Diarrhea in Horses

Problems with diarrhea may be acute or chronic, minor or severe — but they should never be ignored. At the very least, diarrhea is robbing your horse of fluid and electrolytes, putting him at higher risk of dehydration and electrolyte abnormalities. Other consequences include difficulty maintaining weight, low-grade abdominal pain that may interfere with performance or appetite, higher risk of other intestinal problems, compromised intestinal immunity, and possible development of feed ingredient sensitivities.


A wide variety of things can cause diarrhea in your horse. To make matters worse, once a gut upset is severe enough to cause diarrhea to be established, it can quickly have secondary effects that worsen and perpetuate the diarrhea.

Feed Changes and Overfeeding: One of the most common causes of diarrhea is a change in the diet. Just about everyone knows that changes in the type or amount of grain/concentrate fed should be made slowly, but few give much thought to the effects of a new hay or introducing pasture. Hay and grass are primarily processed in the large intestine, where the population of organisms can ferment them to end products called volatile fatty acids (VFAs), which the horse can then absorb and use directly or convert to glucose or fats.

The processing of the complex carbohydrates and fiber in plant foods is a cooperative affair, requiring that many different species of organisms work together to process them in steps. A sudden change in diet can mean that the populations of organisms required to efficiently process a hay or grass with a different composition is inadequate. Diarrhea results.

Food Sensitivity/Allergy: How often sensitivities or allergies to particular feed ingredients or hay types may play a role in diarrhea is not well understood. Some common ingredients, like soy, are known to be prime offenders in other species.

Sensitivities may have a genetic basis or may occur when foals have access to complex food proteins at too early an age. They could also develop a break in the integrity of their gut lining for any reason (ulcerations, parasitism, inflammation). When that happens, complex proteins or carbohydrates in foods come in contact with the immune system of the bowel and can trigger an antibody reaction.

Poor-Quality Diet: Feeds where the fat has gone rancid, or those that have mold growth or bacterial overgrowth can cause diarrhea. In addition, feeds that use generic terms such as ”grain products” in their ingredients list may contain different ingredients in different batches, which amounts to a rapid feed change even if you’re using the same feed brand all the time. For details, see our articles on feed quality in our July and August 2007 issues.

A variety of toxin-producing molds may also infest hays or pastures. These types of problems are more likely to occur in hot, humid weather, but buying feeds beyond their expiration date or feeding improperly cured hays can cause problems year-round.

Drugs: The most common offenders in this category are antibiotics and dewormers. Both oral and intravenous or intramuscular antibiotics may cause enough disruption of the bacterial organisms in the gut to result in diarrhea. Caution should be used with oral antibiotics in adult horses. As a rule, only trimethoprim/sulfa, doxycycline and EPM medications are used orally, and even these aren’t completely safe.

The die-off of beneficial organisms caused by these medications is enough in itself to cause some diarrhea, but an even greater danger is the overgrowth of life-threatening pathogens like Salmonella or Clostridia.

Dewormers don’t directly cause diarrhea, but the die-off of large number of parasites can cause an immune/inflammatory reaction that does.

Phenylbutazone, or ”bute,” can also cause ulceration of the dorsal colon that leads to loss of serum protein into the bowel and often diarrhea as well.

Stress/Exercise: The release of hormones normally triggered by excitement or fear naturally increases how rapidly food flows through the gut (see ”Mechanism of Diarrhea,” page 17). How severe it is depends on how excited the horse is and how healthy and stable his intestinal tract was at baseline.

For the most part, however, once the stressful situation is over, or the horse has become accustomed to it, the diarrhea disappears. Heavy exercise, especially in the heat, may also cause diarrhea as a result of the core body temperature elevating to the point that there is die off of intestinal organisms.

Sand: Diarrhea is a common symptom of sand accumulations in the gut, caused by mechanical irritation to the intestinal lining.

Parasites: Parasitism severe enough to cause diarrhea in an adult horse is rare in this day of effective paste dewormers, however, it is not impossible. Mass emergence of encysted small strongyle larvae is the most common parasitic cause, but unsuspected tapeworm burdens may also be involved.

Altered gut flora: Both very young and very old horses may be prone to diarrhea because of low numbers of organisms in the hindgut, making them more sensitive to diet. Diarrhea from any cause also results in some degree of ”wash out” of organisms from the colon and/or changes in the chemistry of the colon that cause beneficial flora to die off. This can set the stage for chronic diarrhea.

Bacterial: Salmonella, Clostridia and Ehrlichia risticii (Potomac horse fever) are the most common bacterial causes of diarrhea in adult horses. All of these are serious infections that can lead to rapid dehydration, laminitis, even death. Diarrhea caused by antibiotics or anti-EPM drugs may be complicated by overgrowth of these bacteria.

Inflammatory/Cancer: Horses may develop bowel inflammation similar to human Crohn’s disease, an autoimmune condition. Lymphosarcoma, a form of cancer, may also infiltrate the bowel. Both of these causes of diarrhea are relatively rare.


The diagnosis of diarrhea is easy, but determining the exact cause is not. Horses with the sudden onset of profuse, watery diarrhea can dehydrate rapidly and always warrant a call to the vet. You should check the horse’s temperature and the color of the gums since these cases are often bacterial and can become septic or toxic. Do frequent checks of the hoof-wall temperature (see ”Heat Detectors” in August 2007). Icy-cold feet may be in the developmental stages of laminitis, while hot feet and reluctance to walk indicate it has already developed.

Less dramatic changes, ranging from ”cow plop” to mostly formed but softer-than-normal manure, have multiple possible causes. The more serious causes include phenylbutazone-induced colitis, inflammatory bowel disease or cancer, and these will also be associated with progressive weight loss and often edema of the legs or belly.

Prevention and Treatment

Good management and common sense can prevent many of the common causes of diarrhea. Work with your vet to develop a deworming and fecal check schedule that is appropriate for your situation. Avoid sudden changes in grain or hay and introduce pasture access slowly. If the horse is prone to intermittent diarrhea, be sure you are using a fixed formula feed that specifically identifies all ingredients by name rather than generic categories. This will enable you to experiment with different formulations to find one that is well-tolerated.

Our products chart lists common nonprescription diarrhea treatments in several categories. Ration Plus and Forco are prebiotics. For details of studies on Ration Plus, see http://www.rationplus.com/studies.html. Equine Generator is a bacterial probiotic; YeaSacc a Saccharomyces yeast. These products are best used to:

??? Help prevent or treat antibiotic related diarrhea

??? Support intestinal organisms during feed changes

???Help minimize stress and excitement-related diarrhea severity

??? Restore/replace upset gut flora following other types of diarrhea and support populations in older horses.

YeaSacc is particularly helpful for horses on high levels of grain feeding or with grain overload, when the bacterial products should not be used. They won’t help with diarrhea related to toxins, sand, parasites, inflammatory bowel disease or cancer. They may help re-establish good populations after a bacterial diarrhea.

The next category is adsorbent/absorbents. UAA gel is an old stand-by. It is a combination of activated charcoal and absorbent clays. It is active against both fungal/bacterial toxins and irritant chemicals. Bio-Sponge and DiaGel target fungal and bacterial toxins. They’re usually used as part of the treatment for acute diarrheas, but some horses with longstanding soft manure will also respond. The reason for this is unclear but may be related to low-grade, smoldering bacterial infections which these products could neutralize long enough to allow the beneficial bacteria to re-establish control. Although this hasn’t been studied in horses, DiaGel helped reduce the likelihood and severity of diarrhea in sled dogs during a race.

The last two products target diarrheas associated with damage or injury to the horse’s intestinal lining. They are primarily indicated for inflammatory bowel disease, phenylbutazone-related colitis, suspected severe food sensitivity/allergy (after the cause has been removed, of course) and the recovery stages from bacterial infections.

Last but not least we have psyllium husk fiber. Most people think of psyllium, which is found in the human product Metamucil, as a laxative, but intestinal regulator would be more accurate. Even in humans it works for both constipation and diarrhea. Adding psyllium helps resolve many types of diarrhea.

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