What four words from a client make my heart race and my palms sweat? “My horse has diarrhea.” Not what you were expecting? You might think I’d be more concerned by such statements as, “My horse is down, and he can’t get up,” or, “He can’t bear any weight on his leg.” But in reality, it’s diarrhea that gets my full attention almost every time.

The reason is simple. If diarrhea is bad enough to make a client call the veterinarian, chances are, it either means there’s a really sick horse in a barn full of others at risk of getting sick, or it’s a chronic problem that will be almost impossible to diagnose and even more difficult to treat. Either way, there’s a good chance I’ll spend sleepless nights trying to figure out how to help.

Diarrhea is hard to diagnose and frustrating to treat. In a worst-case scenario, it can even be a life-threatening emergency that requires aggressive, immediate treatment. Even if it’s just a soggy mess, solving your horse’s diarrhea problem will require persistence and a heavy dose of patience.

Here, I’ll explain what diarrhea is and how it happens. Next, I’ll outline the factors that can help you determine whether the diarrhea is a serious health threat or simply an annoyance. Finally, I’ll give you the steps to take to protect your horse (and the other horses in your herd) when diarrhea strikes.

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What It Is

Diarrhea is an intestinal disorder that causes frequent fluid manure evacuations. In other words, your horse poops a lot—and that poop is juicy, wet, and messy. Your afflicted horse might leave “cow pie” piles, might pass what appears to be a normal poop followed by a spray of water or, in the worst cases, might actually “spray the walls” with liquid.

But why does diarrhea happen? Normal equine digestion begins the moment your horse takes food in his mouth. He chews to break his meal into small, digestible pieces and mixes feed with saliva that helps to buffer stomach acids.

After swallowing, feed is passed to the stomach, where it’s exposed to acids, liquefied, and passed on to the small intestine where the first real absorption of nutrients begins. If the digestive tract is functioning properly, simple sugars, amino acids (components of protein), and some vitamins and minerals will all be absorbed during the one to three hours feed spends in the small intestine before it passes to the large intestine. The large intestine acts as a kind of fermentation vat, dependent on a healthy population of normal microorganisms (called the microbiota) that help break down large, fibrous feed materials.

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Fermentation in the large intestine produces volatile fatty acids—an all-important source of energy for your horse—as well as vitamins and proteins. The large intestine also plays a critical role in storing and absorbing water from ingested feed. In fact, the large intestine is responsible for absorbing as much as 30 gallons of water every day. It’s a disruption of this function that leads to diarrhea.

If the large intestine isn’t functioning properly, its ability to absorb water will be compromised—meaning all of that excess liquid will be passed out in the manure. The large intestine can be compromised in a variety of ways. For example, excessive ingestion of carbohydrates changes the pH (a measure of acidity), which then upsets the microbiota balance. Ingesting sand can cause physical damage to the cells lining the intestine.

Diarrhea is a symptom, not an actual disease. Diseases that list diarrhea as a symptom range from mild gastrointestinal disruptions to severe, potentially life-threatening bacterial or viral infections. In some situations, such as a senior horse with an aging gastrointestinal tract, mild, chronic diarrhea can be completely normal. The most important question becomes, “How do you tell the difference?”

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Five Key Questions

Your vet’s first priority will be to determine whether your horse’s diarrhea is “potentially deadly” or just a “messy frustration.” The following five questions will help your vet decide.

1. Does he have a fever? A fever (a temperature higher than 101.5 Fahrenheit) can show that your horse is experiencing an infection from either a bacteria or virus. Be aware that his temperature can fluctuate during the day, and that it’s likely to be elevated after exercise. Take your horse’s temperature after he’s been resting quietly for at least 90 minutes. If he has a fever associated with his diarrhea, your vet will be concerned about a more serious underlying cause.

2. Is he exhibiting signs of discomfort? Is your horse pawing, looking at his flanks, or exhibiting other colic symptoms? If he develops diarrhea while acting colicky, he might be experiencing pain due to inflammation in his colon or large intestine (colitis). This type of discomfort is another sign that his diarrhea is likely to have a more serious underlying cause and might require aggressive treatment.

3. How long has it been present? Has your horse had diarrhea on and off for weeks, months, or even years? Or did the diarrhea appear suddenly? Acute onset diarrhea is usually much more concerning, even moreso if it’s accompanied by other signs, such as a fever or discomfort. Chronic, low-grade diarrhea is much more likely to fit into the “annoying mess” category, especially if your horse appears to be feeling fine.

4. Has he been exposed? It may seem obvious, but if your horse develops diarrhea after he’s been exposed to another horse with diarrhea, the risk of an infectious cause increases. And if you have the misfortune of boarding at a facility when a diarrhea outbreak occurs, you can only hope that a vet has already been able to determine an underlying cause, such as an infection or exposure to a toxin. In fact, estimates say that a definitive cause will only be identified in approximately 30 percent of serious colitis cases.

5. What does the lab work show? Your vet is likely to recommend basic lab work, including a complete blood count, to help fully assess your horse’s condition. A low white blood cell count or significant decrease in proteins will increase concerns that there’s a more serious disruption in your horse’s gastrointestinal tract. If an infectious cause is suspected, your vet might order a fecal test in an effort to identify a specific bacteria or virus.

What to Do

If the answers to these questions point toward a serious problem, your vet is likely to sound the alarm, beginning by isolating your horse from others and implementing strict biosecurity measures. Your horse is likely to be hospitalized and treated aggressively. Treatment will include intravenous fluids to prevent potentially life-threatening dehydration. Your vet will try to identify an underlying cause that might guide ongoing treatment.

But what if your horse’s diarrhea is chronic and low-grade, but otherwise he feels fine? In this case, it’s even more unlikely that you’ll be able to discover an underlying cause. Instead, you’ll probably face a list of “can’t hurt, might help” management suggestions, which include the following.

Adjust his diet. Sensitivity to something in the diet is probably the most common underlying cause of diarrhea; it can also be the most difficult to specifically identify and solve. However, reducing your horse’s carbohydrate intake is a good place to start. An excessive amount of carbohydrates can overwhelm the ability of his small intestine to absorb simple sugars. If a too-heavy carbohydrate load reaches his large intestine, it can hurt the bacterial balance that’s essential for proper functioning. Consider eliminating or reducing cereal grains in your horse’s diet, and avoid high-carbohydrate hays.

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Many vets “in the trenches” believe that a large number of horses are specifically sensitive to orchard grass hay and recommend eliminating it from the diet of a horse with diarrhea. If orchard grass isn’t part of your horse’s diet, consider eliminating all “nonessentials” from his daily ration. Strip down his feed to the simplest combination possible, such as one type of hay and a basic vitamin supplement. If his diarrhea improves, try reintroducing one thing at a time to help identify the forage or supplement that might be causing the problem. If you take this route, leave enough time between each change; it might take two to four weeks for you to see improvements.

Manage his medication. Is your horse on any type of regular medication? Specifically, both nonsteroidal anti-inflammatory drugs (or NSAIDs, such as phenylbutazone, flunixin meglumine, or ketoprofen) and antibiotics can contribute to diarrhea. NSAIDs work to control inflammation by blocking prostaglandins (a group of active lipid compounds that have diverse hormone-like effects in animals). NSAIDS can also have a damaging effect on both the stomach and intestines, because some prostaglandins also play a role in protecting the lining of the gastrointestinal tract. Inflammation of the “upper right” portion of the large intestine (right dorsal colitis) is a common complication of long term NSAID use and is a known cause of chronic diarrhea.

Antibiotics can also lead to diarrhea by killing microorganisms that are an important part of normal, healthy functioning of the large intestine. By killing these “good bugs,” “bad bugs” can overgrow, leading to disruption and a lack of normal functioning.

Ask your vet whether you should discontinue any medication that your horse with diarrhea is taking on a regular basis, particularly NSAIDs or an antibiotic.

Control parasites. Parasite larvae burrowed within your horse’s intestinal walls can cause chronic diarrhea. Current recommendations for an effective deworming program include regular monitoring of fecal egg counts and regular, appropriate deworming. Note that if your horse has a negative fecal egg count, he can still be plagued by a heavy parasite burden. Larvae within the intestinal walls that could be contributing to diarrhea don’t lay eggs. But don’t skip fecal egg counts! A high egg count is still a very important indicator of a heavy parasite load, and egg counts remain a critical part of monitoring an effective control program. Review your program with your vet to determine whether intestinal parasites are contributing to your horse’s diarrhea.

Rebalance microbiota. An imbalance in the normal, healthy bacteria that inhabit the large intestine (called microbiota dysbiosis) can be a serious contributor to diarrhea. What about probiotics? In theory, administering a product packed with “good bacteria” should help restore the healthy balance of the gastrointestinal tract, right? In reality, currently available probiotic supplements are more of a “can’t hurt, might help” proposition than a “must do” cure for chronic diarrhea. The research regarding the effectiveness of probiotics in horses is lacking, as is solid knowledge of the ideal formulation of “bugs.” These products don’t require approval to be marketed as a dietary or feed supplement, and aren’t regulated. To be effective, a probiotic must contain adequate amounts of the correct types of organisms, and they must be available in a form that not only survives in the package before you administer it, but also can survive a trip through the gastrointestinal tract and successfully take up residence in the large intestine. That’s a lot to expect. What’s the bottom line? If you want to administer probiotics to your horse with chronic diarrhea, go ahead; these products are generally safe. Just don’t expect a miracle.

What about prebiotics? In contrast to probiotics, prebiotics are supplements designed to support the health of existing gut microbes rather than introducing new ones. This can include everything from psyllium (often used to help clear sand from the intestinal tract) to large carbohydrate molecules called oligosaccharides that provide a source of nutrition and support for a healthy microbiota. Yeast products—most of which contain portions of an organism called Saccharomyces cervisiae—are also considered prebiotics. Studies have shown that yeast products can have wide-ranging, positive effects on the existing microbiota. As with probiotics, there’s limited research about appropriate use of prebiotics in horses. However, most of them “can’t hurt, might help.” Some studies suggest they could benefit a horse with chronic diarrhea problems.

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Protect his tissues. The intestinal tract lining of your afflicted horse is irritated and inflamed. Consider introducing a supplement or medication designed to soothe and protect those irritated tissues. Products include smectite clay (found in Bio-Sponge) and bismuth subsalicylate (found in over-the-counter medications, such as Pepto-Bismol and Kaopectate), as well as sucralfate, a prescription drug. Ask your vet for advice about whether one of these options may help control your horse’s chronic diarrhea, as well as dosing recommendations. 

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