When humans get diarrhea, it’s usually not a big deal. Unpleasant, yes, but most cases resolve in a day or two. Dogs – especially those prone to eating whatever they find – are much the same: a few messy and uncomfortable hours and they are healthy again. (Of course, diarrhea can also be a symptom of something more serious, but most cases are not.) With horses, however, diarrhea – especially when the horse also has a fever – can signal that the animal is suffering from colitis. And that’s a potentially deadly condition.

In a 1991, study at the Ontario Veterinary College, 20 of the 47 horses studied died. Of the 27 that survived the initial illness, three developed chronic laminitis (two of the three were euthanized) and five more developed jugular vein thrombosis.

But that was more than 25 years ago – surely the odds are better now? Yes, they are, depending on the cause of the colitis, but this is still a worrying and potentially fatal disease.

It’s also not uncommon. Dr. Ashely Whitehead, an internal medicine specialist at the University of Calgary Faculty of Veterinary Medicine said: “Colitis is certainly not rare. In the referral clinic here, where I work, we have three cases right now.”

The name colitis describes – for those who understand Latin – exactly what the problem is. The “col” part of the term means the large colon and the “itis” part means inflammation. In other words, horses with colitis have inflamed colons, and that inflammation typically causes diarrhea.

As Dr. Whitehead explained, “In horses, the large colon is very large, and plays an important role in re-absorbing fluid from the intestinal contents the horse has been digesting, among other things. When inflammation affects the lining of the colon, there is a reduction in the fluid absorption – and that fluid ends up heading out the back end of the horse as diarrhea.”

She added that some horses with colitis will lose 100 litres of fluid or more a day, through diarrhea. A normal healthy horse drinks between 25-35 litres each day, meaning that it is almost impossible for the horse to drink enough to replace the lost fluids and intravenous hydration is needed. Otherwise, the horse’s condition can deteriorate very quickly.

And it’s not just fluids that are lost. Once colitis sets in, horses can also lose proteins and electrolytes in the diarrhea. Blood proteins are very important to maintain a normal fluid balance in the body. As proteins are lost in the diarrhea, fluid from within the blood vessels seep out into the surrounding tissues creating edema (swelling). This will compound the dehydration issues and can create a life-threatening situation.


“The list of potential causes is long,” said Dr. Whitehead, “but they all fit into one of two categories: infectious and non-infectious.”

There are a variety of non-infectious causes. Some horses can develop inflammation and diarrhea because they are stressed. Their anxiety speeds up the movement of food through the intestines, so that there isn’t enough time for normal absorption and diarrhea is the result. Usually, once the stress is relieved or the horse adapts to the new situation, everything returns to normal.

Sand ingested along with grass or hay can be another cause. “The sand irritates the wall of the colon, causing irritation of the lining of the gut and swelling, so that the fluids are not absorbed,” explained Dr. Whitehead. Medications, such as antibiotics or anti-inflammatories, can also contribute to diarrheic conditions in horses, as can some toxic plants.

Infectious colitis can be subdivided into bacterial, viral and parasitical. To really understand what is going on when a horse develops an infectious form of colitis, said Dr. Henry Stämpfli of the Ontario Veterinary College in Guelph, you need to know how the horse normally digests food. Dr. Stämpfli, and his co-researchers Dr. Costa Schoster and Dr. Scott Weese, have had a large number of research studies on colitis published in veterinary medicine journals.

“The large colon is the major fermenting chamber of the horse’s gut,” he said. “Bacteria ferment the food and supply energy in the form of short-chained fatty acids. It’s very similar to the rumen in a cow.” In a healthy horse, the mucosa (lining) of the colon and the bacteria, protozoa and other micro-organisms living in the colon all exist in a balanced, symbiotic state.

Added Dr. Stämpfli: “This microbiota is very stable and quite resistant to infections.” However, that balanced state can be disturbed by many factors, and then the more dangerous bacteria are able to take over.

One major disruptor? Antibiotics. Dr. Stämpfli mentioned one study that showed that to infect a healthy mouse so that it was sick with Salmonellosis, you needed to give it about one million Salmonella organisms. If that healthy mouse was first given a dose of streptomycin (an antibiotic), only 10 Salmonella were needed.

“We have shown that antibiotics given intravenously, in the muscle, or orally, will all have effects on the microbiota and some antibiotics have a larger potential for causing dysbiosis – meaning that the balance we talked about is disturbed. The dysbiosis lasted for more than four weeks,” he added. This can happen in humans as well, but horses seem to be more sensitive to this antibiotic side effect than many other mammals.

But other things can cause problems, too. Dr. Stämpfli lists fasting, anesthesia, being transported, and colic as conditions that have been shown to disrupt the healthy microbiota.

One surprising possible risk factor: giving probiotics. “Everyone loves probiotics,” he said. “But I would like to caution this a bit. We published a study in 2016 that showed that foals given a particular probiotic actually suffered from more severe diarrhea than those given a placebo. The problem is that there are very few good studies on the effects of probiotics because probiotics don’t have to go through the rigorous review process that drugs have to go through to be approved.”

Common microorganisms and parasites identified as causes of colitis are:

  • Salmonella spp.
  • Clostridium difficile
  • Clostridium perfringens
  • Potomac Horse Fever – Neorickettsia risticii
  • Equine coronavirus
  • Small strongyles

There are probably others not identified, as between 35 per cent and 50 per cent of the cases of colitis which are tested (through manure samples, blood tests, etc.) to determine the cause yield no result.


A first step when a horse is determined to have infectious colitis is to ensure the animal is isolated, said Dr. Whitehead. “Part of the reason for isolation is to protect the horse from any other diseases that it could catch while in a weakened condition,” she explained. “But even more importantly, it’s to protect other horses, and to protect the humans who are trying to look after it. Many of these infections, such as Salmonella, can be very easily spread to humans or other animals.”

When you are working with a horse that has infectious colitis, you need to take precautions to guard against getting sick yourself. Wear coveralls, boots and rubber gloves when you are cleaning up any diarrhea or when working with the horse. Use separate equipment for the sick horse including tools for mucking out and grooming or feeding supplies. Be sure to dispose of soiled bedding where other animals can’t get into it, and wash all surfaces of the stall, surrounding areas, and equipment with a disinfectant. Frequent handwashing (yes, even though you are wearing gloves) helps to reduce the risk.

Infectious colitis can be challenging – and expensive – to treat. “It is also expensive to diagnose,” said Dr. Whitehead. “A diarrheal panel to identify what is causing the colitis will cost $200 or more, and you may need to do follow up cultures for certain bacteria. Blood work is also very important to monitor organ systems, hydration levels and protein concentrations.” Treatment can be even more costly, averaging $3,000 to $5,000 when intravenous fluids and hospitalization are required, but easily running as high as $15,000 to $20,000 in severe cases with complications or requiring plasma transfusions.

While the care is primarily supportive – managing the symptoms so that the horse’s body can recover – it is intensive, and that’s what makes it so expensive. The horse will likely need intravenous fluids, anti-inflammatories, nutritional support to replace the lost protein and electrolytes, and protection from other potential infections since his immune system is seriously weakened. Depending on the cause and the severity of the condition, the horse may be given some antibiotics such as oxytetracycline for Potomac Horse Fever. Antibiotics are used very cautiously given the link between antibiotic use and colitis.

Many horses with colitis lose their appetite and do not want to eat, but nutritional support is essential; some may have to be given feed by a tube into their stomach or through intravenous administration of special fluids containing amino acids, electrolytes, sugars and lipids.

Fecal transfaunation (aka transplantation) is another treatment that is sometimes used, and additional research is underway to assess its effectiveness. Feces are taken from a healthy horse, turned into a slurry and given to the horse through a tube. The goal is that the health-promoting bacteria from the feces will begin to reproduce in the sick horse’s gut and eventually push out the bacteria causing the colitis, while helping the intestines to heal.

Reducing Colitis Complications

Another goal of the intensive care given at this stage is to reduce the risk of the two most common complications of colitis: laminitis and blood clots (thrombosis).

Laminitis can develop when the inflammation from the colon spreads through the horse’s entire system, explained Dr. Whitehead. The breakdown of the lining of the intestines due to the disease allows toxins and even bacteria to get into the horse’s bloodstream and this causes the horse’s entire body to react. This is known as SIRS or Systemic Inflammatory Response Syndrome. The circulation in the horse’s feet can be affected in SIRS and the inflammation can damage the tissues that normally support the bones inside the hoof. As the coffin bone starts to break away from the hoof wall it rotates and sinks causing severe pain and lameness.

A study by Kullmann and Holcombe et al in 2013 found that treatments to keep the horse’s feet cool (digital cryotherapy or ICE) significantly reduce the risk of developing laminitis. The study reviewed the records of 130 horses with colitis and evidence of systematic inflammatory response. Researchers found that 10 per cent of the horses treated with digital cryotherapy developed laminitis, while 33 per cent of those that were not treated this way developed laminitis. More than half of the horses that developed laminitis were euthanized.

This study also found that horses that developed colitis because of infection with Potomac Horse Fever were at higher risk of laminitis.

The researchers recommended that all four feet of horses with colitis should be treated with cryotherapy from the time they are diagnosed until 24 hours after any signs of SIRS or sepsis (bacterial infection in the blood) have resolved.

Another study compared the effectiveness of seven different approaches to cryotherapy. Methods which immersed the foot in ice water and extended to at least the pastern were found to be the most effective while ice packs placed on the foot or coronary band were least effective.

Horses with diarrhea are also at increased risk of thrombosis, or blood clots, within the blood vessels. These clots can occur secondarily to sepsis and SIRS and due to the loss of anti-clotting proteins through diarrhea. These factors are also compounded by irritation to the blood vessels by intravenous catheters giving the horses necessary fluids or by other physical trauma to blood vessels. There are some medications which may aid in treating blood clots, however, prevention and management of the horse’s inflammation is even more important.

Both of these complications can be very serious and lead to death or the need for euthanasia.

Colitis Prevention Tactics

Given the seriousness of this illness, can it be prevented? Dr. Stämpfli said that a key element is to “feed horses like horses” to maintain optimum gut health, and to use any antibiotics judiciously since antimicrobial use is a major risk factor for some types of colitis.

The importance of the healthy gut is demonstrated by research. A 2016 study found that while the bacterium Clostridium difficile was known to cause colitis, it was also found in the feces of many healthy horses. In one group of horses studied, about 40 per cent of them had the C. difficile in their feces at some point during the year they were followed, and they remained healthy. So, it is not simply the presence of the bacteria in the horse’s system that causes the disease; it’s the imbalance between the “healthy” bacteria and those which can make the horse sick. The researchers found that horses with other health problems, who were hospitalized or given antimicrobials were more likely to develop colitis.

Vaccines are available to help prevent Potomac Horse Fever, but these are bit like the flu shot for humans – they don’t always target the strains that are a problem in your area, and the prevalent strains can change from time to time making the vaccine less effective.

While it’s not a way to actually prevent the disease, horse owners should keep in mind that getting veterinary care as quickly as possible can help reduce some of the complications of colitis and increase the odds of a full recovery.