Diarrhea in horses is recognized as liquid feces, man-ure-crusted tails, splattered stall sides and cow patty-type manure piles rather than well-formed fecal balls.
While the signs of diarrhea are universal, the causes can be varied.
Diarrhea is considered chronic if it has been ongoing for more than two weeks.
The colon absorbs 90 litres of water a day, so you can imagine that any disease process that decreases fluid absorption leads to the production of copious liquid feces.
Diagnosing the cause of chronic diarrhea can be challenging, with only 30 percent of cases successfully diagnosed in living horses.
Read Also
The Western Producer Livestock Report – July 31, 2025
Western Producer Livestock Report for July 31, 2025. See U.S. & Canadian hog prices, Canadian bison & lamb market data and sales insights.
Tests to identify the source of chronic diarrhea include routine blood work, fecal parasite counts, fecal bacterial culture, abdominal ultrasound, rectal palpation and intestinal biopsy.
Autopsies are more successful at identifying the cause but have obvious negative implications for the individual horse.
Salmonella is a frequent cause of diarrhea in horses. It can be challenging to diagnose because the bacteria is difficult to culture in the laboratory and shedding in the feces can be intermittent. Repeated sampling with culture may be required.
In rare cases, other bacteria are associated with chronic diarrhea.
Small strongyles, also known as cyathostomes, embed in the intestinal wall during their larval development.
These larval worms can stop their development for months at time and suddenly emerge in large numbers when the weather is warm, resulting in severe damage to the intestine, diarrhea and weight loss. The encysted larvae are resistant to most of the typical deworming regimes. Unlike cattle, coccidia are rarely implicated.
Horses of any age can get several types of inflammatory bowel diseases. They are not associated with infectious agents and are poorly understood.
These inflammatory diseases can affect absorption of essential nutrients, which is also a contributing factor in weight loss and poor overall performance.
Malabsorption is diagnosed using sugar absorption tests.
Sugar water is delivered to the stomach with a tube, and the amount of sugar in the blood is measured a few hours later. The intestines are not able to properly absorb sugar and other nutrients if blood levels remain low. Response to treatment is poor.
Long-term ingestion of sand has been implicated in chronic diarrhea, but this is less of a concern in Canada. Feeding up off the ground can minimize sand intake. Liver disease, cancer of the intestines and stomach ulceration are less frequent causes of chronic diarrhea.
Older horses are not at increased risk of developing chronic diarrhea. It can lead to weight loss at any age, even with increased appetite.
Loss of protein from the intestines is another contributor to weight loss, but weight loss can be caused by a number of different diseases occurring elsewhere in the body.
Chronic diarrhea can be deadly; 35 to 65 percent of affected horses will die of the condition. The cause of death or euthanasia is often from weight loss.
Treatment is often unsuccessful, depending on the cause of the diarrhea.
Treatments specific to a particular bacterial or parasitic cause can be administered if they are identified. Owners usually try antibiotics, supportive fluids and mineral supplements.
Work is underway to evaluate the entire bacterial population in guts. It appears likely that the persistence of chronic diarrhea in many species may be related to disruption of the normal bacteria, no matter the initiating cause.
Restoration of the normal bacteria through the use of fecal bacterial transplants has been highly successful in treating chronic diarrhea in people infected with bacteria clostridium difficile.
Perhaps fecal transplants will become a common treatment for chronic diarrhea in horses.
People should observe proper hand washing and other hygienic measures when handling affected horses and their wastes.
Many of the bacterial causes of chronic diarrhea in horses can also cause illness in people.
