How significant are hindgut issues? Tim Mair FRCVS discusses the horse’s engine: the large intestine
The horse is a hindgut fermenter, meaning that the large intestine is the site of digestion (fermentation) of ingested fibre. This is where food is converted into energy, which fuels bodily functions such as movement and helps the horse to maintain body heat.
The population of bacteria and other micro-organisms present in the hindgut is responsible for this fibre fermentation. Any major upset in this microbial community – termed dysbiosis – can result in “intestinal upsets”, causing diarrhoea, colic and weight loss.
Conditions affecting the large intestine, sometimes referred to as “hindgut issues”, are often a topic of debate. While scientific evidence regarding the prevalence and significance of certain problems may be lacking, many disorders in this section of the digestive tract are recognised and understood.
The hindgut comprises the caecum, large colon, small colon and rectum. The caecum is a large, blind-ended sac that occupies much of the right side of the horse’s abdomen and functions as a “fermentation vat”. Here, fibrous food material is stored and fermented for several days before passing into the large colon.
Impaction of the caecum with feed material can occur in mares after foaling and in horses who are hospitalised or undergoing surgery and treated with nonsteroidal anti-inflammatory drugs. Signs typically include low-grade colic along with a reduction or cessation of the quantity of faeces being passed.
If an impaction is not recognised or treated, rupture of the caecum can sometimes occur. Since this is usually fatal, it is important to monitor the quantity of faeces passed by these at-risk horses and to feed them a laxative diet such as bran mashes and grass.
From the caecum, food passes into the large colon – a wide, tubular structure measuring up to 3.7 metres in length. The function of the large colon is to continue the fermentation and digestion of fibre, producing volatile fatty acids which are then absorbed into the bloodstream, and to absorb water.
The large colon has a number of U-shaped bends, or flexures. Its convoluted course and a relative lack of any anchoring ligaments make it susceptible to displacements and twists, which are common causes of colic.
Impactions can also occur at the flexure sites where the diameter of the colon narrows, especially the pelvic flexure. Colonic impactions are most commonly seen in horses that are suddenly exposed to management changes, such as alterations to diet or exercise.
Clinical signs are similar to those for caecal impactions. While colonic impactions are usually easier to treat, it can still be intensive and costly, so prevention – that is, feeding at-risk horses a laxative diet – is always the goal.
The final phase
The small colon and rectum, the final parts of the hindgut, are involved with further absorption of water. This is where faecal balls are produced.
Impactions of the small colon and rectum can occur but are fortunately rare. Mild dysfunction of the small colon can be associated with the passage of “faecal water”, whereby horses pass normal, formed faecal balls followed by a volume of malodorous liquid. The precise cause is often uncertain, but it can be associated with age-related damage to the wall of the small colon and rectum, or dysbiosis.
Colitis is defined as inflammation of the colon. Since one of its major functions is the absorption of water, damage to the wall of the colon of the adult horse, caused by inflammation, frequently results in diarrhoea.
The hindgut is not fully developed in foals before weaning; in this age group, diarrhoea is more commonly due to inflammation of the small intestine – termed enteritis.
Mild diarrhoea can occur for many reasons, including a change of diet, stress and access to fresh grass. In these instances, it is usually self-limiting and of little significance.
On occasion, however, colitis can be life-threatening. These cases are often associated with overgrowth by certain species of bacteria, such as Salmonella and Clostridium species, which can invade into the lining of the caecum and large colon, and may produce toxins that damage the large intestinal wall. Some of these diseases can be contagious to other horses and to people.
Another important cause of colitis is larval cyathostominosis, which occurs due to damage to the large intestine by migrating larval stages of the small redworm. Severe colitis can be fatal rapidly unless intensive treatment is instituted early; for this reason, prompt veterinary advice should be sought for any horse who develops watery diarrhoea.
More chronic forms of colitis also occur sporadically. These inflammatory bowel diseases are similar in some respects to those seen in humans, such as Crohn’s disease. In horses, however, they are more common in the small intestine than the large intestine and are usually associated with progressive weight loss. If the large intestine is involved, then chronic diarrhoea ensues.
Some inflammatory bowel diseases can result in increased intestinal permeability. In these cases, the term “protein-losing enteropathy” is more appropriate than the poorly defined and controversial “leaky gut syndrome”, which suggests that the lining of the intestine leaks plasma proteins due to inflammation or ulceration.
Are ulcers to blame?
Ulceration of the stomach, termed gastric ulceration, is a common condition in performance horses. The associated clinical signs can be varied and vague. A horse may become picky with short feeds, eating a little and then walking away. Other signs include grumpiness, loss of weight or coat lustre, decreased exercise tolerance or performance, and sensitivity when the girth is tightened or the abdomen brushed.
Diagnosis of gastric ulceration has become commonplace since the introduction of endoscopes. But what about ulceration of the small intestine or hindgut? Many people believe that ulceration of the hindgut can produce symptoms similar to those of gastric ulceration, but there is currently no easy way to diagnose this problem in the live horse.
Right dorsal colitis causes severe damage to a defined area of the large colon and is usually associated with long-term administration of phenylbutazone. The diagnosis of this condition is often uncertain, unless it is confirmed by surgical exploration and biopsy.
Clinical signs may support a tentative diagnosis but, of course, the reality is that ulceration and damage to other areas of the hindgut could present the same signs – including colic, weight loss, faecal consistency variation and irregular appetite, as well as behavioural issues relating to chronic abdominal discomfort.
Ref: Horse & Hound; 26 November 2020
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