Diarrhea is a common problem in horses as it can be a result of many different causes.  It is typically a sign of infection or dysbiosis in the hindgut.
Diarrhea is defined as the excessive and frequent defection of loose or liquid stool. It can result in imbalanced electrolyte levels and impaired water absorption throughout the intestine.
Because of the physiology of the horse’s colon and cecum, fluid loss with diarrhea can quickly reach harmful levels, leading to dehydration and other health problems.
Equine diarrhea is a symptom and not a disease in and of itself. It is a sign that something is wrong. However, up to 50% of cases of diarrhea go undiagnosed.
Wondering why your horse has diarrhea? Typical practices such as changing feed, stall confinement, and high grain consumption can contribute to the onset of diarrhea. Bacterial or viral infection can also cause this symptom to develop.
While not as common, diarrhea can also be a symptom of something more serious such as inflammatory bowel disease or cancer (i.e. gastrointestinal lymphoma).
Often, diarrhea is transient and resolves itself within a few days. In prolonged cases of diarrhea, the root cause is usually treatable. However, untreated chronic diarrhea with excessive water loss can result in serious complications such as colic.
Defining Diarrhea in Horses
Horses have a unique and complex digestive system. Diarrhea most commonly results from dysfunction in the hindgut rather than the stomach or small intestines.
Equine diarrhea can present differently with various levels of stool softness and water excretion, ranging from non-formed (cow-pat) stools to liquid defecation.
The fecal excretion can be projectile or involuntary leakage down the hind legs. The latter could even result in skin scalding. It often gives off an offensive smell.
Diarrhea can be classified as acute or chronic. Acute diarrhea occurs suddenly but is often short-lived, whereas chronic diarrhea persists for several weeks or months or is recurring.
Acute VS Chronic Diarrhea
Acute diarrhea occurs over one or two days but is often resolved independently. It usually does not require medical attention, although you may need to adjust their diet to ensure proper hydration and to prevent nutrient loss. 
Chronic diarrhea is present for more than four weeks. This type of diarrhea often requires medical attention from a veterinarian. Hydration, protein, and electrolytes should be increased for horses with chronic diarrhea. 
Diarrhea does not have to be present daily for four weeks to be considered chronic. It may come and go over the four (plus) weeks.
Consequences of Diarrhea
Diarrhea in itself is considered a messy and unpleasant problem to the horse owner. However, many complications can also arise due to frequent diarrhea in your horse.
Direct consequences of equine diarrhea can include: 
- Water loss and dehydration
- Pain and discomfort in the girth and anal region
- Reduced nutrient absorption
- Nutrient deficiencies
- Loss of proteins
- Reduced appetite
- Poor performance or low energy
Although rare, in extreme cases diarrhea can be fatal if not managed properly or if the underlying condition is not treated.
Determining the cause of your horse’s diarrhea episodes can help reduce and prevent incidences. Here we briefly review 25 potential reasons why your horse may have equine diarrhea.
25 Reasons Your Horse May Have Diarrhea
Horses can develop diarrhea for a number of different reasons. The most common causes of diarrhea are linked to dietary management, bacterial infection, toxins, or viruses.
Diarrhea can also be a symptom of several different diseases and disorders, some of which are discussed below.
Diet is one of the biggest causes of diarrhea. Certain feeding practices and feeds can cause dysbiosis of the hindgut, which results in a shift in the hindgut microbiota. Quick or abrupt changes to diet can also shock the microbiome, causing imbalances in the microbial populations which affects nutrient digestion.
Diarrhea linked to dietary management is often treated with changes to your horse’s feeding plan and does not usually require medications. During bouts of diarrhea, it is critical to ensure adequate hydration and to maintain electrolyte balance.
1) Over- or Under-Feeding
Horses that are eating too much may be putting excess strain on their gastrointestinal system or surpassing the capacity of the small intestine to digest starch. Horses that are underfed may experience nutrient deficiencies that impair gut health and interfere with cell turnover in the intestinal lining.
Over- and under-feeding can also have their own set of health consequences (i.e. equine metabolic syndrome, ulcers, instability of the microbiome).
2) Nutrient Imbalances
For example, excess magnesium, particularly in the form of magnesium sulfate can induce diarrhea. In fact, it is commonly used as a laxative to draw water into the intestinal lumen and increase the rate of passage. 
3) Forage Type
Not all hays or forages are created equal and some forages are known to increase the risk of gut problems.
High-quality fresh grass or grass hays are recommended for horses recovering from serious gastrointestinal issues. Hay that is less fibrous and easily digestible will cause less irritation through the digestive tract. This typically means second-cut grass hays are preferable over first-cut hay. 
High-quality alfalfa hay can be used to promote food intake in horses with low appetites. However, in general, alfalfa hay should not represent more than 10-20% of your horse’s forage intake.
You can learn more about hay selection in this article.
4) Poor Grass/Forage or Hay Quality
Inconsistent hay quality can be a major factor in diarrhea episodes for horses. The hay you have available to your horse may change depending on the time of harvest, soil conditions, and hay maturity. All of these factors can alter the nutrient composition and water holding capacity of forage.
Ensuring consistent hay quality can prevent diarrhea episodes.  Make hay changes gradually so as to allow the microbiome time to adjust.
If travelling, consider using hay cubes which provide more consistent quality.
5) Feeding Mouldy Forage, Hay, or Concentrates
Mouldy hay is common in hot and humid conditions, but can cause serious problems for your horse’s gut and overall health. Mould is a source of mycotoxins that can disrupt your horse’s microbiome and causes diarrhea.
Avoid exposure of feedstuffs to moisture by storing forage or hay in a cool, dry, and dark environment. Throw out any mouldy feed to reduce the risk of toxins entering your horse’s digestive system.
6) Abrupt Changes to Your Feeding Regimen
The horse’s digestive system is very sensitive to changes and the microbiome is easily disrupted when new feeds are introduced. Abrupt changes to your feeding regimen can disrupt the delicate balance of microbes in the digestive tract and result in diarrhea. 
Horses are hindgut fermenters. They rely on bacterial populations in the hindgut to ferment fibres found in forage so they can extract energy and nutrients from cellulose.
When new hays are introduced, this can alter the microbial populations in the cecum and colon which can cause diarrhea.
Make changes gradually over a one-to-two-week period. These include changing your horse’s forage or hay type, switching their concentrate source, or even adjusting mealtimes.
7) High Intakes of Non-Structural Carbohydrates (NSC)
Grains and concentrates (i.e. oats, barley, maize) are high in NSC and are associated with looser stool. Non-structural carbohydrates include starches and sugars.
The horse’s small intestine only has a limited ability for digesting NSC and excess starches can enter the hindgut, resulting in a condition known as hindgut acidosis.
Diets that are too high in NSC can cause dysbiosis and lead to diarrhea. Limit grains and concentrates when possible and provide fibre-rich grass/forage or hay as the majority of your horse’s diet. 
Stress can lead to hormonal changes in the horse that can result in many digestive problems such as diarrhea. Hormonal responses may alter a horse’s appetite and result in reduced feed intake. Hormones can also affect how the digestive system processes food.
Stress-induced diarrhea is most commonly acute unless the stressor persists, in which case diarrhea can become chronic.
The best way to prevent stress-induced diarrhea is to eliminate or reduce exposure to the stressor. Feeding management can help reduce the impact of stress-related diarrhea when this is not an option.
Digestive health supplements may benefit horses exposed to stressful conditions more frequently, such as athletic or working horses. Ensuring proper hydration and electrolyte balance is also critical for these horses.
8) Stall Confinement
Stall confinement of more than 12 hours per day can reduce colonic motility.  This slows the movement of food and liquid through the gut.
9) Travel and Trailering
Travel can be stressful for horses and results in similar physiological effects as stall confinement.  Horses that are trailered often have inconsistent access to food and water while in transit.
Horses are usually confined while traveling and can’t express normal species-appropriate foraging behaviours. This lack of movement results in decreased gut motility and reduced movement of digesta through the colon.
Horses are also greatly affected by new environments, changes in routine and changes in social grouping. Horses that are being transported may require additional dietary support in the form of probiotics or other supplements to minimize the risk of gut problems.
10) Environmental Stress
Environmental stress can also negatively affect digestion and lead to diarrhea.  Examples of environmental stressors can include loud sounds or unusual movements, changes in housing conditions, and changes to social dynamics.
Research shows that horses experiencing a spike in stress hormones like cortisol are more likely to experience digestive issues including increase risk of gastric ulceration, colic, and diarrhea. 
These effects are mediated by inflammatory pathways that affect the microbiome and pH levels of the digestive system.
11) High-Intensity Exercise
Regular exercise is great for digestive health as it can encourage the transit of feed through the digestive tract and promote hydration. However, too much exercise can lead to gut issues.
Intense exercise may contribute to diarrhea by causing food to move rapidly through the intestines. When gut motility is increased, absorption of nutrients and water from feed is compromised, which can overwhelm the hindgut with digesta. 
12) Anesthesia Recovery
Research shows that stress from anesthesia may impact the microbiome of horses, resulting in bouts of diarrhea.
The effects of anesthesia on digestive health are not well understood. Horses receiving anesthesia typically also undergo fasting and transportation, so it is difficult to isolate the effects of anesthesia in the scientific literature. 
Bacterial infections can cause dysbiosis, resulting in diarrhea and other symptoms such as fatigue, weakness, and loss of appetite.
Horses with bacterial infections may require intravenous supply of fluids and electrolytes to replenish fluid levels and prevent dehydration.
Horses may require treatment with antibiotics or prescription non-steroidal anti-inflammatory drugs.
Use of antibiotic medications can also contribute to diarrhea as these drugs kill off both good and bad bacteria in the gut. Probiotic supplements can help to replenish the microbiome following use of antibiotics. 
Salmonella infection is one of the most commonly diagnosed causes of diarrhea in adult horses.  This pathogenic bacteria releases toxins that cause inflammation in the intestinal lining.
Your horse may become infected by Salmonella through feed or water contamination, eating feces that contain the bacteria, or contact with other infected animals.
Horses with Salmonellosis may exhibit mild signs such as fever, weakness, low appetite and watery stools. In severe cases, diarrhea may be bloody and fatality can occur.
Clostridium is another pathogenic bacteria that can cause diarrhea in horses. These bacteria grow in the small and large intestines and the rectum of the horses. 
Horses may be exposed to the bacteria through the feces of a horse treated with antibiotics, the feces of a foal with diarrhea, or at a veterinary hospital.
Internal parasites can be a serious problem for horses, resulting in diarrhea, weight loss, weakness, fatigue, and subcutaneous edema.
If you suspect your horse is affected by parasites, consult with your veterinarian. Treatment should be administered as soon as possible to minimize adverse effects.
Regular deworming and appropriate management strategies can help to reduce parasite loads in horses.
15) Larval Cyathostominosis
Over 40 cyathostominosis species can infest the large intestine of horses resulting in watery stools and diarrhea. 
The larvae from cyathostomes (small strongyles) become embedded in the large intestine encased within cysts. Larval cyathostominosis occurs when the larvae emerge from the cysts and trigger an immune reaction that causes damage to the intestinal lining.
Grazing horses can ingest these parasites if they are foraging near manure piles and become infected. Cyathostominosis infections may require anti-inflammatory drugs, fluid therapy, and other medications.
Environment and Toxins
Toxins and other substances in your horse’s environment can result in diarrhea if consumed in large amounts.
Certain poisonous plants contain chemicals that can disrupt gut function. Excess use of medications can also result in toxicosis.
16) NSAID Toxicity
Prolonged use of Non-Steroidal Anti-Inflammatory Drugs (NSAIDs) at high doses can result in NSAID toxicity. 
Excessive use of NSAIDs can result in increased intestinal permeability, right dorsal colitis, and protein deficiencies, all of which can result in diarrhea.
16) Sand Ingestion
Sandy pastures, paddocks, and stalls can lead to sand ingestion in your horse. Sand can accumulate in the large intestine and disrupt the microbiome, potentially resulting in diarrhea. 
In serious cases, sand ingestion can lead to impaction and sand colic.
17) Toxic Plants:
- Raw linseed oil
- Castor oil
Disease and Disorders
In some cases of diarrhea, the underlying cause can be a disease or disorder that affects the gastrointestinal system. Consultation with a veterinarian is required to diagnose the problem.
18) Free Fecal Water Syndrome
Horses with FFW can experience two distinct phases of defecation: a solid phase in which solid feces are eliminated and a liquid phase in which free water runs out of the anus before, during, or after the solid phase.
The cause of fecal water is unclear; like diarrhea, it may be a sign of leaky gut or dysbiosis.
19) Inflammatory Bowel Disease (IBD)
Horses with IBD experience inflammation in the lining of the small intestine. If the large intestine is also affected, diarrhea can occur.
20) Hindgut Ulcers
Ulcers are extremely common in horses, affecting between 60-90% of pleasure and performance horses. They can occur in the esophagus, squamous or glandular regions of the stomach, small intestine, and hindgut. 
Diarrhea can sometimes occur in conjunction with gastric ulcers, but this symptoms is more commonly associated with hindgut ulcers.
21) Gastrointestinal Lymphoma (Cancer)
This disease is extremely rare in horses, but can cause diarrhea. Horses affected by gastrointestinal lymphoma can exhibit symptoms such as weight loss, poor appetite (anorexia), lethargy, abdominal pain and diarrhea.
Diarrhea in Foals
Diarrhea is extremely common in foals as the immune system and gut microbiome are not yet well-established. 
According to one study, 20% of foals experience diarrhea within the first six months of life.  During the first few weeks of life, foals are particularly susceptible to infections, disease, and gut disturbances.
Some cases of diarrhea in foals may resolve on their own without medical care or treatment. However, it is important to consult with your veterinarian to rule out other more serious causes of diarrhea, particularly if the foal shows loss of appetite or a dull affect.
22) Foal Heat Diarrhea
Within the first four to fourteen days of life, foals may develop foal heat diarrhea or scours. This condition is believed to be caused by changes in the foal’s gastrointestinal tract with the addition of feed to the diet.
Foals may also come into contact with bacteria through ingestion of the mare’s manure (coprophagy), resulting in changes to the foal’s gut flora. Foal heat scours will typically resolve over time as the hindgut microbiome is established.
Foals younger than six months old are at high risk of contracting rotavirus, a contagious virus that is one of the most common reasons why foals experience diarrhea.
Exposure to contaminated feces is the main route of transmission for rotavirus. If your foal has diarrhea, keeping it away from other foals and horses can help to minimize the risk of contagious transmission.
24) Rhodococcus euiq
Foals are susceptible to infection by Rhodococcus equi bacterium, which live in the soil. This pathogen can cause pneumonia in foals. 
Approximately one-third of foals affected by Rhodococcus equi experience diarrhea as a symptom. Foals showing respiratory or gastrointestinal signs of R.equi infection should be isolated from the herd to minimize spread of the virus.
25) Transitioning from Milk to Feed
As the foal begins to consume solid foods such as hay, forage, and eventually grains and concentrates, their digestive system needs to adjust. This transition is sometimes associated with diarrhea, especially if the foal’s diet is changed abruptly.
Foals may also develop diarrhea or loose stools as a result of feeding a milk replacer or cow’s milk. 
Forages, grains and concentrates should be introduced as “creep feed” before the foal is weaned to help ease the transition away from milk and minimize digestive upsets related to weaning.
Diagnosing Diarrhea in Horses
Not every bout of diarrhea warrants a visit from your veterinarian. However, sometimes diarrhea is a sign of a bigger problem that requires veterinary care.
Monitor your horse closely for the first 24 hours of a diarrhea episode. If unaccompanied by other symptoms, the loose stools may resolve on their own without needing further attention.
If 24 hours have passed and your horse is still experiencing diarrhea, seek out veterinary care. Unresolved diarrhea can cause dehydration, loss of electrolytes, reduced nutrient uptake and other serious problems.
Check to see whether your horse is exhibiting any of the following signs or symptoms alongside diarrhea:
- Reduced energy levels
- Lower appetite
- Stereotypic behaviours
- Poor skin and coat health
- Lameness or resistance to exercise
- Weight loss
Symptoms such as these could indicate an underlying issue.
Treatment and Management of Diarrhea
Treatment of equine diarrhea typically involves addressing the underlying causes of this condition and supporting fluid balance during recovery.
Closely monitor your horse for any worsening of symptoms. This condition can become dangerous if it is associated with excessive water loss and dehydration.
Fresh, clean lukewarm water should be available to your horse at all times. Provide plain loose salt to promote thirst and rehydration.
Diarrhea can impair nutrient absorption from your horse’s diet and lead to electrolyte loss. Feeding an electrolyte supplement may be required to restore mineral balance.
Ideally, add these electrolytes directly to water to encourage drinking. Some horses do not like flavoured water so it is a good idea to provide drinking water without added electrolytes as well.
Mad Barn’s Performance XL: Electrolytes supplement contains a balanced profile of electrolytes to support your horse’s hydration status.
Diet and Routine
If your horse experiences diarrhea, it is a good opportunity to review their feeding plan and daily management. A well-balanced diet and species-appropriate lifestyle can help to prevent future disturbances to your horse’s gut health.
You can read our article on How to Feed a Horse with Diarrhea for a complete list of strategies to support your horse’s recovery.
- Remove starch-rich feed or grains from the diet
- Provide fibre-rich feeds such as beet pulp or psyllium
- Limit vegetable oil supplementation
- Consider supplementation of probiotics and yeast
- Feed many small meals throughout the day
- Ensure adequate protein intake
Chronic diarrhea can lead to hypoproteinemia – low levels of protein in the blood. This is a consequence of protein loss from the blood into the intestines when they are inflammed.
Horses with chronic diarrhea should have their diet reviewed by an equine nutritionist to address potential nutrient deficiencies and weight loss.
Stress is a major contributing factor to digestive dysfunction in horses. Try to minimize stress where possible, make changes to routine gradually, and consider reducing high-intensity exercise or travel.
A veterinarian may recommend intestinal absorbents or anti-diarrhea compounds to manage diarrhea. These can include activated charcoal, bismuth subnitrate, and kaolin.
Consider adding a gut health supplement to your horse’s feeding plan. Mad Barn’s Optimum Digestive Health is a complete digestion and immune supplement for horses.
Optimum Digestive Health contains a combination of probiotics, prebiotics, and yeast to support nutrient absorption and stabilize gut flora. This supplement is also a source of toxin binders to prevent harmful substances from being absorbed.
Looking for advice on feeding a horse with diarrhea? You can receive a free consultation from our equine nutritionists by submitting your horse’s diet online.
Is Your Horse's Diet Missing Anything?
Identify gaps in your horse's nutrition program to optimize their well-being.
- Lewis L. Equine Clinical Nutrition: Feeding and Care. Williams & Wilkins, PA, USA. 1995.
- OMAFRA Poisoning of Horses by Plants.. 2021
- Rush B. Rhodococcus equi Pneumonia in Foals. Merck Vet Man. 2014.
- McKenzie H.C. Disorders of Foals. Equine Intern Med. 2018.
- Rolfe R.D. The role of probiotic cultures in the control of gastrointestinal health. J Anim Nutr. 2000.
- Hillyer M. A practical approach to diarrhoea in the adult horse.. Clin Prac. 2004.
- Cipriano-Salazar M. et al. The dietary components and feeding management as options to offset digestive disturbances in horses. J Equine Vet Sci. 2019.
- Goncalves S. et al.Risk factors associated with colic in horses. Vet Res. 2002.
- Geor R.J. et al.How to minimize gastrointestinal disease associated with carbohydrate nutrition in horses. AAEP Proceedings. 2007.
- Magdesian K.G. Nutrition for critical gastrointestinal illness: feeding horses with diarrhea or colic.Vet Clin North Am Equine Pract. 2003.
- Baverud V. Clostridium difficile diarrhea: infection control in horses. Vet Clin North Am Equine Pract. 2004.
- Garber A. et al. Factors influencing equine gut microbiota: Current knowledge. J Equine Vet Sci. 2020.
- Pagan J.D. et al. Exercise affects digestibility and rate of passage of all-forage and mixed diets in Thoroughbred horses. J Nutr. 1998.
- Anderson M.S. et al.Risk factors for colic in horses after general anaesthesia for MRI or nonabdominal surgery: absence of evidence of effect from perianaesthetic morphine. Equine Vet J. 2010.
- Lindroth K. Free faecal liquid in horses. Faculty of Vet Med and Anim Sci. 2020.
- Kalck K.A. Inflammatory bowel disease in horses. Vet Clin North Am Equine Pract. 2009.
- Mair T. et al.Manual of Equine Gastroenterology.Saunders Ltd. 2002.
- Baverud V. et al.Clostridium difficile: Prevalence in horses and environment, and antimicrobial susceptibility. Equine Vet J. 2010.
- Williams C.A.Specialized dietary supplements. Equine Appl Clin Nutr. 2013.
- Allaart J.G. et al. Effect of lactobacillus fermentum on Beta2 Toxin production by Clostridium perfringens. Appl Envrion Microbiol. 2011.
- Malinowski, Karyn. Stress Management for Equine Athletes. Rutgers Equine Science Center. 2004.
- Stewart, A. Foal diarrhea: causes, diagnosis and treatment (Proceedings). DVM360. August 1, 2011.
- Freeman, DE. et al. Comparison of the effects of intragastric infusions of equal volumes of water, dioctyl sodium sulfosuccinate, and magnesium sulfate on fecal composition and output in clinically normal horses. Am J Vet Res. 1992.