Diarrhea is a common problem in horses as it can be a result of many different causes. [1] It is typically a sign of of disrupted function or motility of the hindgut, including dysbiosis or infection.

Diarrhea is defined as the excessive and frequent defecation of loose or liquid stool. It can rapidly result in imbalanced electrolyte levels due to extensive sections of the hindgut that have impaired absorption and/or increased secretion of water into the gut.

Equine diarrhea is a symptom and not a disease in and of itself. It is a sign that something is wrong. However, the cause of diarrhea goes undiagnosed in up to 50% of cases.

Defining Diarrhea in Horses

Horses have a unique and complex digestive system. Diarrhea in adult horses most commonly results from dysfunction in the hindgut rather than the stomach or small intestines. In comparison, foal diarrhea can arise from stomach or small intestinal disorders.

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.

Acute VS Chronic Diarrhea

Diarrhea can be classified as acute or chronic. Acute diarrhea occurs suddenly, it may be transient and resolve itself or become prolonged and chronic.

Acute diarrhea

Acute diarrhea lasts less than two weeks and is often resolved independently. Often, diarrhea is transient and resolves itself within a few days or even a single bowel movement. An example of transient or isolated diarrhea is horses who develop diarrhea when shipped or away from home at competitions. This is caused by epinephrine (adrenalin) accelerating gut transit time.

Acute diarrhea may not require medical attention if it resolves in a day or two, although you may need to adjust their diet to ensure proper hydration and to prevent nutrient loss. [17]

Although it often resolves itself, there is potential for rapid dehydration. In some cases, diarrhea can be associated with endotoxemia (toxins in the blood) or laminitis.

Inform your veterinarian if your horse develops diarrhea, especially if the horse experiences:

  • Projectile liquid diarrhea
  • Loss of appetite
  • Elevated pulse (heart rate)
  • Elevated temperature
  • Signs of abdominal pain
  • Discolored mucus membranes such as the gums

Acute diarrhea is more likely to be infectious and require medical attention. [25]

Chronic diarrhea

Persistent diarrhea lasts two to four weeks. Chronic diarrhea is present for more than four weeks. [24] This type of diarrhea often requires medical attention from a veterinarian. Hydration, protein, and salt should be increased for horses with chronic diarrhea. [17]

In prolonged cases of diarrhea, the root cause is usually treatable. However, untreated diarrhea with excessive water loss can be accompanied by serious complications such as colic, dehydration, loss of condition, hypoproteinemia (low blood protein), edema, and electrolyte disturbances.

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.

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Consequences of Diarrhea

Diarrhea in itself is considered a messy and unpleasant problem for 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 abdomen 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. Foals in particular are at high risk of complications and adult horses with infectious diarrhea are more ill than with other types.

Determining the cause of your horse’s diarrhea episodes can help reduce and prevent incidences. Here we briefly review 25 potential scenarios where your horse could have diarrhea.

25 Reasons Your Horse May Have Diarrhea

Horses can develop diarrhea for a number of different reasons. The most commonly identified 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 fermentation.

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. obesity, ulcers, instability of the microbiome).

2) Osmotic Diarrhea

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

This is called osmotic diarrhea and can occur in adult horses and foals. In foals, it commonly occurs when feeding commercial milk replacers that are not digesting it well. In adult horses, there are anecdotal reports of diarrhea developing from overconsumption of salt (sodium chloride).

Other compounds in the diet can also induce osmotic diarrhea. For example, excess magnesium, particularly in the form of magnesium sulfate (epsom salts) 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. It is likely the sulfate component of this molecule that has this effect. [23]

High sulfate in drinking water is well known to cause diarrhea in other farm animals. There is a documented case of high sulfate and high salinity (salt) water on a farm in Canada causing severe diarrhea and even death in horses. [26]

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 young cuts of grass hays are recommended for horses recovering from serious gastrointestinal issues. Hay cut at a early growth stage is less fibrous than mature hay. This makes it more easily digestible and less likely to cause irritation through the digestive tract. [10]

High-quality