Diarrhoea in adult horse

Diarrhoea can be caused by many things, from changes in grass composition to serious infections.

Severity can range from faeces being a bit too soft, to a more life-threatening, profuse, undigested, watery diarrhoea. If your horse has diarrhoea, it may lose a lot of fluid and electrolytes. Call your vet to make sure your horse doesn’t become dehydrated and find out what has caused it.



What can cause diarrhoea?

Diarrhoea is the result of one or several of the following:

  • increased motility of the intestine so that contents are rushed through without being further digested and absorbed
  • a decreased ability of the gut to absorb
  • increased secretion or loss of electrolytes and water by the gut itself

Changes in feed

The horse’s hind gut has its own microflora that breaks down fibre to generate energy sources and vitamins for the horse. The horse’s health depends on this microflora of bacteria, fungi and protozoa. The microflora adapts to the feed that’s presented to them. However, when the horse’s diet changes, it takes a few weeks for the microflora to adapt. Feed changes should therefore be made very gradually. When sudden changes in the diet occur, this can disrupt the microflora and cause diarrhoea. Changes may be made by us (changing from hay to grass or baileage, increasing the amount or type of hardfeed, etc) or sometimes nature is the cause of it (changes in grass composition, fruit falling from trees into the paddock). Once the microflora has adjusted to the diet change, the diarrhoea should resolve. To prevent digestive issues, we advise you to introduce dietary changes gradually over the course of 2 weeks.

The microflora in the hind gut especially has difficulties coping with sugar and carbohydrates. Sugar and carbohydrates are normally digested and absorbed in the small intestine. When there is too much for the intestine to handle, it end up in the large intestine. This upsets the microflora and causes diarrhoea. Common examples are spring grass, horses on high-grain diets, or when a horse breaks into the feed room and munches up on hard feed. Be careful in these cases, because these situations can also lead to laminitis.



When a horse has teeth problems, such as hooks, diastemas or excessive transverse ridges, it can’t chew its food properly. Because the fibre and hard feed isn’t broken down into small enough pieces before it enters the digestive system, enzymes have a hard time breaking these big pieces down. The food doesn’t get digested properly and attracts more fluids. This can lead to diarrhoea or faeces with larger pieces of fibre in it, often accompanied by a watery fraction (a “splash” at the end). Horses should have their teeth looked after by a good dentist every year.

Fig 1 Hooks and an excessive transverse ridge, preventing the horse from being able to grind down food
Fig.1: Hooks and an excessive transverse ridge, preventing the horse from being able to grind down food.


Every horse has worms. Worms only become a problem when there are too many of them. This can be due to more worms on the pasture, or a lapse in the horse's immunity and general health. In such cases worms can cause diarrhoea. The biggest representative is the small strongyle, a.k.a. Cyathostominae or “red blood worm”. These worms can incapsulate in the intestinal wall and break out again, causing puncture holes and inflammation in the gut. This can make your horse very sick, diarrhoea and set the scene for other problems, such as Salmonella.

If a recent faecal egg count (FEC) has shown that your horse is clear, it is unlikely that diarrhoea is caused by worms. However, if no FEC has been done, there can be many reasons why a horse that has been wormed may still have a high worm burden. In some cases, when a horse has a very high worm burden, worming may have some risks. The best thing is to have a FEC done once every season, to guide you on whether worming is necessary to keep your horse in good health, and to see whether problems might be expected.



Stress (or nervous excitement), for example due to transport or competition / being on unfamiliar grounds, can cause diarrhoea. This is a reflex of the nervous system.



Salmonella is the most common bacteria that causes diarrhoea. Severity varies from low grade chronic soft faeces to acute profuse diarrhoea with fever and possible toxaemia and death. Horses can be carriers of the bacteria, which means that they can carry the bacteria without getting sick. When a carrier becomes debilitated (immunity goes down) the bacteria can seize its opportunity to cause diarrhoea and spread to the environment. Other horses in that environment can then pick up the bacteria and develop diarrhoea as well. Foals are more susceptible than horses. Septicaemia can, particularly in foals, lead to arthritis/polyarthritis, and/or pneumonia. Laminitis is considered a possible complication of salmonellosis in horses. As we’ve mentioned before, worm problems may predispose to oppertunistic bacteria, so if a horse has a salmonella infection this might not be the only issue at hand. Salmonella is not only transmissible (infectious) to other horses, but to people as well. Care should therefore always be taken when handling a horse with diarrhoea.


Colitis and “Colitis X”

Colitis means inflammation of the colon. It can be caused by different things, such as bacteria (Salmonella or Clostridium species), worms (Cyathostominae) or ulcers (as a side-effect of non-steroidal drugs such as phenylbutazone a.k.a. “bute”). Stress, low immunity or illness can predispose to these things. Colitis often results in colic and/or diarrhoea.

Colitis X is a form of peracute colitis, meaning that it happens very fast. Historically, “Colitis X” was a term used for cases of severe acute colitis in horses in which the cause was unknown. Colitis X was characterized by sudden onset of severe, profuse diarrhea and the development of hypovolemic shock, often resulting in death. Death may occur within three hours of the onset of clinical signs, sometimes before diarrhea has developed, and the problem in the colon is only observed at necropsy. More commonly, there is rapid development of abdominal pain, explosive diarrhea, and dehydration, which can be severe if left untreated. The abdomen may appear bloated (with fluid and/or gas) and the gastrointestinal sounds can vary from hyper to hypomotile. Other clinical signs of severe colitis are those attributed to the onset of hypovolemic shock and/or toxins in the blood.



A horse usually ingests small amounts of sand, which is transported through the digestive tract along with other intestinal contents, until it is expelled with the faeces. However, if sand uptake is higher than normal, or the gut’s motility is decreased, sand can accumulate in the large intestine. It may take years before enough sand is accumulated to causes obvious signs. Sand can act as sand-paper, abraiding the intestine, which can cause colitis and diarrhoea. A large amount of sand can also be very heavy, so that the intestine pulls on the roof of the abdomen, which may cause pain during activities (such as riding). Some tips to make sure your horse doesn’t accumulate too much sand: make sure you offer hay from a non-sandy ground or surface; don’t let your horse graze from pastures that are too short, so that they don’t pull out the roots of the grass when they graze; check your horse's faeces for sand.

Fig 2 - In extreme cases, sand may be noticed with the naked eye
Fig.2: This horse with diarrhoea had sand in its intestine. Sand can be seen on this slightly more formed ball of faeces. However, this is not usually the case. There is a simple DIY test to heck if your horse has sand in its faeces.


When a horse loses too much fluid and electrolytes, its condition can deteriorate quite rapidly. It is therefore important to find the cause and treat it. Your vet ask you questions about worming, dental care, dietary changes, things that could have caused stress, any drugs that might have been used

and whether any other animals are affected as well. Your vet will also do a clinical exam to assess the horse's temperature, hydration status and the gut’s motility. A faecal sample may be taken to assess for worms, sand and/or bacteria.



Whatever the cause, diarrhoea can cause loss of fluids, electrolytes, protein and energy.

Depending on the type of diarrhoea, your vet may give you dietary advise and/or prescribe probiotics to balance out the gut’s microflora. If teeth or worms are the problem, these will be addressed as well. If the diarrhoea is more profuse, your vet may give your horse an oral product that prevents bacteria’s toxins from doing harm. If your horse seems more severely affected, it may require IV fluids and/or other medication. In some cases a horse may need to be referred for constant monitoring, blood tests and intensive therapy. Endotoxaemia (toxins in the blood) can lead to laminitis, so in cases of severe diarrhoea, the horse's feet should be monitored. Preventative measures may be warranted.


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