Colitis in horses is inflammation of the large intestine, and it has several distinct causes: bacterial infections, parasites, pain medications, antibiotics, viral infections, and even ingested sand or soil. In many hospitalized cases, no specific cause is ever identified. Understanding the different triggers matters because each one damages the colon through a different mechanism, produces slightly different symptoms, and calls for a different management approach.
Bacterial Infections
Two bacteria account for the majority of identified infectious colitis cases in horses: Salmonella and Clostridioides difficile.
Salmonella enterica serovar Typhimurium is the strain most commonly linked to intestinal disease in horses. Other serovars that cause clinical illness include Anatum, Newport, and Angona. Horses can pick up Salmonella from contaminated water, feed, or environments shared with infected animals. Stress, transport, surgery, and crowded conditions increase the risk of both infection and shedding. A horse can carry Salmonella without showing signs, then develop explosive watery diarrhea, fever, and dehydration when its immune defenses dip.
Clostridioides difficile works differently. This bacterium produces toxins that directly damage the colon lining. The two primary toxins, known as toxin A and toxin B, are what actually cause disease. Some strains produce an additional third toxin that may make the illness worse. C. difficile colitis often follows antibiotic use, which wipes out the normal gut bacteria that keep this organism in check. Diagnosis depends on detecting the toxins in feces, not just finding the bacterium itself, because some horses carry C. difficile harmlessly.
Antibiotic-Triggered Colitis
Horses have an enormous, delicately balanced fermentation system in their hindgut. Antibiotics can destroy that balance. Many classes of antibiotics are implicated: tetracyclines, cephalosporins, fluoroquinolones, trimethoprim-sulfonamides, and metronidazole among them. The problem isn’t the antibiotic directly irritating the colon. Instead, the drug kills off beneficial bacteria, allowing harmful organisms like C. difficile to multiply unchecked.
This is one of the reasons veterinarians are cautious about prescribing oral antibiotics to horses. Even appropriate, necessary antibiotic treatment carries a real risk of triggering colitis, which can be more dangerous than the original condition being treated. The risk varies by drug, dose, and individual horse, but it’s a consideration with nearly every antibiotic course.
NSAID-Induced Right Dorsal Colitis
Nonsteroidal anti-inflammatory drugs, particularly phenylbutazone (commonly called “bute”), can cause a specific form of colitis localized to the right dorsal colon. This condition, called right dorsal colitis, is a protein-losing disease characterized by severe swelling, thickening, and ulceration of the colon wall.
The right dorsal colon is uniquely vulnerable for several reasons. It has a smaller diameter than other parts of the colon, slower transit time, and a blood supply that makes it more prone to reduced blood flow. These factors mean the drug stays in contact with the tissue longer and the tissue is less able to tolerate the insult. Phenylbutazone blocks enzymes that normally protect the gut lining’s blood supply and its ability to buffer acids. Without that protection, the tissue develops blood clots in its tiny vessels, loses blood flow, and ulcerates.
This type of colitis typically develops after prolonged use or higher-than-recommended doses of NSAIDs, but some horses are sensitive even at standard doses. Signs include diarrhea, weight loss, low protein levels in the blood, and sometimes colic. Recovery can be slow and requires complete withdrawal of all NSAIDs along with dietary changes to rest the colon.
Small Strongyle Parasites
Cyathostomins, or small strongyles, are the most common internal parasites in horses worldwide, and their larvae can cause devastating colitis. Horses ingest the larvae while grazing. The larvae travel to the cecum and large colon, where they burrow into the intestinal wall and form cysts. Here’s the critical part: those encysted larvae can remain dormant in the gut wall for up to three years before emerging.
When large numbers of larvae emerge simultaneously, a condition called larval cyathostominosis, the result is severe inflammation of the cecum and colon. The mass emergence tears through the intestinal lining, creating widespread ulceration, tissue death, and hemorrhage. Pathology studies of affected horses describe thickened intestinal walls studded with small white nodules (the cysts), extensive ulceration, bacterial overgrowth in the damaged tissue, and blood clots forming in local blood vessels.
What makes this condition particularly frustrating is that standard deworming kills adult worms in the gut but may actually trigger larval emergence. Researchers have hypothesized that removing adult worms disrupts a delicate balance between the parasites and the horse’s immune system, potentially unleashing a wave of emerging larvae while simultaneously removing the anti-inflammatory influence that the established adult worm population had on the host. The result is a “perfect storm” of inflammation and microbial disruption.
Potomac Horse Fever
Potomac horse fever is caused by Neorickettsia risticii, a bacterium with an unusual life cycle. Horses become infected by accidentally swallowing aquatic insects, particularly caddisflies, that harbor the organism. The bacteria live inside tiny parasitic flatworms (trematodes) that infect the insects. When a horse drinks from a trough where insects have drowned, or eats hay near lights that attract insects at night, it can ingest enough of the organism to become sick.
The disease causes fever, depression, decreased appetite, and diarrhea that can range from mild to profuse and watery. It occurs seasonally, peaking in warmer months when aquatic insect populations are highest, and tends to cluster near rivers, streams, and other freshwater sources. Despite the name, it’s been identified far beyond the Potomac River region.
Equine Coronavirus
Equine coronavirus is a recognized cause of colitis in adult horses. Affected horses typically present with fever, loss of appetite, and lethargy, with most also developing watery, high-volume diarrhea. Some develop colic signs as well.
The virus is highly contagious and spreads through the fecal-oral route. Infected horses shed the virus in their feces for two to five weeks, which means a single case in a barn can easily spark an outbreak. Most horses recover, but the prolonged shedding period makes isolation and biosecurity critical. One retrospective study noted that in the majority of colitis cases presenting to referral hospitals, no infectious agent is ever identified, suggesting that equine coronavirus and other pathogens may be underdiagnosed.
Sand Ingestion
Horses that graze on sandy soil or eat hay off the ground in sandy paddocks gradually accumulate sand in their large intestine. Sand is heavy and abrasive. It settles in the lowest parts of the colon, mechanically irritating and eroding the mucosal lining. Over time, this chronic irritation produces inflammation, diarrhea, weight loss, and an increased risk of sand impaction colic.
This cause is largely geographic. Horses in coastal areas, desert regions, or anywhere with sandy soil are at highest risk. Management changes, such as feeding hay in elevated feeders or on rubber mats, and providing psyllium supplements to help move sand through the gut, are the primary prevention strategies.
Why the Cause Often Goes Unidentified
One of the most frustrating aspects of equine colitis is that even with thorough testing, a specific pathogen or trigger is never pinpointed in a large percentage of cases. The colon’s response to injury is fairly uniform regardless of the cause: inflammation, fluid loss, diarrhea, and pain. Multiple causes can also overlap. A horse on antibiotics for a wound infection might simultaneously be exposed to Salmonella in a hospital environment, while also harboring encysted parasites. Sorting out which factor tipped the balance is often impossible.
What makes colitis in horses particularly serious, regardless of cause, is the sheer volume of fluid the equine large intestine processes daily. When the colon’s ability to absorb water is compromised, horses can become dangerously dehydrated within hours. Rapid fluid loss, electrolyte imbalances, and the risk of bacterial toxins entering the bloodstream through a damaged gut wall are what make acute colitis a veterinary emergency rather than a wait-and-see situation.