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That strange gut feeling

This article has appeared previously with Equestrian Life. To see what's in our latest free digital issue, click here.

It is always important to have a stethoscope in your first aid kit. © Shutterstock

It is always important to have a stethoscope in your first aid kit.

© Shutterstock


By Dr Maxine Brain

Horses are simple-stomached herbivores designed to graze pastures for extended periods of the day and night. That is, until humans started looking after them.

Domestication and modern management practices now used in horse practice have altered the ideal functioning of the gut, potentially compromising the overall health and wellbeing of the horse.  Management practices such as feeding one or two large meals a day, or feeding large amounts of grain with little or no grazing, can damage the intestinal tract. From the formation of stomach ulcers to the production of undesirable acid levels in the hindgut, these changes can affect the health of the horse.

Here I want to concentrate on the negative effects that damage to the hindgut can have on the horse. The horse’s hindgut includes the caecum, which is a large, closed sac-like structure, a small and a large colon.  It extends from the small intestine to the rectum and allows for digestion and absorption of food materials as they pass along its length.  The large intestine is responsible for a lot of the fluid and electrolyte absorption that occurs from the gut.  The microflora in the caecum consist of numerous bacteria, protozoa and anaerobic fungi, all of which are extremely important for the normal functioning of the gut.  The caecum is home to multiple bacteria, some which are important for fibre digestion; others are responsible for the formation of essential vitamins that the horse requires.

The population of microflora in a normal gut are accustomed to an environment of insoluble fibres that the resident bacteria ferment to produce certain short-chain fatty acids (SCFA).  Horses need substantial fibre to produce enough SCFAs to maintain a healthy functioning body. The gut microflora are also not accustomed to the presence of starch in the hindgut, and this is where problems arise from large changes in the diet resulting in starch flowing into the hindgut.

In simple terms, when large amounts of grain are fed suddenly to the horse, the small intestinal cells and enzymes are overwhelmed with the amount of starches and sugars, causing large amounts of these to spill over into the hindgut.  The bacteria in the hindgut, when confronted with these new substrates, begin to produce lactic acid or lactate, instead of the desirable SCFA such as butyrate, which they should produce.  The lactic acid then decreases the pH level in the gut (makes it more acidic), which results in the death of the “good” bacteria and the overgrowth of less desirable bacteria, including some that are potentially fatal.  Lactate can also reduce or stop the absorption of the SCFAs that are produced.  These compound the negative effects, as normally these SCFAs, once absorbed, would offer some protection to the intestinal mucosa. The result is a damaged intestinal wall that does not function as normal.  This allows bad bacteria and the toxins they form to be absorbed into the body and prevent normal absorption of fluid and electrolytes from the gut.  This can lead to a number of issues such as colic, diarrhoea, endotoxaemia and laminitis, all of which have the potential to be fatal.


Colic refers to any form of abdominal pain (see the Jan-Feb issue of Equestrian Life).  In cases of hindgut colic caused by grain overload or sudden changes to diet, the pain is usually a result of the fluid or gas that accumulates in the hindgut, causing stretching of the gut wall and stimulation of the pain receptors.  Often these cases of colic can be resolved medically with no long-term effects, however, there is some thought that the large amounts of gas that accumulate in the caecum and colon can result in the displacement of the gut or, in extreme cases, a twisted bowel.


If large amounts of fluid accumulate in the hindgut and the peristaltic movement of the gut is still functioning, the fluid is passed out through the rectum, producing diarrhoea or “scouring”.  The severity of the diarrhoea and how long it continues will depend on the degree of damage to the intestinal wall and whether there is fluid being drawn from the body into the gut, as well as lack of absorption of fluid back into the body.  In severe cases the horse can become significantly dehydrated and require intravenous fluids to combat the loss of fluid.  Along with this fluid loss, the absorption of toxins and bacteria can occur which can result in a medical emergency.


In the normal healthy gut, “bad” bacteria are often kept under control by the “good” bacteria, so while they may be present in small numbers, they produce no ill effects.  In cases where the pH drops and the acid levels become higher, some of the “bad” bacteria are able to multiply and grow in numbers and exert their deleterious effects on the body. These bacteria can gain entry into the body or release toxins which are absorbed into the blood (endotoxaemia), causing the horse to become extremely ill.  They further damage the gut wall resulting in more fluid being drawn into the lumen of the gut, which clinically is seen as profuse watery diarrhoea. This diarrhoea can be bloody and very smelly when present.  Due to its infectious nature, these horses need to be kept in isolation and strict hygienic measures need to be implemented. The main two bacterial offenders are Salmonella spp and Chlostridium spp, and it should be noted that salmonella in particular can cause illness in humans.


Endotoxins are a form of toxin associated with gram-negative bacteria. When these toxins are absorbed through the gut wall into the bloodstream, they circulate through the body, producing deleterious effects on the heart and vascular system which, if allowed to progress, can damage multiple organs. Horses initially will have a fever and high heart rate, but as the circulation begins to shut down, symptoms such as cold extremities, toxic mucous membranes and limb oedema become apparent. This can lead to renal (kidney) failure, liver failure, loss of motility of the gut, laminitis and death.


It is well documented that feeding large amounts of grain to horses can cause laminitis. This is primarily a result of the excess grain or starch reaching the hindgut, allowing the bad bacteria to multiply and cause endotoxaemia. Essentially what happens is there is a breakdown of the highly vascular structures which form the attachment of the hoof wall to the pedal bone (P3). As the bonds break down, the hoof wall is no longer firmly adhered to the P3 and the bone either begins to rotate inside the hoof wall or begins to sink down towards the sole of the foot. In cases where there are large amounts of endotoxins absorbed into the circulation, such as with grain overload or salmonella, these changes in the foot can occur overnight resulting in severe rotation or sinking. In cases where there is a much smaller dose of endotoxin absorbed, the clinical signs may be much more mild and be detected as slightly increased pulses to the foot or mild shifting of weight between the limbs. Treatment for laminitis will depend on the severity of the condition and the underlying cause (which can involve a multitude of other issues unrelated to the hindgut and thus not mentioned here). 

There are a number of products on the market designed to reduce the risks associated with hindgut acidosis which should help prevent some of the negative issues just discussed.  Whilst most of these products are beneficial in the day-to-day management of the horse, there is no substitution for good feeding practices such as adequate fibre (hay), reduced access to large amounts of grain in a single meal and no sudden changes in diet composition.







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