Foals commonly experience diarrhea at 1-2 weeks of age by ingesting the mare’s and other adult horse’s manure. Bacteria then settles in the foal’s intestines. This process is required to establish the foal’s healthy, normal gut microbiome [2].
• Stress during the birth process: Foaling stress may reduce oxygen levels in the lining of the intestinal tract and could cause necrotizing enterocolitis, essentially leading to severe damage to the mucosa of the gut for a newborn foal. This leads to the absorption of bacteria into the circulatory system and toxemia that can cause organ failure and death [1].
• Bacterial or viral infection:
  • Clostridia and salmonella infections may be carried by foals and adult horses without showing clinical signs of diarrhea. These infections may cause body-wide illness and can localize in joints, causing infectious arthritis.
  • Rhodococcus, a bacteria that most commonly causes lung infections in foals 30 days of age and older, also causes enterocolitis in about 30% of affected foals [3].
  • Foals 3-12 months old may be affected by a bacteria called Lawsonia intracellularis that most commonly causes thickening of the intestine. The infection is slow-acting and results in poor body condition, lack of weight gain, and at times, diarrhea [2].
  • Rotavirus is the most common viral cause of diarrhea in foals less than 30 days of age that can range in severity. In a survey of 233 foals with diarrhea, 20% had rotavirus [4].
• Parasites: Ingestion of strongyle larvae (Strongyloides westeri) from the mare’s milk may cause mild to serious foal diarrhea, but may be preventable with proper deworming practices.

Senior Horses Experiencing Diarrhea

Age alone does not predispose a horse to having diarrhea. However, certain conditions (like PPID, Cushing’s Disease) may reduce an older horse’s tolerance for, and recovery from, an infection. The immune system of older horses is slower to respond to infections and may react with either a poor or exaggerated inflammatory response [5].

If your senior develops diarrhea, his risks are higher than a younger stablemate’s. A study found that horses with colitis that were 20 years or older were 15x more likely to pass away than horses 2-12 years old [5].

As with foals, older horses should be evaluated by your veterinarian right away to ensure it does not lead to dehydration, toxemia, or organ dysfunction.

Diagnostics for Diarrhea

Getting to the bottom of why a horse is having diarrhea can be challenging for both veterinarians and horse owners. A diagnosis begins with a thorough history, identifying your horse’s:

• Age
• Existing medical conditions and medications
• Current feeding program (hay, pasture, grain, and supplements)
• Turnout and exercise schedules
• Recent changes to diet, workload, or management
• Exposure to new horses or new facilities

In addition, owners should be prepared to give a description of the current problem to their vet, which includes answering:

• When the loose stool first started and what it looks like?
• What other signs have been observed?
• A list of all the treatments given thus far and their results.
• Whether any other horses in the barn or herd are sick?

Blood Tests

After a complete physical examination, your vet may recommend certain tests to help rule in or out specific causes. For example, bloodwork can be used to:

• Assess red blood cells for the presence of anemia and white blood cells for the presence of infection
• Measure the degree of inflammation in the body
• Evaluate the health of organs such as the kidney and liver
• Check for acid-base or electrolyte abnormalities
• Tests for bacterial toxins

Fecal Tests

Several fecal tests may be performed to check:

• Bacterial culture
• Count parasite eggs
• Looking for the presence of sand

Imaging and Other Procedures

• Vets may also perform an abdominocentesis (or belly tap) to directly check the fluid in the abdomen.
• Ultrasound and radiography (x-rays) are sometimes used to image specific structures.
• Your horse’s stomach and duodenum could be scoped or visually assessed, and samples taken of the tissue at the same time. Samples or biopsies can even be taken of rectal tissue during palpation.
• An intestinal absorption test measures the efficiency of sugar absorption from the intestine as a gauge of how well the intestinal cells are doing their job. This test is simple and easily done at the barn.

Treatment and Management of Horses with Diarrhea

Which treatments are given highly depends on the cause and whether it’s an acute, severe bout or a mild, chronic diarrhea.

Reducing Stress and Gastric Ulcers

Many ailments are caused by stress, and for horses, stress can make them prone to developing ulcers.

Changes in your horse’s environment or daily schedule, like how long he gets turned out and with a herd or alone, may be enough to disrupt normal digestive processes and impact the population of beneficial microorganisms in the hindgut. Abrupt shifts in the type, amount, or frequency your horse is fed can also be stressful. And shipping, whether on the trailer for a short drive down the road or loading onto a plane to fly cross country, is a stress trigger for many horses.

Try to minimize the occurrence of these stressful events in your horse’s life. Non-prescription, FDA-approved Ulcergard can be helpful as a preventative for equine gastric ulcers in times of stress, like travel. Also, oral supplements that contain beta glucan and hyaluronan may be administered to protect the stomach and hindgut [6].

Properly Controlling Parasites

Many different types of animals will have diarrhea if they are infested with parasites. A fecal egg count test can help your vet identify the type and number of worms present. Based on those results, your vet can recommend a dewormer and schedule for administering it that’ll be most helpful for getting rid of ascarids, strongyles, and other worms that may be present.

Feeding Programs to Support GI Health

When the goal is simply to re-establish normal intestinal microflora and encourage regular, firm manure, better results may be achieved in your horse’s regular home versus a clinic. At this stage, it’s commonly recommended to try dietary trials that either eliminate suspect feedstuffs or add in components known to be easy for horses to digest and absorb.

Here is a list of products you could try giving your horse one at a time with your veterinarian’s advice, and at least for 30 days:

• Probiotics, prebiotics, and yeast: to reinoculated and feed the “good bugs”
• Psyllium: in case the diarrhea is caused by inflammation from sand ingestion
• Digestive enzymes: such as amylase, lipase, cellulase and protease
• Hindgut buffer: encapsulated or protected sodium bicarbonate
• Digestive support: products with L-glutamine, Licorice, oat fiber and oil
• Plant extracts: adaptogens to help normalize the body’s systems

Slowly introducing a new food (over 7-14 days) can help your horse’s GI tract better adjust to the change.

Medications for Horses with Diarrhea

If none of these over-the-counter treatments work, you may need to speak with your vet about starting prescription products. Here are a few that have worked for some horses:

• Steroids: prescription immune suppressants
• Antihistamines: prescription products that may work best with steroids
• Antibiotics: prescription only since some antibiotics can worsen the situation!
• Rheaform: a prescription product that reduces motility in the colon
• Lomotil: prescription anti-diarrhea medication
• Imodium: over-the-counter anti-diarrhea medication
• Opiates: prescription narcotics related to morphine

When Diarrhea May Lead to a Stay at the Equine Hospital

In order to replace intravenous fluids and electrolytes; address pain, inflammation, and endotoxemia; and promote repair of intestinal tissue, it may be necessary to refer an extremely sick horse to a hospital for more intensive, round-the-clock nursing care and observation.

Infectious diseases require immediate veterinary care, and your horse should be quarantined from others if the virus is contagious. Horses may also require intensive care if the cause of their diarrhea is colitis, or inflammation of the colon.

The primary cause of frequent, loose stool in horses is never fully discovered in well over half of all cases. This means that treatments may be mainly supportive in nature and not specific.

Preventing a Bout of Diarrhea

It can take time to find the right combination of forage, grain, supplements, and medication that will help maintain formed fecal balls for some horses with chronic diarrhea.

For some horses, the situation may never be fully resolved with treatments only providing temporary relief before the diarrhea returns. In these cases, owners must make sure their horse always has access to fresh, clean water to maintain hydration and that they have a system for keeping the hind legs and tail clean so as not to attract flies or cause skin lesions.

Key Takeaways

• Diarrhea that’s sudden and severe can be life-threatening.
• For all situations in which a horse’s manure is not normal, it is a good idea to talk to your veterinarian.
• Serious or simple causes of diarrhea must be ruled out and a systematic approach taken to improving defecation.

A related condition called Free Fecal Water Syndrome (FFW), is when a horse passes normal, solid manure separately from fecal liquid. Chronic diarrhea and FFW may occur concurrently and can be frustrating in terms of both your horse’s health and their appearance. Learn more about the causes, treatment and management of FFW in this article.

Evidence-Based References

1. Magdesian, K. Gary. “Neonatal Foal Diarrhea.” Veterinary Clinics of North America: Equine Practice, vol. 21, no. 2, Aug. 2005, pp. 295–312. https://doi.org/10.1016/j.cveq.2005.04.009.
2. Mallicote, Martha, et al. “A Review of Foal Diarrhoea From Birth to Weaning.” Equine Veterinary Education, vol. 24, no. 4, Jan. 2012, pp. 206–14. https://doi.org/10.1111/j.2042-3292.2011.00358.x.
3. Reuss, Sarah M., et al. “Extrapulmonary Disorders Associated With Rhodococcus Equi Infection in Foals: 150 Cases (1987–2007).” Javma-journal of the American Veterinary Medical Association, vol. 235, no. 7, Oct. 2009, pp. 855–63. https://doi.org/10.2460/javma.235.7.855.
4. Frederick, Jeremy, et al. “Infectious Agents Detected in the Feces of Diarrheic Foals: A Retrospective Study of 233 Cases (2003–2008).” Journal of Veterinary Internal Medicine, vol. 23, no. 6, Oct. 2009, pp. 1254–60. https://doi.org/10.1111/j.1939-1676.2009.0383.x.
5. McFarlane, Dianne. “Immune Dysfunction in Aged Horses.” Veterinary Clinics of North America: Equine Practice, vol. 32, no. 2, Aug. 2016, pp. 333–41. https://doi.org/10.1016/j.cveq.2016.04.009.
6. Slovis, Nathan M. “Polysaccharide Treatment Reduces Gastric Ulceration in Active Horses.” Journal of Equine Veterinary Science, vol. 50, Mar. 2017, pp. 116–20. https://doi.org/10.1016/j.jevs.2016.11.011.