Diarrhea in a horse can be a minor, short-lived change in manure—or the first visible sign of infectious disease, toxin exposure, intestinal inflammation, or another serious disorder. Treat sudden watery or persistent diarrhea as a veterinary problem, especially when the horse has a fever, seems systemically ill, or other horses are affected. Because the same stool change has many possible causes, appearance alone cannot identify the disease.
Call your veterinarian promptly for triage. Keep the horse under observation, prevent contact with other horses when infectious disease is possible, and do not start a generic medication or supplement in an attempt to stop the diarrhea.
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When diarrhea is an emergency
Contact a veterinarian immediately when diarrhea is watery, profuse, persistent, or accompanied by fever or other signs of illness. The urgency is higher if the horse has acute gastrointestinal abnormalities, marked depression or weakness, signs of dehydration, or another horse on the property is sick. The American Association of Equine Practitioners (AAEP) advises biosecurity measures and on-premises quarantine when one or more horses have acute gastrointestinal disease with fever, or when multiple febrile horses are affected, whether or not respiratory or neurologic signs are also present. See the AAEP acute infectious diarrhea field guidelines.
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- One horse with fever and acute diarrhea: call the veterinarian and separate that horse while you wait for instructions.
- Several horses with diarrhea or fever: treat the situation as a possible outbreak; stop moving horses on or off the premises until your veterinarian advises otherwise.
- Diarrhea plus systemic illness: report every sign, including reduced appetite, weakness, unusual behavior, respiratory signs, or neurologic changes.
Do not wait for the manure to develop a particular color or consistency. A serious cause can look different from one case to the next.
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Soft manure is not always the same as true diarrhea
Merck Veterinary Manual notes that some otherwise healthy horses pass soft feces after a feed or pasture change—such as moving onto lush pasture or eating more alfalfa—or after temporary stress such as travel or a veterinary visit. That limited situation may resolve without becoming a medical problem. It does not apply automatically to watery, ongoing, or foul diarrhea, or to a horse with fever or other illness. Read Merck’s owner guidance on intestinal disorders other than colic in horses.
Record when the change began, how often manure is passed, recent feed or pasture changes, travel, medications, and whether the horse is eating, drinking, and behaving normally. These details help a veterinarian distinguish a transient soft-feces episode from acute intestinal disease.
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What can cause diarrhea in a horse?
Diarrhea is a symptom, not a diagnosis. Merck describes acute diarrhea in connection with infectious disease, toxin ingestion, sand colic, medications, and colitis-X; chronic diarrhea also has a broad differential. The AAEP’s 2025 field guideline separates important adult-horse possibilities into infectious and noninfectious groups. The list below is a framework for a veterinarian, not a checklist for diagnosing your horse at home.
| Category | Examples identified in veterinary guidance | Why the distinction matters |
|---|---|---|
| Infectious | Clostridioides difficile, Clostridium perfringens, equine coronavirus, Neorickettsia risticii and N. findlayensis, Salmonella species, and cyathostomiasis | Some agents can spread between horses, and some have zoonotic implications. Isolation and practitioner-directed testing may be needed. |
| Noninfectious | NSAID-associated right dorsal colitis, toxins (including blister beetle, arsenic, oak, and ionophores), sand or gravel consumption, neoplasia, and inflammatory bowel disease | These problems require different investigations and treatments; antibiotics or dewormers are not interchangeable solutions. |
| Context-dependent or other acute causes | Dietary transitions, temporary stress, medications, toxin exposure, sand-related disease, and colitis-X are described in Merck’s discussion of intestinal disorders. | A recent change may be relevant, but it should not be used to dismiss severe or persistent diarrhea. |
There is no reliable “diarrhea color chart” that tells an owner which disease is present. Similar intestinal signs occur with many disorders, and the cause can be difficult to determine. The AAEP guideline supports selecting differentials, samples, and tests according to the individual case.
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Can horse diarrhea spread to other horses or people?
Yes, some causes have contagious or zoonotic implications. The AAEP specifically identifies equine coronavirus, equine rotavirus, Salmonella species, and Clostridioides difficile among infectious agents of concern. You cannot determine the risk from manure appearance alone.
Immediate biosecurity steps
- Separate the sick horse. Move it to an area away from shared stalls, paddocks, water, feed, and high-traffic routes if this can be done safely. If moving it would create danger, isolate it in place and tell the veterinarian.
- Stop unnecessary horse movement. Do not haul, compete, sell, or introduce horses until your veterinarian has assessed the situation.
- Limit people and equipment. Assign care to as few people as practical, use dedicated buckets and tools, and clean hands and footwear after handling the horse or manure. Follow the veterinarian’s instructions for disinfectants and waste handling.
- Notify the veterinarian about contacts. Provide a list of horses that share a barn, turnout, water source, trailer, event, or caretakers so exposure can be assessed.
People who are pregnant, very young, elderly, or immunocompromised should avoid handling manure from a potentially infectious horse unless their physician and veterinarian advise otherwise. Wash hands thoroughly after any contact.
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What your veterinarian will do
A veterinarian will combine the history and physical examination with a case-specific differential. Expect questions about onset, feed and pasture changes, travel, medications (including nonsteroidal anti-inflammatory drugs), deworming, toxin access, other sick horses, and recent contact with outside horses.
Assessment and testing
- The veterinarian evaluates hydration, temperature, cardiovascular status, abdominal findings, manure characteristics, and signs affecting other body systems.
- When infectious disease is possible, the practitioner chooses appropriate samples and tests rather than ordering every test indiscriminately.
- Diagnostic laboratories should be contacted for current collection, packaging, and shipping guidance. Sample collection and testing should be directed by the veterinary team, not improvised by an owner.
- Results may need to be interpreted alongside examination findings; a negative or positive result does not replace clinical judgment.
The AAEP created its field guideline to help practitioners select diagnostics and improve recognition of equine infectious disease. Toby L. Pinn-Woodcock, DVM, DACVIM, chair of the AAEP Field Guidelines Subcommittee and assistant clinical professor at Cornell University College of Veterinary Medicine, said: “Our goal is to provide practitioners with comprehensive and accessible resources to ease diagnostic selection and increase the diagnosis of equine infectious disease.” The announcement was published by AAEP on May 27, 2025, in its notice about the guideline.
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What to do before the veterinarian arrives
- Call and describe the situation clearly. State when diarrhea started, whether it is watery, how frequently it is occurring, the horse’s temperature if you have been trained to take it safely, and every other sign you have noticed.
- Provide safe access to clean water. Do not force water or fluids into a weak or uncooperative horse. Ask the veterinarian whether feed should be adjusted while the examination is pending.
- Remove hazards. Prevent access to known toxins, spoiled feed, contaminated water, and areas where the horse may consume sand or gravel. Preserve labels or packaging from any suspected substance for the veterinarian.
- Do not medicate on guesswork. Do not start antibiotics, anti-diarrheal drugs, dewormers, NSAIDs, electrolytes, probiotics, or supplements unless your veterinarian specifically directs their use for this horse. A product that is reasonable in one diagnosis can be ineffective or harmful in another.
- Track changes. Note manure episodes, water intake, appetite, temperature readings, urination, attitude, and any new sick horses. Keep the record available for the veterinarian.
Do not attempt to diagnose a pathogen from a photograph or a manure sample sent without veterinary direction. The useful next step is a professional examination and an appropriate testing plan.
How to think about the pattern without self-diagnosing
Owners can organize information for the veterinarian by using four comparison axes. These categories help communicate urgency; they do not identify a disease.
| Pattern | Questions to answer | Action |
|---|---|---|
| Acute versus ongoing | Did the change begin suddenly, or has abnormal manure continued or recurred? | Sudden, severe, or worsening diarrhea warrants prompt veterinary contact; ongoing cases need evaluation even when the horse appears bright. |
| Single horse versus multiple | Is any other horse febrile, depressed, or passing diarrhea? | Multiple affected horses increase concern for an infectious event and require coordinated isolation and veterinary advice. |
| Local gut sign versus systemic illness | Is there fever, weakness, appetite loss, respiratory disease, neurologic change, or other acute gastrointestinal abnormality? | Systemic signs raise urgency and strengthen the need for biosecurity. |
| Possible exposure | Was there a new feed or pasture, travel, medication, toxin access, or sand/gravel exposure? | Tell the veterinarian exactly what changed and when; do not assume the most recent change is the cause. |
After the first episode: monitoring and prevention
Follow-up depends on the diagnosis. A horse with a brief soft-feces episode after a documented feed or stress change may return to normal, while a horse with infectious colitis, toxin exposure, inflammatory disease, or medication-associated injury may need ongoing treatment and monitoring. Ask when the veterinarian wants a recheck and which changes should trigger an emergency call.
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For acute infectious diarrhea, use the AAEP’s 2025 field diagnostic guideline PDF with your veterinarian. It emphasizes practitioner-led selection of differentials, samples, and tests and recognizes the herd-health and zoonotic consequences that make rapid communication important.
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