The Burning Question
Horse & Hound|November 26, 2020
How significant are hindgut issues? Tim Mair FRCVS discusses the horse’s engine: the large intestine
Tim Mair

The horse is a hindgut fermenter, meaning that the large intestine is the site of digestion (fermentation) of ingested fiber. 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 fiber fermentation. Any major upset in this microbial community – termed dysbiosis – can result in “intestinal upsets”, causing diarrhea, 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.

BLOCKED PIPES

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 hospitalized or undergoing surgery and treated with nonsteroidal anti-inflammatory drugs. Signs typically include lowgrade colic along with a reduction or cessation of the number of feces being passed.

If impaction is not recognized or treated, rupture of the caecum can sometimes occur. Since this is usually fatal, it is important to monitor the quantity of feces 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 meters in length. The function of the large colon is to continue the fermentation and digestion of fiber, 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.

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