It is a sensation you hope to experience only within the comfort of your own home: your stomach begins to churn, you scramble to the closest bathroom and experience a bout of diarrhea. You drink some anti-diarrhea medicine and feel confident you will soon recover.

When it comes to horses though, simply waiting for signs of diarrhea to pass can spell disaster. In fact, the massive loss of fluids, electrolytes and protein that often result from an episode of equine diarrhea can rapidly escalate into a costly medical emergency.

“When signs of diarrhea are present, phoning your veterinarian as quickly as possible is always a good rule of thumb,” explained Dr. Ashley Whitehead, DVM, BSc, DVSc, DACVIM and assistant professor, Equine Clinical Sciences at the University of Calgary’s Faculty of Veterinary Medicine.

“Colitis [inflammation of the colon] is a very serious condition and can sometimes be fatal. Treatment needs to be started as soon as possible, especially since diagnostic tests take time to process.”

Equine Digestive System

The digestive tract makes up a large portion of a horse’s anatomy. In fact, food travels roughly the length of a hockey rink’s blue line before exiting through the rectum.

After passing through the stomach, ingesta (eaten food) quickly travels through the small intestine where enzymes help digest and absorb sugar, starch, protein and fat. It is also here that fat-soluble vitamins, calcium and phosphorus are absorbed into the body.

Millions of tiny villi, finger-like formations, line the intestines and increase the surface area. These cells also secrete enzymes and absorb nutrients and water.

Digesta (as food is referred to at this point in the digestion process) then enters the hindgut, a large fermentation vat that makes up the majority of a horse’s digestive system.

This is where proteins, carbohydrates and cellulose are turned into nutrients and where a perfect balance of fluid absorption and secretion takes place, thereby maintaining hydration.

When pathogens make their way into the digestive tract, they can cause inflammation and partial destruction of the all-important villi system, which means nutrients, water and electrolytes can no longer be absorbed by the body and are suddenly lost through the feces.

“When the absorption of water is compromised, the patient can lose as much as 100 litres of fluids per day,” said Dr. Whitehead.

“Correcting and then maintaining fluids in a dehydrated horse can escalate costs very rapidly,” she explained. “Although this treatment can add up to a couple of thousand dollars, one has to realize that without proper hydration levels and supportive care, a horse with colitis faces the possibility of severe electrolyte imbalances, rapid weight loss, laminitis and kidney failure.”

Early Warning Signs

As with many disruptions to equine health, a loss of appetite and depression are two of the earliest warning signs. These are frequently followed by fever and lethargy.

“One of the key signs is an increase in the quantity and frequency of watery manure,” said Dr. Whitehead. “As soon as there is a noticeable change in a horse’s feces, it is important to immediately to consult your veterinarian. Although episodes of diarrhea occur frequently in the human body, it is a rare condition in horses and is almost always a cause for concern.”

The warning signs of dehydration are another good reason to reach for the phone. “Sometimes, colitis is present without obviously watery manure,” noted Dr. Whitehead. “Assessing a horse in the early stages of dehydration can greatly reduce the severity of the disease.”

Specifically, Dr. Whitehead recommends ensuring mucous membranes such as the horse’s gums are moist and slippery. “Tacky and squeaky gums denote a lack of moisture, which is often a red flag when it comes to dehydration,” she said.

“I also like to pinch the skin on the eyelid to see how quickly it bounces back. If it is slow to return to normal and remains in a tented position for more than two seconds, it is usually a good indicator of dehydration.

“Also, due to poor perfusion of blood during sickness, a horse’s extremities such as the limbs and ears become cold and this is another important sign that should be reported to your veterinarian immediately.”

Common Causes

Diarrhea can be broken down into either infectious or non-infectious categories.

The most common infectious agents in horses include Salmonella, Clostridium, Neorickettsia risticii (Potomac Horse Fever), Equine Coronavirus and parasites such as small strongyles.

Non-infectious diarrhea has its own sets of causes, including the use of antibiotics, which may disrupt normal bacteria in the gastrointestinal system and non-steroidal anti-inflammatory drugs, which can cause inflammation and ulceration of the large colon. In addition, some plants, such as buttercups or Jimsonweed, may contain toxins or products that irritate the lining of the gut, leading to diarrhea. In a nervous horse, simple anxiety can also change intestinal activity and lead to watery feces. Horses that consume excessive sand or dirt with their feed may also develop colitis from the irritation and inflammation caused by the abrasive properties of both substances.

Diagnostic Tools

Blood tests, fecal samples and a thorough physical exam are the primary tools used to arrive at a diagnosis.

A specialized test called a polymerase chain reaction (PCR) is also used to detect the presence of pathogenic (harmful) fecal organisms or their toxins’ genes such as from Salmonella, Clostridium, Neorickettsia risticii and Equine Coronavirus.

Ultrasounds are also sometimes used to measure the thickness of the intestinal walls, which is an indicator of the level of inflammation.

According to the American College of Veterinary Internal Medicine, despite the current variety of diagnostic tools, a cause for diarrhea cannot be established in approximately 50 per cent of cases.

“Undifferentiated diarrhea [with an unidentified cause], is very frustrating for both veterinarians and owners,” said Dr. Luis Arroyo, DVM, DVSc, PhD, DACVIM, associate professor at Guelph University’s Ontario Veterinary College.

“Current tests are effective, but what I see as the problem is that we are very focused on the bug, the microbe without factoring in the entire community of micro-organisms that are present,” he said.

“Currently, once a fecal sample is sent to the lab, it is grown in a very selective, very rich media,” explained Dr. Arroyo. “Invariably, we ask for tests that are going to look for known bacteria that grow in those conditions. But perhaps the bacteria that is actually causing the diarrhea is not going to favoured in those in vitro conditions and it will be missed.”

Although in its infancy, Dr. Arroyo is working on a research idea that could one day revolutionize the way tests are conducted. “My objective is to identify the characteristics that exist in an entire healthy gut microbial ecosystem,” he said. “At that point, we could potentially look at the cause of disease from the perspective that the entire community is being turned upside down rather than simply trying to identify one specific pathogen.”


“The most important part of treatment is to support the horse’s body and help restore its hydration, electrolytes and proteins,” explained Dr. Whitehead.

Intravenous fluid therapy helps restore fluids as well as electrolytes, but the extensive amounts often required means this is best performed in a clinic or hospital.

Blood protein levels decrease in horses with colitis, as they often stop eating. The inflammation caused by colitis will also decrease the intestines’ ability to absorb proteins from the feed and, in some cases, protein may leak out of intestinal tissues and into the feces.

“If a horse cannot maintain or replenish their blood protein levels by eating a high-quality feed, they may require a plasma transfusion,” said Dr. Whitehead. “This treatment may also help correct dehydration and restore other elements such as antibodies, immune chemicals and clotting factors.

“One indicator of protein loss during an episode of colitis is the presence of edema [swelling] in tissues, especially in the limbs and under the belly.”

Since antibiotics can cause colitis through alteration of the good intestinal bacteria, their use is reserved for select cases such as Potomac Horse Fever.

“It is a delicate balance,” explained Dr. Whitehead. “Killing good bacteria can invite pathogens, which is why we must be very selective when choosing antibiotic use.”

Anti-inflammatory and pain medication as well as human-grade Pepto-Bismol® may also be administered while treating colitis.

Innovative treatments such as fecal transplantation, feeding bits of manure from a healthy horse on the farm, can also be quite helpful once a horse has been stabilized but continues to suffer from diarrhea.

“This method reintroduces good bacteria that the horse’s digestive system is familiar with,” said Dr. Whitehead. “There is also smectite clay [an absorbent clay mineral product] that is useful in preventing some toxins from causing damage in the intestines.”


“We have a good rate of success if we have the opportunity to see a horse in the early stages,” said Dr. Arroyo. “Horses that improve quickly, within three to five days, usually make a full recovery. If the case goes beyond that, the likelihood of recovery drops dramatically.

“Laminitis is a high risk in colitis cases as well as thrombophlebitis [inflammation of the veins]. The longer a horse has diarrhea and is being treated with a catheter, the higher the risk.”


Although simple, one of the best preventative measures when it comes to diarrhea is barn cleanliness and instilling good biosecurity measures. “Keeping water buckets clean and removing manure from paddocks and pastures is an effective way to control the spread of pathogens and parasites,” said Dr. Whitehead.

Dr. Alison Moore, DVM, DVSc, DACVIM, DACVSMR, lead veterinarian, Animal Health and Welfare with the Ontario Ministry of Agriculture, Food and Rural Affairs added that it is also important to segregate new arrivals for a two-week period. “When you have new horses coming onto the property, it is important to keep them separate and ensure you are monitoring their health,” said Dr. Moore. “Diarrhea can also be caused by a heavy parasite load, which is another reason to segregate new horses.

“Horses that come on the site with a high parasite load will contaminate the pasture and if they come in with drug-resistant parasites, they will deposit those in the field which is a potential problem and, if in a high enough load, may cause diarrhea in susceptible individuals.”

Dr. Moore recommends performing a fecal egg count upon arrival and then another in two weeks’ time to assess resistance and whether the new horse is a high shedder of parasite eggs.

Another preventative measure is to monitor a horse’s potential exposure to Salmonella from other species found in farm areas. “Salmonella can be transmitted from chickens, ducks [all poultry] and other barn birds as well as rodents,” added Dr. Moore. “Owners should keep mice, all rodents, in fact, out of feed since it is transmitted by rodent feces, which is then ingested by the horse.

“Birds and poultry also transmit the disease,which means owners should ensure there are no bird nests or roosting areas right above stalls or near drinking and feed tubs.”

Since sand can also lead to either colitis or colic, feeding hay off the ground is another simple yet effective preventative measure. “The presence of sand in a horse’s environment is very easy to test for,” added Dr. Moore. Placing manure in a jar or sealable bag, adding water and then shaking the contents are easy steps to performing a home test. Once the mixture settles overnight, sand will be visible in the bottom if present.

To protect against Potomac Horse Fever, assessing barn environments is key. “What many people forget to consider is snails can harbour the parasite causing PHF and these can sometimes be attracted by wet areas in barns,” Dr. said Moore. “If, for example, you have a leaky barn and have wetness under your stall mats, you can get snails. Wash-stalls are another area to monitor.”

Although vaccines are available to protect against the disease, “there is varied data regarding the effectiveness of the vaccines,” added Dr. Whitehead. “As a result, vaccinating against it is only recommended where the disease is more prominent.”


Equine diarrhea may be a relatively rare condition, but it also has the unfortunate distinction of being potentially deadly. As such, a “better safe than sorry” approach is always a good game plan.

Maintaining updated records of your horse’s normal vital signs is also helpful. “As with every health crisis, the more information we as veterinarians can compile in the face of a potentially serious disease, the better,” concluded Dr. Whitehead.

For more information on biosecurity practices and to help prevent diarrhea, visit and the Equine Guelph Biosecurity Calculator at


One of the most common infectious diseases requiring veterinary care in young foals is diarrhea.

Currently, no regional data exists to identify the bacteria, viruses and parasites affecting the health of foals in Western Canada.

“It is necessary to identify regionally important pathogens so that veterinarians can direct treatment and prevention strategies,” explained Dr. Ashley Whitehead, DVM, BSc, DVSc, DACVIM, assistant professor, Equine Clinical Sciences at the University of Calgary’s Faculty of Veterinary Medicine.

What is known is that Necrotizing Enterocolitis, a condition causing severe destruction of the small intestines and colon in foals, appears to be common in Alberta.

Over the next two years, Dr. Whitehead will be conducting a study to evaluate various farm-based risk factors for infectious foal diarrhea and hopes to isolate infectious agents in mare/foal pairs as well as in their environment.

Fecal and blood samples from diarrheic foals and their mares throughout Western Canada are being requested. For more information please contact Dr. Ashley Whitehead at