Bit & Bridle

Equine Diarrhoea

Equine diarrhoea is a common and potentially serious clinical sign in horses of all ages. It refers to the production of loose, watery, or unformed faeces at a frequency or volume greater than normal. Whilst a brief episode of mild loose droppings is rarely a cause for concern, persistent or severe diarrhoea represents a significant veterinary emergency in adult horses and foals alike, with the potential for rapid dehydration, electrolyte imbalances, and life-threatening endotoxaemia. Understanding the causes, warning signs, and appropriate response to diarrhoea is an essential part of responsible horse ownership.

Other Names

  • Loose droppings
  • Scours (particularly in foals)
  • Enteritis (when accompanied by gut inflammation)
  • Colitis (large bowel diarrhoea)

What Is Equine Diarrhoea?

Diarrhoea is a symptom rather than a disease in itself — it is the result of disrupted gut function from any one of numerous causes. In horses, the large intestine (caecum and large colon) is primarily responsible for water absorption and fermentation of fibrous feed. When this process is disrupted — by infection, toxins, dietary changes, motility disorders, or systemic illness — the result is watery, malformed faeces.

Diarrhoea can be acute (sudden onset, lasting days) or chronic (persisting for weeks to months). It can range from mildly loose droppings to profuse, watery faeces passed almost continuously. The degree of systemic upset associated with diarrhoea is highly variable — a horse with profuse watery diarrhoea and systemic signs of endotoxaemia (dark mucous membranes, elevated heart rate, painful abdomen) is a critical emergency requiring immediate hospitalised care.

Causes

The causes of equine diarrhoea are diverse. Broadly, they can be divided into:

Infectious causes:

  • Salmonella spp. — one of the most serious causes; can cause explosive, blood-tinged diarrhoea with severe systemic illness and is zoonotic
  • Clostridium difficile and C. perfringens — particularly associated with antibiotic-associated diarrhoea
  • Lawsonia intracellularis (equine proliferative enteropathy) — an important cause in weaned foals and yearlings
  • Equine Coronavirus (ECoV) — increasingly recognised as a cause of diarrhoea and systemic illness
  • Cyathostomiasis (small redworm emergence) — mass emergence of encysted cyathostome larvae from the gut wall in late winter/spring is a significant cause of sudden, severe diarrhoea in adult horses
  • Strongylus vulgaris and other parasites

Non-infectious causes:

  • Dietary change — abrupt changes in feed type, quality, or quantity, introduction of fresh spring grass
  • Stress — transport, competition, changes in routine, weaning
  • Antibiotic use — disruption of the normal hindgut microbiome, particularly with broad-spectrum antibiotics
  • NSAIDs — overuse of phenylbutazone or other NSAIDs can cause right dorsal colitis, a specific inflammatory condition of the large colon
  • Sand ingestion — accumulation of sand in the large colon causes chronic irritation and intermittent diarrhoea
  • Inflammatory bowel disease (IBD) — a heterogeneous group of conditions causing chronic intestinal inflammation
  • Lymphoma or other neoplasia — large intestinal tumours are uncommon but cause chronic diarrhoea with weight loss
  • Hepatic disease — liver failure can cause diarrhoea as part of the broader metabolic disturbance
  • Antimicrobial-associated diarrhoea

Symptoms and Signs

The presenting signs vary widely depending on cause and severity. Look for:

  • Loose, watery, or unformed faeces — from mildly soft droppings to profuse water
  • Increased frequency of defecation
  • Faecal scalding of the hindquarters (skin irritation from persistent loose faeces)
  • Colic signs — abdominal discomfort, kicking at the belly, repeated lying down
  • Dehydration — sunken eyes, tacky gums, poor skin tent, dark urine
  • Depression, anorexia, and lethargy
  • Fever — often present with infectious causes
  • Tachycardia (elevated heart rate) — a key indicator of systemic compromise
  • Abnormal mucous membrane colour — pale, congested (brick red), or purple-tinged gums indicate severe systemic illness
  • Weight loss — particularly prominent in chronic diarrhoea
  • Ventral oedema — protein-losing enteropathy causing fluid accumulation under the belly

Diagnosis

Because diarrhoea has many causes, a systematic diagnostic approach is needed:

  • History and physical examination: Vaccination and deworming history, recent diet changes, antibiotic use, travel, and contact with new horses are all relevant
  • Faecal culture and sensitivity: For bacterial pathogens including Salmonella (requires multiple samples — three on consecutive days for greatest sensitivity) and Clostridium spp.
  • PCR panels: Rapid molecular testing for multiple pathogens simultaneously (Salmonella, ECoV, Clostridium, Lawsonia, rotavirus in foals)
  • Haematology and biochemistry: Leucopaenia, neutropaenia, elevated fibrinogen, hypoproteinaemia, and electrolyte disturbances are common findings. Liver enzyme elevation may indicate hepatic involvement.
  • Faecal egg count and larval culture: To assess parasite burden and identify cyathostomiasis
  • Faecal sand assessment: Settle a faecal sample in water — sand sinks to the bottom of the container. Auscultation of the ventral abdomen may reveal "sand sounds."
  • Abdominal ultrasound: Assess gut wall thickness, motility, and the presence of free fluid
  • Rectal biopsy: In chronic diarrhoea cases, biopsy of rectal mucosa can diagnose inflammatory bowel disease and some forms of neoplasia
  • Glucose absorption test: Assesses small intestinal absorptive capacity in horses with suspected malabsorption

Treatment

Treatment depends entirely on the underlying cause but general principles include:

  • Fluid and electrolyte replacement: The cornerstone of treatment for significant diarrhoea. Oral electrolyte solutions can be used for mild cases; IV fluid therapy is required for moderate to severe dehydration or systemic compromise.
  • NSAIDs: Flunixin meglumine is preferred for its anti-endotoxic properties as well as its analgesic and anti-inflammatory effects
  • Bismuth subsalicylate: Useful as a gut protectant and mild antimicrobial in certain cases
  • Probiotics and prebiotics: To support restoration of the normal hindgut microbiome
  • Di-tri-octahedral smectite (Bio-Sponge): Binds bacterial toxins in the gut lumen — useful in Clostridium-associated diarrhoea
  • Antibiotics: Targeted antibiotic therapy based on culture and sensitivity for confirmed bacterial causes. Empirical use of antibiotics in diarrhoea is controversial as it can worsen dysbiosis and risk antibiotic-associated colitis.
  • Psyllium: For sand-associated diarrhoea — helps bind and move sand through the colon
  • Dietary management: Offer high-quality, highly digestible hay; remove rich or fermentable feeds; avoid pasture access temporarily in acute cases
  • Biosecurity: Isolate any horse with infectious diarrhoea immediately and implement strict hygiene protocols to prevent spread

Prevention

Preventative measures reduce the risk of diarrhoea significantly:

  • Maintain a strategic deworming programme using faecal egg counts and targeted treatment — particularly addressing encysted cyathostome larvae with a moxidectin-based product in late autumn/early winter
  • Make all dietary changes gradually — a minimum of 7–14 days for feed changes
  • Implement quarantine protocols for all new horses arriving on the property
  • Avoid routine use of NSAIDs at high doses or for prolonged periods
  • Use antibiotics judiciously and only under veterinary supervision
  • Provide access to adequate salt and water to maintain good hydration
  • Manage grass access carefully in spring to avoid sudden exposure to rich pasture

Prognosis

Prognosis ranges from excellent (mild dietary diarrhoea that resolves with management changes) to grave (salmonellosis with severe systemic involvement, right dorsal colitis with extensive mucosal necrosis, or cyathostomiasis with massive larval emergence). Chronic diarrhoea cases require thorough investigation to determine the underlying cause before a meaningful prognosis can be given. Early intervention significantly improves outcome in all but the most severe cases.

When to Call the Vet

Call your vet immediately if your horse shows:

  • Profuse watery diarrhoea — particularly if accompanied by colic signs
  • Abnormal gum colour (pale, dark red, or purple/grey)
  • Elevated heart rate (above 48 bpm at rest)
  • Fever alongside diarrhoea
  • Rapid deterioration in demeanour or responsiveness
  • Blood in the faeces
  • Multiple horses in the same yard affected simultaneously

For mild loose droppings without systemic signs, monitor closely and call your vet within 24 hours if there is no improvement. For related conditions, see Equine Colic and Equine Coronavirus (ECoV). Our horse terminology guide on the equine digestive system explains the anatomy relevant to understanding these conditions.

Frequently Asked Questions

Can diarrhoea in horses be caused by stress?

Yes. Stress is a well-recognised cause of transient loose droppings in horses — competition, transport, weaning, and changes in routine can all trigger a brief episode. Stress-related diarrhoea is typically mild and self-limiting, resolving once the stressor is removed. However, persistent diarrhoea should not be attributed to stress alone without ruling out infectious or other primary causes, particularly if other horses are affected or the horse is systemically unwell.

Is equine diarrhoea contagious to humans?

Some causes of equine diarrhoea are zoonotic, most notably Salmonella spp. Basic precautions should always be taken when handling a horse with diarrhoea: wear gloves, wash hands thoroughly, avoid eating or drinking in the stable, and change footwear before entering other areas. If you are immunocompromised, pregnant, or caring for young children, discuss appropriate precautions with your doctor.

Why does spring grass cause diarrhoea?

Fresh spring grass is high in rapidly fermentable carbohydrates (fructans) and has a high water content. When horses are suddenly exposed to large quantities after a winter on hay-based diets, the change disrupts the hindgut microbiome, leading to rapid fermentation changes, acidosis, and loose droppings. Gradual introduction to spring pasture — starting with short grazing periods and increasing slowly over 2–3 weeks — prevents this problem.

What is right dorsal colitis and how is it caused?

Right dorsal colitis (RDC) is a specific inflammatory condition of the right dorsal colon — a portion of the large intestine — caused primarily by overuse of NSAIDs, particularly phenylbutazone (bute). NSAIDs inhibit prostaglandin synthesis, which is important for maintaining the protective mucous barrier of the gut. Long-term or high-dose use leads to mucosal ulceration and inflammation. Signs include intermittent colic, loose droppings, weight loss, ventral oedema, and low blood protein. Treatment involves strict NSAID withdrawal, a high-fibre low-starch diet, and specific gut protectants.