Diarrhoea is usually of little concern, but if severe may point to something more serious. Lesley Barwise-Munro MRCVS outlines possible causes


Diarrhoea refers to the passing of faeces with a greater than normal water content.

It is often due to a simple dietary upset, but can also occur due to more serious causes.

Diarrhoea associated with a disease process in the gastrointestinal tract can range from intermittent soft to persistent liquid projectile faeces. Yet mild forms can progress by destabilising the balance of healthy organisms in the normal large intestine.

In the case of dietary upset, the faeces are fairly normally formed but with a separate water phase. This is not usually harmful to the horse, who will often respond to good dental care and dietary changes: improving fibre quality, switching from haylage to complete hay or a hay/haylage mix, or using alternative fibre sources such as beet pulp or alfalfa.

Feeding live yogurt, stomach tubing with fresh droppings from a healthy horse or even caecal contents from a deceased horse can help; probiotics have variable success. While not every bout of diarrhoea is a cause for alarm, its presence — especially if severe or sudden in onset — could indicate a more worrying problem.

Grain overload

This can sometimes occur if a horse escapes into the feed room. Excessive intake of starch-based feed can cause colic signs and sudden-onset diarrhoea, sometimes leading to colitis. Laminitis often follows within 24-48 hours.

Various tests will determine the level of dehydration and damage, and guide your vet on best treatment options. Stomach tubing with mineral oil, Epsom salts or activated charcoal can be beneficial.

Frog support bandages, intravenous NSAIDs and aspirin may help prevent laminitis, but this remains a long-term risk for a horse recovering from colitis caused by grain overload.

Sand Enteropathy

There’s a risk that sand may collect in the colon if horses are fed and/or grazed on sandy surfaces or drink from sand-based water sources. It can cause impaction or diarrhoea that may worsen with bowel wall inflammation or perforation.

Sand in the colon can be heard grating through a stethoscope or visualised with an ultrasound scanner. A sand sedimentation test (faeces mixed with water to separate the sand) aids diagnosis. Removing the horse from the sand source is crucial.

Stomach tubing with psyllium mucilloid helps carry sand out of the gut.

NSAID Toxicity

This may occur after prolonged or high-level use of a drug such as bute (phenylbutazone). Loss of appetite and low-grade colic are among the signs, with ventral oedema (gathered fluid) along the belly or sheath in geldings — diarrhoea is not always present. Lab tests and other investigations confirm diagnosis, but better to discuss with your vet before considering long-term treatment. Non-steroidal anti-inflammatory drugs (NSAIDs) should be withdrawn and supportive therapy given if required.

Corona Virus

This emerging intestinal disease affects mainly younger animals in the winter months. Diarrhoea, fever, anorexia and neurological signs may be present.


An inflammation and/or infection in the abdominal cavity, this can cause depressed behaviour, mild and often intermittent colic, a raised temperature and chronic diarrhoea. The specific cause is unknown, but diagnosis is confirmed by testing a sample of peritoneal fluid.

Aggressive treatment with broad spectrum antibiotics and non-steroidal anti-inflammatories is required or sometimes more complex therapy, including surgery.

Colitis, including salmonella

Salmonella is associated with colitis, but is not the only cause. It can be highly infectious if salmonella is involved, posing an additional risk to humans in contact with the horse. Depression and colic signs may develop before diarrhoea. The horse’s temperature and heart and respiratory rates may be elevated, his gums bright red and his extremities cold. It can be rapidly fatal.

Repeated faecal cultures may be needed to confirm salmonella as the cause of illness. Blood tests will normally show a profound drop in white blood cells at an early stage and are also necessary to establish fluid requirements and severity of endotoxaemia — the presence of toxins in the bloodstream that can lead to shock, laminitis and even death.

Intensive nursing and the administration of intravenous fluids and other medical help will give the horse the best chance of survival.

Clostridial colitis

This can result from an upset in the balance of bacteria — notably Clostridium perfringens and Clostridium difficile — in the horse’s gut. This life-threatening colon disruption can be diagnosed with specific lab tests and requires a similar level of intensive support to a salmonella case.

Inflammatory Bowel Disease (IBD)

IBD, including lymphosarcoma, is a complex but well-recognised condition in horses.

It is the collective term for intestinal diseases primarily affecting the small intestine and causing malabsorption of food products. Signs include progressive weight loss, intermittent diarrhoea, colic, poor appetite, depression and sometimes skin lesions.

A glucose absorption test can confirm the presence of a condition causing malabsorption — but not the type. A thickened bowel wall can usually be seen on an ultrasound scan and biopsies can be taken to identify the cause.

Treatment options range from high steroid doses to feeding highly digestible, palatable feeds or electrolyte, mineral or vitamin supplements — all with varying success. Medical treatments are limited and include steroids, occasionally chemotherapeutic agents or other substances to improve gut/health mobility.


Cyathostomosis is inflammation of the bowel wall, caused by mass emergence of redworm larvae. More common in animals between one and five years old, it typically occurs in late winter or spring — sometimes after worming. Signs include diarrhoea, sometimes following weight loss, and a raised temperature. Faecal worm egg
counts at this stage are often negative, but tiny red larvae may be seen in the faeces or on the vet’s rectal glove. Ultrasound scanning can detect inflammation at an early stage and assist with diagnosis.

Colitis can follow, requiring hospitalisation for intensive treatment. Key preventative measures include keeping pasture clean with regular poo-picking, sometimes harrowing or grazing with sheep or cattle, monitoring worm burdens with egg counts and worming as required.

Ref: Horse & Hound; 21 May 2015