Diarrhea in Horses

What is diarrhea?

Diarrhea means the production of feces that are softer than normal. Normal equine feces are produced in formed, non-offensive smelling, greenish-brown, semi-solid portions that will break up in the hand, revealing varying degrees of fibrous content depending upon diet. The softness of diarrhea varies from non-formed (‘cow-pat’) to liquid (like colored water) and there may be an offensive smell. Liquid diarrhea may be produced apparently involuntarily in a projectile manner (‘pipe stream’) or may involuntarily run down the hind legs (incontinence), resulting in skin ‘scalding’.

What is the significance of diarrhea?

Diarrhea is a relatively common condition in the horse and in most instances is transient. Due to the anatomy of the adult gastrointestinal tract it is normally only conditions that affect the large bowel and cecum that result in diarrhea. In foals, before their large bowels are fully competent, i.e., before three months of age, small intestinal conditions can be involved and foals can quickly become very ill.

Diarrhea may represent a simple digestive ‘upset’, e.g., following a sudden change in diet, when it causes no other significant illness. When caused by infection, intestinal parasitism or other significant gastrointestinal or metabolic abnormality, the horse may be clinically ill and need veterinary attention. Horses that are ill with diarrhea may show a variety of other symptoms that may include colic, inappetance, increased thirst, depression, weight loss, dehydration, pot belly, poor coat and dull eyes. Cases associated with cyathostomiasis (small strongyle worms), some cancers or other causes of protein loss can develop edema i.e., filling of the lower legs, ventral chest and abdomen.

What causes diarrhea in adult horses?

In many cases of adult equine diarrhea, the horse recovers before the cause is known. The horse’s intestines, especially the large intestines and cecum, contain large numbers of ‘normal’ bacteria, of many different types, that are an essential part of the normal digestive processes. If the normal ‘balance’ of these bacteria is upset by a sudden change in diet or antibiotic treatment, digestion is upset and diarrhea may follow. When the normal bacterial population reorganizes and returns to normal, digestion and fecal consistency also return to normal. Treatment with probiotics (see later) may help a speedy return to normality and, if used early, before diarrhea develops, may sometimes help prevent diarrhea.

There are specific conditions that do require diagnosis and specific treatment. 

Infections, such as salmonellosis (infection with Salmonella spp. bacteria) can cause symptoms ranging from acute, severe diarrhea to chronic, more insidious diarrhea and weight loss. Some symptomless carriers of salmonellosis only develop signs of diarrhea following treatment with antibiotics or hospitalization for surgery. Some species of salmonella bacteria can infect susceptible (especially young and aged) humans and animals and positive cases should be put into isolation. Rarely, other bacteria such as Clostridium spp. can cause acute enteritis (inflammation of the small intestine), that can be rapidly fatal before diarrhea is produced. Less acute cases develop a foul smelling watery diarrhea.

Intestinal Parasites are an important cause of diarrhea in young and debilitated older horses, the most severe associated with cyathostomiasis. Cyathostomes are small strongyle worms and during their lifecycle they migrate through and encyst (curl up and ‘hibernate’) in the wall of the cecum and large intestine. They cause no problems while encysted but during migration and particularly during emergence they cause considerable damage to the lining of the large intestine. This can result in profuse watery diarrhea and marked loss of water and protein, causing serious illness.

Stress associated with long transportation in hot conditions, general anesthesia and surgery may cause colitis (inflammation of the large intestine, i.e., colon) and this may result in massive fluid losses and the development of toxemia (formation, release and reabsorption of toxins or tissue poisons) secondary to damage to the gut wall. Horses with acute colitis are severely ill and require intensive care. Some cases are fatal. In some individuals, lower-grade stress e.g., pre-performance ‘nervousness’ may cause loose feces or even diarrhea, as is well recognized to affect some people.

Less common causes of diarrhea include intestinal neoplasia (cancer) and granulomatous enteritis (a chronic, i.e., long term, inflammation of the small intestines). Both may result in damage to and malfunction of the intestines, resulting in failure of reabsorption of water by and loss of protein through the intestines (malabsorption syndrome). This results in the production of fluid feces and progressive weight loss in affected horses that may become life-threatening.

How are the causes of diarrhea diagnosed?

In most cases it is obvious when a horse has diarrhea but often the difficulty is in diagnosing the cause. A veterinarian will perform a clinical examination and take a history, that will include medical and worming history, the duration of diarrhea and dietary changes, exposure to stress or surgery or other medical treatments and any history of other illness. Blood and fecal samples may be collected for laboratory analysis. The fecal sample will be examined for parasitic worm eggs and cultured for bacteria. It may be useful to perform a rectal biopsy. This is accomplished in the standing sedated horse. In chronic or severe cases it may be necessary to take large or small intestinal biopsies either by laparotomy under general anesthesia or laparascopically in the standing sedated horse.

How is diarrhea treated?

Many cases of diarrhea in adult horses are associated with transient digestive disturbances, do not require treatment and resolve quickly in a day or two. If the diarrhea is severe or symptoms persist then treatment is required. It may be necessary to commence treatment prior to a diagnosis of cause being made. Symptomatic treatment includes removing any obvious cause or exacerbating problems such as green grass, rich feed and antibiotic treatment. Water and electrolytes should be administered by allowing and encouraging access to drinking water to which electrolytes have been added. Always supply an alternative supply of drinking water without electrolytes in case the horse does not like the flavored water. If the horse is not drinking or is dehydrated, fluids can be administered by stomach tube or by intravenous drip. Intestinal absorbents and anti-diarrheals such as activated charcoal, montmarillonite, bismuth subnitrate, codeine and kaolin may be administered by stomach tube as necessary. If there is pain (colic) associated with the diarrhea then analgesics may be used, such as Buscopan (a spasmolytic) or flunixin (a non-steroidal anti-inflammatory and analgesic). Once the cause of the diarrhea is known, specific treatment should be introduced. In cases of cyathostomiasis, corticosteroids are sometimes given alongside symptomatic treatment and larvicidal (high) doses of appropriate anthelmintics. These cases may also require intensive nutritional (especially protein) and fluid supplementation and antibiotic medication, as they are often severely debilitated and secondarily infected. In cases of salmonellosis and other bacterial infections, there is still some debate as to whether antibiotics should be used, as they are often ineffective and may promote antibiotic resistance in the bacterial population. Other conditions that require specific treatment and management include granulomatous enteritis and neoplasia.

Severe cases of diarrhea and colitis are life-threatening and require aggressive intensive care. Fluid and electrolytes are lost in large amounts and a combination of reduced digestion and absorption and increased loss of protein due to intestinal inflammation can result in profound weight loss and debility in a very short period of time. If the lining of the gut is severely damaged or, in certain intestinal bacterial infections the lining of the gut, is so severely damaged that the horse can become toxemic, i.e., toxins produced by bacteria in the gut are absorbed into the blood stream. The toxins can damage large intestinal blood supply irreparably, the gut wall dies and these horses develop shock and die or require euthanasia quite quickly despite treatment.

How is diarrhea prevented?

Diarrhea is best prevented by good management, with provision of good quality pasture and feed and any changes in diet made gradually. A strategic parasitic worm control program appropriate to the management is essential, with twice yearly (spring and autumn) anthelmintic dosing specifically to control small strongyle worms (cyathostomes) and tapeworms. Contact with known Salmonella spp. carriers should be avoided.

Some horses tolerate stress less well than others and it is difficult to predict which horses may develop diarrhea or colitis following stressful conditions or situations such as competitions or surgery but one must always be aware of the possibility.

When should I call my veterinarian?

You should call your veterinarian if your horse appears unwell (depressed, dehydrated, behaving abnormally or in pain), the mucous membranes in the eyes and mouth appear congested (red rather than pink), or if the diarrhea has persisted for more than 2-3 days.