Diarrhea can affect horses of all ages, breeds and sizes, resulting in dehydration, weight loss, poor nutrient absorption or electrolyte imbalance.
Diarrhea is described as the increased excretion of liquid or semi-solid feces. Cases of equine diarrhea can range in severity from mild episodes to serious and long-term episodes, which may require veterinary attention.
An imbalance of the gut microflora (known as dysbiosis), is a key factor in the development of diarrhea in horses. This is most often caused by dietary factors, although there are many reasons why a horse may develop diarrhea. 
This article describes the common causes of diarrhea, the complications associated with this condition, as well as methods to treat and manage non-infectious diarrhea in horses.
Common Causes of Diarrhea
Non-infectious causes include, but are not limited to:
Infectious causes of diarrhea in horses may include:
- Bacterial infection:
- Clostridium difficile
- Clostridium perfringens
- Potomac horse fever (PHF)
- Viral infection:
- Equine Coronavirus
- Parasitic infection:
- Larval cyathostomiasis
Non-infectious Causes of Diarrhea
There are many proposed non-infectious causes for diarrhea in horses. Ultimately, anything that shifts the microbial population in the horseâ€™s gut can predispose your horse to diarrhea.
Commonly described risk factors which may be influenced by management practice are listed below:
When diarrhea is observed in a horse, diet is often suspected as the most likely cause. 
The physiology of the equine gastrointestinal tract is specialized to the fermentation of fibre in the hindgut, composed of the cecum and colon.
Microbes in the horse’s hindgut ferment fibre in forages to produces volatile fatty acids (VFAs), which serve as a valuable energy source for the horse.
Additionally, the small intestine of the horse is limited in its capacity to digest starch from carbohydrate-rich meals. It contains limited quantities of the enzyme that breaks down starch (alpha-amylase). 
It is clear from the physiology of the equine gastrointestinal tract that horses have evolved to consume a diet high in fibre, and low in starch.
However, equine athletes have very high nutritional demands and increased daily energy requirements. To meet these increased requirements, competition horses are often given large quantities of commercial feeds, with high starch and sugar content.
Meals that contain large volumes of starch-rich feeds will likely overwhelm the small intestineâ€™s ability to digest starch,Â resulting in the delivery of starch to the cecum.
As starch is rapidly fermentable, the cecum will produce increased lactic acid, causing the pH in the cecum to decrease and become more acidic. This can ultimately alter the hindgut microflora. 
The increased acidity in the hindgut leads to a decreased ability for the horse to ferment and digest fibre as well as a decreased utilization and absorption of short-chain fatty acids (SCFAs) for energy.
A low pH in the hindgut may also lead to disruption of the intestinal mucosal barrier and hindgut ulcers.
Abrupt Changes to Diet:
A sudden change in the horseâ€™s diet is a major risk factor for the development of diarrhea. This is a common reason why horses admitted for hospitalization develop diarrhea. 
Changes in the diet may include an increased consumption of grain or increased turnout and grazing time.  Horses evolved to eat a fairly uniform diet and it can take time for the bacterial population in the microbiome to adjust to feed changes.
Fructans, a type of fermentable sugar found in grass may also be responsible for microbial changes in the hindgut. 
Horses very commonly ingest fructans. However, the fructan content in grass changes depending on weather and season.
Whether due to seasonal variation or changes in grazing patterns, a sudden increase in fructan consumption may result in the same adverse outcomes as seen with high consumption of concentrate feed. 
Ingestion of Toxic Plants and/or Substances:
- Castor beans & castor oil
- Raw linseed oil
- Propylene glycol
The administration of antibiotics is a recognized risk factor for the development of acute diarrhea in horses. The association of antibiotic treatment and equine diarrhea is estimated to be 22-28%. 
Every class of antibiotics has been implicated in the development of diarrhea.  There has been no published research on the relative risks of different classes of antibiotics.
Antibiotics disrupt the normal balance between the different bacterial species of the intestinal microflora. This may alter the metabolic function of the bacteria, causing changes in the metabolism of carbohydrates and VFAs.
Ultimately, this may result in the increased secretion and decreased absorption of water in the colon. The resulting dysbiosis may further result in the overgrowth of potential pathogens and inflammation in the colon. Â These pathogens can include salmonella and clostridium.
Infectious Causes of Diarrhea
More research has been published on the infectious causes of equine diarrhea than the dietary causes. Bacterial infections, viruses and parasites can all contribute to diarrhea in horses.
- Abrupt changes in feed
Overall, any changes in microflora, intestinal motility, and VFA production can allow Salmonella to proliferate within the gastrointestinal tract.
Salmonella releases toxins and causes an inflammatory reaction, impairing the intestinal mucosal barrier, and negatively affecting absorption and secretion of water within the colon. 
Salmonella is consider zoonotic and is transmitted through fecal-oral route. Horses may become infected through contact with feces from contaminated horses or feces of other animals that shed Salmonella, such as birds and rodents. Outbreaks appear to be more common in veterinary hospitals. 
Along with diarrhea, horses affected with salmonellosis may present with: 
- Anorexia (lack of appetite)
Clostridia (c. difficile and c. perfringens) are bacteria which may cause diarrhea in horses. They are associated with dysbiosis stemming from antibiotic use in horses.
Another risk factor for the proliferation of clostridial bacteria appears to be the withholding of roughage. This may occur in horses awaiting general anesthesia or in those that are ill and have low appetite. 
The normal production of VFAs appears to be key in the prevention of clostridial overgrowth.
Horses may become exposed to Clostridia bacteria at a veterinary hospital, through the feces of a foal with diarrhea, or through the feces of a horse or foal recently treated with antibiotics. 
C. difficile and C. perfringens can be found in small amounts as part of the normal microflora. However, populations of this bacteria greatly increase in horses with diarrhea. 
Diagnosing clostridia as a cause for equine diarrhea may prove difficult, due to its presence in the microflora of healthy horses.
Potomac Horse Fever (PHF)
Potomac horse fever (PHF) is an infectious disease first reported in 1979, along the Potomac River in Maryland, USA. Â It is caused by the oral ingestion of the bacteria Neorickettsia risticii.
Neorickettsia risticii has been identified in aquatic insects and in the secretions of freshwater snails. 
This disease is not thought to be contagious, and its distribution is dependent on the season and geographic area. Risk factors for the development of PHF include grazing near water, including freshwater rivers, lakes and creeks. 
Complications Associated with Diarrhea
Intestinal inflammation can lead to increased water secretion, and reduce water reabsorption by the colon. This will result in diarrhea, and an increased volume of water lost in feces by the horse.
It has previously been calculated that a 500 kg (1100 lb) horse with diarrhea may have a daily fecal fluid loss that is 36 litres greater than a healthy horse. 
Horses with diarrhea may quickly become dehydrated if fluid intake is not increased to replenish lost fluids.
The hindgut is the primary site of electrolyte absorption. Inflammation in the hindgut results in decreased absorption of electrolytes, increased fecal losses, and possible anorexia or inappetence (loss of appetite). These factors may cumulatively result in electrolyte imbalance. 
Electrolytes have many important functions within the body, such as involvement in nerve and muscle function, influencing thirst and hydration, and maintaining optimal fluid balance within cells.
Diarrhea and intestinal inflammation may inhibit efficient nutrient absorption, which refers to the gastrointestinal uptake of nutrients from food in the diet. 
Additionally, any inappetence or anorexia that may co-exist along with diarrhea may be limiting or further causing imbalances in the horseâ€™s nutrient levels.
Dietary Management of Diarrhea
If your horse presents with diarrhea, consider making changes to its diet to support healthy gut function.
The general dietary strategy described for a horse suffering from diarrhea includes: 
- Limit or avoid commercial feeds high in starch
- Limit or avoid the supplementation of vegetable oils
- Limit pasture grazing
- Provide forage as a source of highly fermentable fibre through a good quality grass or alfalfa hay
- Supplement with probiotics
- Increase water intake
- Feed electrolytes (provided in water)
- Feed additional source of easily fermentable fibre (ie: psyllium or soya hulls)
Horses with diarrhea should be provided with electrolytes in order to offset any potential electrolyte imbalance.
Electrolytes are especially important in horses that are dehydrated. Providing water, without electrolytes, may cause the horse to become even more dehydrated and electrolyte depleted. 
Electrolytes can be provided through oral supplementation or in the form of loose salt. Salt blocks should not be used as they do not encourage enough sodium consumption to replenish electrolyte levels.
It is generally recommended that electrolytes be provided in the drinking water. Providing electrolytes without water may draw water out of the extracellular fluid and into the digestive tract. 
Commercial electrolyte supplements are more effective than salt alone because they contain a full spectrum of electrolyte minerals and not just sodium chloride. Additionally, electrolytes containing sucrose and flavour may encourage horses to drink more when supplemented in their drinking water. 
Mad Barn’s Performance XL: Electrolyte supplement offers a convenient and effective way to re-establish electrolyte balance and prevent dehydration.
Probiotics are live microorganisms supplemented in an animalâ€™s feed. Probiotics are generally used to improve or maintain the community of beneficial bacteria within the gastrointestinal tract.
These bacterial organisms naturally populate the horse’s gut and confer a protective immune effect as well as supporting digestion.
Overall, studies investigating the effectiveness of probiotics for managing diarrhea in horses have had inconsistent results, and are dependent on the underlying disease and the strains of bacteria being investigated.  Some probiotic protocols have been found effective while others have not.
In foals, the provision of five different strains of Lactobacillus bacteria was successful in decreasing the incidence of diarrhea. 
Studies in other animal species have also demonstrated that Lactobacillus bacteria may prevent the intestinal binding of pathogenic bacteria, such as Salmonella.
Overall, supplementing probiotics in horses is considered safe. 
Both Mad Barnâ€™s Visceral+ and Optimum Digestive Health gut health supplements contain numerous strains of Lactobacillus bacteria, in addition to Saccharomyces cerevisiae yeast, and other ingredients that support gut health.
Prevention of Non-Infectious Diarrhea
In order to support optimal gastrointestinal health and function for our horses, stability in the microbial gut population must be maintained.
To best prevent non-infectious causes of diarrhea, the following strategies are recommended.
Avoid abrupt dietary changes
Changes in diet should be gradual. This includes a two-week transition between commercial extruded feeds, by gradually introducing greater quantities of the new feed. Exposure to grazing should also be increased gradually.
Limit quantities of dietary starch fed
Horses fed more than 2 grams of starch per KG of bodyweight in one meal may experience adverse gastrointestinal effects. This is equivalent to 1000 grams of starch in a single meal for a 500 KG (1100 LB) horse.
If your horse requires large amounts of commercial feed and dietary starch to support their high energy requirements, it is recommended to divide their feed into several small meals throughout the day as opposed to one or two large meals.
Support gastrointestinal health
Considering adding supplements designed to support digestive health to your horse’s feeding program. There are many effective gut supplements that can help to maintain normal immune function, strengthen the intestinal mucosal barrier, stabilize the gut microflora and support hindgut health.
Supplements should provide a high CFU count (colony forming units) of at least 5 billion CFU per serving. Many equine supplements provide lower CFU counts and should be avoided as they will likely be ineffective.
Limit or reduce stress whenever possible
Keep feeding and turnout schedule as consistent as possible. Intense training and travel to competitions can increase your horse’s stress.
Changes in social grouping and herd population are another major stressor for horses.
Get your horseâ€™s diet analyzed
A diet evaluation can help you determine if you are overfeeding grain or if there are other ways to support the gastrointestinal health of your horse through their diet.
Submit your horse’s diet for analysis online and our nutritionists can provide a complimentary report.
Diarrhea is often frustrating for both horse owners and veterinarians as the underlying cause may not be obvious and may be difficult to determine.
Dietary management aimed at supporting your horseâ€™s gastrointestinal health and their gut microflora will assist in managing and preventing future cases of non-infectious diarrhea in horses.
Is Your Horse's Diet Missing Anything?
Identify gaps in your horse's nutrition program to optimize their well-being.
- McKinney, C.A. et al. The fecal microbiota of healthy donor horses and geriatric recipients undergoing fecal microbial transplantation for the treatment of diarrhea. PloS one. 2020.
- Cohen, N. and Woods, A. Characteristics and risk factors for failure of horses with acute diarrhea to survive: 122 cases (1990-1996). J Am Vet Med Assoc. 1999.
- Frederick, J. et al. Infectious agents detected in the feces of diarrheic foals: a retrospective study of 233 cases (2003â€“2008). J Vet Intern Med. 2009.
- Durham, A.E. Intestinal disease. In: Equine Applied and Clinical Nutrition. 2013.
- Feary, D.J. and Hassel, D.M. Enteritis and colitis in horses. Vet Clin Equine. 2006.
- Oliver-Espinosa, O. Diagnostics and treatments in chronic diarrhea and weight loss in horses. Vet Clin Equine. 2018.
- Garber, A. et al. Factors influencing equine gut microbiota: Current knowledge. J Equine Vet Sci. 2020.
- Dyer, J. et al. Molecular characterisation of carbohydrate digestion and absorption in equine small intestine. Equine Vet J. 2002.
- Richards, N. et al. Examination of the use of exogenous ?-amylase and amyloglucosidase to enhance starch digestion in the small intestine of the horse. Anim Feed Sci Tech. 2004.
- Shirazi?Beechey, S.P. Molecular insights into dietary induced colic in the horse. Equine Vet J. 2008.
- Clarke, L.L. et al. Feeding and digestive problems in horses: physiologic responses to a concentrated meal. Vet Clin North Am Equine. 1990.
- De Fombelle, A. et al. Feeding and microbial disorders in horses: 1-Effects of an abrupt incorporation of two levels of barley in a hay diet on microbial profile and activities. J Equine Vet Sci. 2001.
- Drogoul, C. et al. Feeding and microbial disorders in horses: 2: Effect of three hay: grain ratios on digesta passage rate and digestibility in ponies. J Equine Vet Sci. 2001.
- Hussein, H.S. et al. Effects of cereal grain supplementation on apparent digestibility of nutrients and concentrations of fermentation end-products in the feces and serum of horses consuming alfalfa cubes. J Anim Sci. 2004.
- Geor, R.J. and Harris, P.A. How to minimize gastrointestinal disease associated with carbohydrate nutrition in horses. In: Proceedings of the Annual Convention of the AAEP. 2007.
- Longland, A.C. and Byrd, B.M. Pasture nonstructural carbohydrates and equine laminitis. J Nutr. 2006.
- Cohen, N. et al. Dietary and other management factors associated with equine colic. J Am Vet Med Assoc. 1999.
- Hoffman, R.M. et al. Hydrolyzable carbohydrates in pasture, hay, and horse feeds: direct assay and seasonal variation. J Anim Sci. 2001.
- Traub-Dargatz, J.L. et al. Epidemiologic study of salmonellae shedding in the feces of horses and potential risk factors for development of the infection in hospitalized horses. Journal Am Vet Med Assoc. 1990.
- Williams, C.A. Specialized dietary supplements. In: Equine Applied and Clinical Nutrition. 2013.
- Barr, B.S. et al. Antimicrobial?associated diarrhoea in three equine referral practices. Equine Vet J. 2013.
- Staempfli, H.R. et al. Use of bacitracin in the prevention and treatment of experimentally-induced idiopathic colitis in horses. Can J Vet Res. 1992.
- McGorum, B.C. and Pirie, R.S. Antimicrobial associated diarrhoea in the horse. Part 1: Overview, pathogenesis and risk factors. Equine Vet Educ. 2009.
- Hird, D.W, et al. Case-control study of risk factors associated with isolation of Salmonella saintpaul in hospitalized horses. Am J Epidemiol. 1984.
- Weese, J.S. A review of equine zoonotic diseases: risks in veterinary medicine. In: Proceedings of the Annual Convention of the AAEP. 2002.
- Owen, R.A. et al. Effects of transportation, surgery, and antibiotic therapy in ponies infected with Salmonella. Am J Vet Res. 1983.
- Kim, L. et al. Factors associated with Salmonella shedding among equine colic patients at a veterinary teaching hospital. J Am Vet Med Assoc. 2001.
- Mair,T.S. et al. Manual of equine gastroenterology. 2002.
- BĂ¤verud, V. et al. Clostridium difficile associated with acute colitis in mature horses treated with antibiotics. Equine Vet J. 1997.
- BĂĄverud, V. et al. Clostridium difficile: prevalence in horses and environment, and antimicrobial susceptibility. Equine Vet J. 2003.
- Weese, J.S. et al. A prospective study of the roles of Clostridium difficile and enterotoxigenic Clostridium perfringens in equine diarrhoea. Equine Vet J. 2001.
- Borriello, S.P. Pathogenesis of Clostridium difficile infection. Journal Antimicrob Chemother. 1998.
- Madigan, J.E. et al. Transmission of Ehrlichia risticii, the agent of Potomac horse fever, using naturally infected aquatic insects and helminth vectors: preliminary report. Equine Vet J. 2000.
- Cymbaluk, N.F. Water. In Equine Applied and Clinical Nutrition. 2013.
- Coenen, M. Macro and trace elements in equine nutrition. In Equine Applied and Clinical Nutrition. 2013.
- Zeyner, A. and Harris, P.A. Vitamins. In: Equine Applied and Clinical Nutrition. 2013.
- Lindinger, M.I. and Ecker, G.L. Gastric emptying, intestinal absorption of electrolytes and exercise performance in electrolyte-supplemented horses. Exp Physiol. 2013.
- Rose, R.J. A physiological approach to fluid and electrolyte therapy in the horse. Equine Vet J. 1981.
- Danel, J. and Merkies, K. Discrimination and selection by horses of sucrose solution. J Equine Vet Sci. 2009.
- Randall, R.P. et al. Response of horses to sweet, salty, sour and bitter solutions. J Anim Sci. 1978.
- Schoster, A. et al. Probiotic Use in Horses â€“ What is the Evidence for Their Clinical Efficacy? J Vet Intern Med. 2014.
- Yuyama, T. et al. Evaluation of a host-specific Lactobacillus probiotic in neonatal foals. J Appl Res Vet M. 2004.
- Desrochers, A.M. et al. Efficacy of Saccharomyces boulardii for treatment of horses with acute enterocolitis. J Am Vet Med Assoc. 2005.
- Jouany, J.P. et al. Effect of live yeast culture supplementation on apparent digestibility and rate of passage in horses fed a high-fiber or high-starch diet. J Anim Sci. 2008.
- Medina, B. et al. Effect of a preparation of Saccharomyces cerevisiae on microbial profiles and fermentation patterns in the large intestine of horses fed a high fiber or a high starch diet. J Anim Sci. 2002.
- Collado, M.C. et al. Probiotic strains and their combination inhibit in vitro adhesion of pathogens to pig intestinal mucosa. Curr Microbiol. 2007.
- Bayoumi, M.A. and Griffiths, M.W. In vitro inhibition of expression of virulence genes responsible for colonization and systemic spread of enteric pathogens using Bifidobacterium bifidum secreted molecules. Int J Food Microbiol. 2012.
- Allaart, J.G. et al. Effect of Lactobacillus fermentum on beta2 toxin production by Clostridium perfringens. Appl Environ Microbiol. 2011.
- Medellin-PeĂ±a, M.J. et al. Probiotics affect virulence-related gene expression in Escherichia coli O157: H7. Appli Environ Microbiol. 2007.