Diarrhea in Horses

Key Takeaways

Diarrhea in horses is stool that contains more than the usual amount of water, ranging in consistency from loose “cow plop” defecation to projectile defecation of mostly liquid.

• Underlying causes of diarrhea vary widely, and include infectious agents, parasites, ingestion of toxins or irritants, and shifts in the intestinal flora (aka “gut bacteria”)

• In foals, underlying causes include foal heat, Rhodococcus equi, and rotavirus

Foals with any form of diarrhea require immediate veterinary attention

Adult horses with severe diarrhea or diarrhea lasting longer than 24 hours, or with fever or signs of dehydration, require immediate veterinary attention

• Diagnostic tools include physical examination, bloodwork, and fecal analysis

• Treatments include IV fluids, intestinal protectants, anti-inflammatory medications and pain relievers

• Due to the possibility of infectious agents, horses with diarrhea must be isolated until examined by a veterinarian

A Closer Look: What is Diarrhea in Horses?

Horses’ colons are both large and sensitive, therefore diarrhea is common, and in some cases, concerning. If left untreated, the loss of excessive fluids and electrolytes due to diarrhea creates severe and serious shifts in the horse’s body, which in extreme cases can be fatal.

Due to the sensitive nature of the horse’s gut, diarrhea can be caused by changes in routine that are not necessarily linked to an underlying condition that requires treatment. Changes in routine that may cause diarrhea include:

• Stress

• Travel

• Recent deworming

• Competition or other unusual exertion

• Changes in feed

• Changes in management 

The severity of diarrhea in horses varies widely depending on the underlying cause, the frequency of defecation, and the amount of water lost with each bowel movement. Diarrhea can be acute (lasting for a short span of time) or chronic (continuing over long spans of time). In some cases, it is intermittent (occurring every once in a while). Depending on these factors, length of time that the diarrhea continues, and the underlying condition causing the diarrhea, severity ranges from mild to life-threatening. Severe cases that are left untreated, can lead to enough water loss to cause reduced blood volume, electrolyte imbalance, and eventually death.  

In severe cases, diarrhea can lead to endotoxemia, a condition in which bacterial toxins in the bloodstream damage internal organs. Consequences of endotoxemia include:

• Hemorrhage

• Liver failure

• Kidney failure

• Shock

• Laminitis

• Death

Possible Causes

The potential causes of diarrhea in adult horses vary widely. In general, the cause of diarrhea is a change in the function of the gastrointestinal tract that affects the motility or absorption. Causes of dysfunction include:

Infectious agents including:

• Bacteria such as salmonella and Clostridium

• Viruses such as coronavirus

• Parasites such as larval cyathostomiasis

Toxins including:

• NSAIDs

• Arsenic

Cantharidin 

Altered intestinal flora

Conditions stemming from systems outside of the GI tract including:

• Chronic liver disease

• Intra-abdominal abscesses

Inflammatory conditions including inflammatory bowel disease

Tumors, particularly of the gastrointestinal tract

Ingestion of irritating substances such as sand

In foals, conditions causing diarrhea include:

• Foal heat

Rhodococcus equi

• Rotavirus

Risk Factors

Horses that have one or two incidences of loose stool and are otherwise bright and healthy do not require veterinary attention. Horses with loose stool for more than 24 hours, or are lethargic, dull, unwilling to eat, have fever, or are showing signs of dehydration require immediate veterinary attention. Signs of dehydration include:

• Low urine output

• Skin tenting (skin that does not resume its normal position when pinched)

• Sunken eyes

• Dry or sticky mucous membranes

For adult horses with chronic or intermittent diarrhea, prompt veterinary attention is required to identify any underlying causes.

Foals with diarrhea require immediate veterinary attention in all cases, because they are highly susceptible to dehydration from diarrhea.In many cases, untreated diarrhea in foals is fatal. Many of the conditions that cause diarrhea are contagious, both to other horses and, in some cases, to humans. Horses with diarrhea must be isolated from other animals. Humans in contact with horses in isolation must wear protective gear. It is also recommended that sick horses be handled and managed after all other horses on the property to prevent cross contamination.

Testing and Diagnosis

Diagnosing the underlying cause of diarrhea is challenging and the results are often inconclusive. Diagnostic tools include:

• Physical examination

• Fecal cultures

• Fecal testing

• Blood work

• Ultrasound of the abdomen

• Abdominocentesis: sampling the fluid within the abdomen

• Biopsy of the intestinal tract

Treatment does not depend on confirming a diagnosis, since for many underlying causes the goal is the same: maintain or restore appropriate hydration and electrolyte balance. Some conditions such as bacterial or parasitic infections have specific treatments that aid in recovery. 

In general, treatment plans include:

• IV fluids

• Antibiotics

• Anti-inflammatory medications

• Anti-parasitic medications

• Oral intestinal protectants such as activated charcoal, smectite, or bismuth subsalicylate

• Pain medications

• Fecal transfaunation (transfer of bacterial flora from a healthy horse)

There is little evidence that probiotics are useful for resolving or preventing diarrhea.

Similar symptoms

Fecal water syndrome is a condition in which water runs out of the anus before, during, or after defecation. Unlike diarrhea, there are no apparent health concerns with this condition. Fecal water syndrome is particularly common in senior and geriatric horses.

Associated Symptoms

Symptoms observed alongside diarrhea depend on the underlying cause. They include:

• Fever

• Dehydration

• Lethargy

• Dullness

• Reduced appetite

• Tacky or dry mucous membranes

• Purple or red gums

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