Diarrhea is a common problem in horses as it can be a result of many different causes. [1] 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. [17]

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. [17]

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: [1]

  • 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.

Dietary Management

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

Diarrhea may be a sign that your horse is being over- or under-fed. [1][6]

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

Imbalances of electrolytes such as magnesium, potassium, sodium, and chloride can also result in diarrhea.

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. [23]

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. [10]

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. [9] 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. [7][9]

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. [6][7]

Diet is the most common cause for diarrhea in horses. Management involves making adjustments to feeding practices and ensuring adequate hydration, electrolyte balance, and in some cases feeding additional protein.



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. [12] This slows the movement of food and liquid through the gut.

Research shows that stall-confined horses also consume less, even when forage or hay is readily available. These factors can result in digestive problems such as ulcers, colic and diarrhea.

9) Travel and Trailering

Travel can be stressful for horses and results in similar physiological effects as stall confinement. [12] 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. [12] 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. [21]

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.

Frequent, intense or long exercise bouts can incr