What Are the Causes of Diarrhea in Horses?


With a large intestine of between 70 to 80 feet it's little wonder that the modern-day horse's digestive system is easily disrupted: Grain, medications, nominal pasture grazing, illness and competing all can affect the sensitive balance of the equine digestive tract and lead to diarrhea, or scours. There is always a cause behind the symptom of watery manure in adult horses, so take diarrhea as a sign that something is wrong. The exception to this is in newborn foals; foal heat diarrhea is normal and should not be treated unless signs of distress accompany the condition. Contact your veterinarian if diarrhea persists more than 8 to 12 hours, regardless of cause.

There is always a cause behind the symptom of watery manure in adult horses.

Dietary Disruptions

Many horses are extremely sensitive to changes in their diet. Specific examples are adding or increasing grain, changing the type of grain, changing hay type and even a difference in the quality of hay or pasture grass. These changes can upset the balance of microbes within your horse's colon. Some microbes may die off due to the change, while others may flourish. The colon becomes inflamed, a condition known as colitis. This inflammation can result in acute, or rapid onset, and typically temporary diarrhea. As your horse's system regains a healthy bacteria balance in response to the new food type or quantity, his stools become normal. This is why any food changes should be made gradually. Ideally, you should make changes over a period of two weeks or more to ease the adjustment and minimize negative health issues that can accompany diarrhea, such as dehydration.


  • Never purchase hay from an unknown source. Inspect hay carefully for stickers, weeds and especially mold, which is toxic to horses.

Infectious Disease Agents and Illnesses

Keep your horse's vaccinations up to date to prevent or minimize his risk of catching a serious illness that can lead to diarrhea, such as Potomac horse fever, but remember that vaccine protection is not 100 percent. Bacteria found in soil and water, such as salmonella and clostridium, can cause enteritis, an inflammation of your horse's small intestine, and resulting diarrhea if the bacteria amount consumed is excessive or if your horse's immune system cannot tolerate these bacteria. Certain illnesses and bacteria are more prevalent in specific regions of the country. Your veterinarian is likely aware of what types of bacteria are common in your area.

Enterocolitis is when both your horse's small intestine and colon are inflamed.

If your horse has diarrhea accompanied by other symptoms that point to an illness, such as a fever, your veterinarian likely will take blood and fecal samples to try to pinpoint the exact cause. Since many bacteria can be shed through manure and transmitted to other horses, separate your horse from others during diagnosis and treatment. Wear protective boots and gloves when around a horse with diarrhea, and clean your outwear thoroughly.

A colicking horse can experience diarrhea, particularly sand colic, which occurs when sand accumulates in your horse's colon.


  • Many horses with healthy immune systems can tolerate small amounts of bacteria without issue. Adding probiotics to your horse's diet will help his natural defense mechanisms against deadly bacteria and keep his digestive system healthy.

Antibiotics and Other Medications

Antibiotics disrupt the microbe balance in your horse's system, so adding probiotics if he is on antiobotic therapy can minimize his chances of diarrheal episodes. Use any nonsteroidal anti-inflammatory drugs, or NSAIDs with discretion, as they also can cause diarrhea. Long-term use of these drugs, such as phenylbutazone and flunixin meglumine, can create a life-threatening condition in your horse called right dorsal ulcerative colitis, where both ulcers and inflammation are present in your horse's colon.


  • Phenylbutazone is commonly referred to as bute. Banamine is the common brand name for flunixin meglumine.

Worm Infestation

A worm overload also can cause diarrhea in horses, so keep your horse on a deworming regimen that is suitable for your area. Ask your vet what protocol she advises. Some advise rotating types of dewormers based on what parasites are prevalent during a specific season. Performing a fecal count can help you design a custom deworming program for your horse that doesn't overload his system with unnecessary deworming medication.

Stress and Schedule Changes

Horses can experience stress just like people do. Some horses are unfazed by schedule disruptions, new locations and experiences, while others do better with a more rigid daily schedule and minimal changes. Help your horse transition to changes by exposing him to new situations on a regular basis, such as trailering him to new locations. This is particularly important if you will be attending competitions, which can be especially stressful for many horses. It also will make necessary trips easier on both you and your horse, such as vet clinic visits.

Some horses become stressed if confined to a stall due to injury or illness. Ask your veterinarian if you can walk him by hand one or more times a day.


  • Most horses do better with a living situation that is natural to them, such as large amounts of turnout and pasture grazing.

Foal Heat Diarrhea

Diarrhea in newborn foals 7 to 10 days old is very different from diarrhea in adult horses. Typically, as long as the foal is alert, nursing and otherwise healthy, you don't need to be concerned. The term "foal heat" is used because the diarrhea typically occurs when the mom, or dam, goes into her first heat or estrus cycle after giving birth. This also happens to be the time when the foal's digestive system is undergoing significant changes, which leads to the onset of diarrhea. Keep your foal's behind clean and apply a protective coating, such as petroleum jelly, under the tail to prevent irritation. If you become worried about the amount or length of diarrhea, consult your veterinarian.

No Known Cause

If your horse succumbs to chronic diarrhea00284-5/abstract?cc=y=) that your veterinarian can't definitively diagnose, work with your vet or an equine nutritionist on dietary and lifestyle changes. This might include a reduction or elimination of grain in his diet, allergy tests, more frequent and smaller meal sizes and increased turnout.

Diarrhea Care

Regardless of cause, it's important that a horse with diarrhea consume plenty of water so he doesn't become dehydrated. Adding salt or electrolytes to his diet can encourage him to drink. Your horse's gums should be pink and moist. When you press gently on his gums with your finger, you should briefly see a white spot that immediately turns back to pink. If it remains white, you need to increase his fluid intake. Talk to your vet about adding fiber such as psyllium. Administer any prescribed medications diligently.


  • If the diarrhea is not serious, your vet may approve a dosing regimen of Pepto Bismol.

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