Colitis in horses is inflammation of the large intestine, primarily the colon and cecum, that causes profuse watery diarrhea and rapid fluid loss. It is one of the most serious gastrointestinal emergencies in horses, with hospitalized cases surviving to discharge roughly 67% of the time. The condition can strike suddenly, and a horse can deteriorate within hours as fluid pools inside the inflamed gut instead of being absorbed normally.
What Happens Inside the Gut
A horse’s large colon is where most water and electrolytes are absorbed from digested food. When the colon’s lining becomes inflamed, that absorption process breaks down. Fluid that would normally pass back into the bloodstream instead stays trapped in the intestine, a process called intraluminal sequestration. Within several hours of onset, the horse begins passing large volumes of liquid stool.
The damaged intestinal lining also becomes more porous. Bacterial toxins that would normally stay contained in the gut leak into the bloodstream, triggering a body-wide inflammatory response. This is what makes colitis so dangerous: it’s not just a bout of diarrhea, it’s a cascade of dehydration, toxin exposure, and electrolyte imbalance that can overwhelm the horse’s cardiovascular system.
Infectious Causes
Several bacteria and other organisms are responsible for most infectious colitis cases. The major players include Salmonella, Clostridioides difficile, and Clostridium perfringens. Salmonella spreads through the fecal-oral route when horses ingest contaminated feed, water, or come into contact with contaminated surfaces, equipment, or even caretaker hands. C. difficile spores are picked up from the environment, food, or water. Once inside the gut, the spores germinate and release toxins that damage the colon wall.
Potomac horse fever is another well-known cause. It’s triggered by an intracellular bacterium called Neorickettsia risticii, which horses pick up by accidentally eating infected insects or snails, often near streams or ponds. The disease peaks in summer and early fall, though weather patterns can shift the timing. Potomac horse fever cases carry a particularly high laminitis risk: in one study of 85 hospitalized horses, 42% of those with Potomac horse fever developed laminitis, compared to 15% of salmonellosis cases.
Equine coronavirus can also cause colitis, though it tends to carry a lower risk of complications than the bacterial causes.
Non-Infectious Causes
One of the most common non-infectious forms is right dorsal colitis, caused by overuse of anti-inflammatory pain medications (NSAIDs) like phenylbutazone, commonly known as “bute.” These drugs work by blocking enzymes involved in inflammation, but they also reduce the protective mechanisms that keep the gut lining healthy. The right dorsal colon is especially vulnerable.
Phenylbutazone carries the highest risk of gut damage among commonly used equine NSAIDs, followed by flunixin meglumine (Banamine). Toxicity typically occurs at high doses or with prolonged use, but some horses are inherently more sensitive and can develop problems even at recommended doses. This makes it important to use these medications only as directed and for the shortest effective duration.
Other non-infectious triggers include sudden diet changes, sand ingestion in horses grazing on sandy soil, and stress from transport or hospitalization.
Signs to Recognize
Colitis can progress from subtle to severe in a matter of hours. Early signs include depression, loss of appetite, and fever. The horse’s heart rate and respiratory rate will climb, sometimes before any diarrhea appears, because fluid is already pooling inside the colon and inflammatory toxins are entering the bloodstream.
The hallmark sign is profuse, watery diarrhea that often has a strong, foul smell. Dehydration follows quickly. You may notice dry or sticky gums that look brick-red or dark pink instead of their normal salmon color. If you press a finger against the gum and release, the color takes noticeably longer to return (this is capillary refill time, and anything over two seconds suggests poor circulation). The skin may also lose its elasticity. If you pinch a fold of skin on the neck and it stays tented instead of snapping back, the horse is significantly dehydrated.
Some horses show signs of abdominal pain (colic) before or alongside the diarrhea, including pawing, looking at their flank, or rolling.
How Colitis Is Diagnosed
Veterinarians use a combination of physical exam findings, bloodwork, and fecal testing to diagnose colitis and identify the cause. Blood tests often reveal a dangerously low white blood cell count, particularly neutrophils, which signals that the immune system is being overwhelmed by toxins crossing the damaged gut wall.
Fecal testing has become more sophisticated in recent years. Diagnostic labs now offer enteric disease panels that use PCR (a DNA-based detection method) to screen for multiple pathogens in a single fecal sample, including Salmonella, C. difficile, C. perfringens, and Neorickettsia risticii. Some labs also use ELISA testing to detect specific bacterial toxins, and enriched bacterial cultures to identify Salmonella serovars. Blood samples may be tested by PCR for Potomac horse fever specifically. Despite all available tools, a cause is never identified in a significant number of colitis cases.
Treatment and Fluid Replacement
The cornerstone of colitis treatment is aggressive fluid replacement. A horse with acute colitis can lose tens of liters of fluid into the gut in a short period, so intravenous fluids are almost always necessary. Maintenance fluid needs for an adult horse run around 40 to 60 milliliters per kilogram of body weight per day, but a horse with active colitis needs far more to replace ongoing losses. For a 500-kilogram horse, that baseline alone is 20 to 30 liters daily before accounting for the extra fluid being lost through diarrhea.
Electrolytes are a critical part of treatment. Colitis disrupts levels of potassium, calcium, and sodium, all of which are essential for normal heart and muscle function. Fluids are supplemented to correct these imbalances. In milder cases or during recovery, fluids can also be given through a nasogastric tube directly into the stomach, typically 4 to 6 liters every 4 to 6 hours.
Beyond fluids, treatment targets the specific cause when one is identified. Horses with endotoxemia (bacterial toxins circulating in the blood) receive medications to combat the systemic inflammatory response. Supportive care also includes monitoring for and preventing complications.
Laminitis and Other Complications
Laminitis, the painful and potentially devastating inflammation of the tissue connecting the hoof wall to the bone inside, is the most feared complication of colitis. In a retrospective study of 85 hospitalized horses with acute colitis, 20% developed laminitis during their hospital stay. The risk varies dramatically by cause. Potomac horse fever carried the highest risk at 42%, while horses with equine coronavirus colitis had a 0% laminitis rate in the same study.
Laminitis develops because the toxins flooding the bloodstream damage the delicate blood supply to the hooves. Horses showing elevated heart rates and immature white blood cells at the time of admission tend to have worse outcomes, likely reflecting a more severe systemic inflammatory response. Aggressive cryotherapy (icing the hooves) is commonly used as a preventive measure during the acute phase of colitis.
Other complications include protein loss through the damaged gut wall, blood clotting abnormalities, and kidney damage from prolonged dehydration.
Survival and Recovery
Survival depends heavily on how quickly treatment begins and what caused the colitis. The overall short-term survival rate for horses hospitalized with enterocolitis is around 67%, notably lower than the roughly 90% survival rate for horses hospitalized with other types of colic treated medically. Horses that avoid laminitis and respond to fluid therapy within the first 24 to 48 hours have a much better outlook.
Recovery from colitis is a gradual process. The gut lining needs time to heal and the microbial community in the hindgut needs to reestablish. During recovery, small frequent feedings of high-quality forage are recommended. Start with about half a kilogram of good-quality grass hay or alfalfa four to six times daily, gradually increasing the volume over several days as the horse tolerates it. Alternatively, short periods of grazing (20 to 30 minutes, several times a day) or a highly digestible pelleted senior feed can work well. Alfalfa or alfalfa-timothy hay mixes may support better recovery than straight grass hay, likely due to their higher digestibility and protein content.
Grain and concentrates should be avoided for at least 10 to 14 days after the acute episode, since excess starch can further disrupt an already compromised hindgut microbial community.
Prevention on the Farm
For infectious forms of colitis, basic biosecurity makes a real difference. Any new horse arriving at your property should be quarantined for 30 days, kept separate from resident horses with its own set of feed buckets, water buckets, grooming tools, and pitchforks. If a horse becomes sick, isolate it immediately and prevent nose-to-nose contact with others. Use a dedicated set of equipment for the sick horse, and set up a footbath at the entrance and exit of the isolation area.
Visitors should wear clean shoes and clothes, or at minimum spray their shoes with disinfectant before entering barn areas. During local outbreaks, footbaths at the farm entrance are a practical step.
For NSAID-related colitis, the best prevention is cautious use of pain medications. Use the lowest effective dose for the shortest time possible, and be aware that some horses are unusually sensitive. If your horse needs long-term pain management, discuss safer alternatives or protective strategies with your veterinarian. Avoiding abrupt diet changes and managing sand intake in sandy-soil regions also help reduce risk of non-infectious colitis.