Diarrhea in horses is caused by water content in manure due to increased secretion and decreased absorption of water in the gastrointestinal tract. The appearance of diarrhea can range from soft-formed fecal balls to completely watery. The cecum and large colon are responsible for most of the water absorption in the digestive tract; the colon can absorb 20-30% of the horse’s body weight in water, says Elizabeth MacDonald, BVMS, MS, Dipl. ACVIM (LAIM), clinical instructor at Virginia-Maryland College of Veterinary Medicine in Leesburg, Virginia.
Diarrhea in the adult horse can be acute or chronic and can have an infectious, parasitic, toxic, or many other potential causes, said MacDonald. “Acute diarrhea is not the one pile of loose manure that your horse has right off the trailer at a horse show,” she noted. “Acute diarrhea is the sudden onset of persistent diarrhea, while chronic often persists for a month or more.”
Infectious Causes of Diarrhea
Infectious causes of diarrhea can include Salmonella spp., Clostridium, Neorickettsia risticii (which causes Potomac horse fever), and equine coronavirus. Salmonella spp. is a Gram-negative anaerobic bacterium that is ubiquitous in the environment and spread through exposure to infected fecal material from other animals such as birds and rodents, and it can cause rapid deterioration in the horse. “About 2% of horses are carriers of this Salmonella but show no signs,” said MacDonald.
Clostridium is a Gram-positive, spore-forming, obligate anaerobic bacterium that produces exotoxins that cause severe intestinal inflammation. Risk factors include antibiotic administration, food deprivation, and other stressful conditions, said MacDonald. Horses with Clostridium often become very systemically ill, with a high fever and inflamed colon, and might not regain normal colon function if they recover.
N. ristic colonizes with a flatworm host that infects freshwater snails and aquatic insects; horses typically become infected when inadvertently consuming the insects. Potomac horse fever is an infectious disease that is not contagious between horses and has a preventive vaccine, said MacDonald.
“Infected horses are at risk for laminitis and will often develop a fever and diarrhea,” she says.
“Equine coronavirus is an RNA virus which is transmitted through the fecal-oral route; therefore, horses must ingest the feces from an infected horse,” or consume feed that has come in contact with contaminated equipment or surfaces. There is no evidence of transmission of equine coronavirus to humans.
Toxic Causes of Diarrhea
Antibiotic or NSAID (non-steroidal anti-inflammatory) administration, cantharidin (a substance produced by blister beetles), and arsenic are all potential toxic causes of acute diarrhea. NSAIDs can cause right dorsal colitis in horses if an inappropriate dose is administered or the horse is sensitive to the medication, said MacDonald. “Often these horses will present with diarrhea and low protein,” she said.
Cantharidin toxicosis occurs in some parts of the United States and can cause oral and intestinal ulcers, as well as diarrhea. “These beetles are found in alfalfa when it is flowering, and if cut during peak flowering time, they may be crushed when the alfalfa is baled, contaminating it with their toxin,” said MacDonald.
Arsenic poisoning typically happens when a horse ingests pest-control products, herbicides, or other products that contain arsenic. Contaminated water sources can also be to blame.
Other Causes of Diarrhea
Parasitic diarrhea can be caused by Strongyleus vulgaris and small strongyles (cyathostomins). This is not often picked up on a fecal egg count because the test cannot detect larvae that have encysted in the colon wall, said MacDonald. “These horses may also have mild colic signs along with diarrhea, but we often make this diagnosis based on the horse’s deworming history and ruling out other causes,” she said. Veterinarians might suspect parasites in horses with diarrhea that have not been dewormed with an anthelmintic.
Carbohydrate overload can also cause diarrhea in horses. “This is typically seen in the horse that breaks into the feed room and eats half a bag of grain, causing rapid fermentation and overwhelming the buffering systems in the digestive tract,” said MacDonald. Carbohydrate overload can cause the horse to become systemically ill and develop laminitis, she added.
Sand enteropathy is most common in horses fed on sandy soils or bluestone or those kept in dry-lots. The sand builds up in the digestive tract over time and might even become visible in fecal balls, said MacDonald.
Dysbiosis occurs secondary to any change to the gut microbiome. “Stress, hospitalization, dietary changes, antibiotics, and environmental changes can all disrupt the gut microbiome,” said MacDonald. “These horses do not usually become systemically ill.”
Inflammatory bowel disease occurs when abnormal cells infiltrate into the intestinal wall—they can be inflammatory cells or neoplastic (cancerous) cells. “Diarrhea can be a sign of cancer in the gastro-intestinal tract and is an option to explore when other causes have been ruled out,” said MacDonald.
Knowing When to Worry
“Unfortunately, the cause of equine diarrhea can only be determined in about 50% of cases,” MacDonald noted. Diagnostic methods that can be used include fecal egg counts, Salmonella culture, fecal PCR (polymerase chain reaction) panel, ultrasound, abdominal radiographs, and biopsy, she added.
“If the horse has a fever, is not eating, has watery diarrhea, seems dull and depressed, or has diarrhea that persists for six to 12 hours or more, I recommend that the horse be seen immediately,” said MacDonald. Diarrhea in horses is much more dangerous than in humans, she added. “Diarrhea in horses can become fatal rapidly. Always call your veterinarian if you are unsure.”
Biosecurity is a concern when diarrhea is present because pathogens such as coronavirus and Salmonella can spread through a barn very quickly. To protect the other horses in the barn, sick horses should be isolated, especially if fever, lethargy, and diarrhea are present.