Sunday, December 19, 2010

The Equine Digestive System 5 - Cecum

(I've always been fascinated by all things equine, including their anatomy and how their various body systems work.  As a project to learn more about these amazing animals, I've decided to do a series of posts on the equine digestive system.  As these come along, they'll be added to the sidebar.  Please keep in mind that I am neither a vet nor an expert on these topics - if you have corrections and additions or resources to direct us to, please put that in the comments.  Also, you may find it helpful to visit this article, which has excellent pictures of the digestive system and the location of the various organs.)

After the food exits the small intestine, it enters the cecum.  The equine cecum is an anatomical equivalent of the human appendix, but in the horse it is an essential organ of digestion, being a fermentation and break-down vat for the digestion of the tough, structural carbohydrates such as cellulose that compose such a large fraction of the ideal equine diet.  The cecum is a large, muscular organ with its entry from the small intestine and exit to the large intestine at the same end.  It constitutes approximately 15% of the total digestive tract, is about four feet long and holds 28-32 quarts.  Unlike the rapid transit of the foregut - esophagus, stomach and small intestine - the food matter stays for up to 7 hours in the cecum to allow sufficient time for mixing by the muscular walls and fermentation by the bacteria and other microorganisms that reside there.   The breakdown and bacterial fermentation of structural carbohydrates like cellulose produces, among other things, certain fatty acids which are an essential source of energy for the horse.

The cecum has very much the same function as the rumen in ruminant animals, although its location in the equine hindgut (rather the foregut as in ruminants) has some important implications for the horse's diet and digestive efficiency.  Since the fermentation process occurring in the cecum occurs after the portion of the horse's digestive system where most nutrients are most easily absorbed - the small intestine - the horse's digestive system is not quite as efficient at extracting nutrients as a ruminant's is. 

Also, because the cecum is in the hindgut, if a meal is too large or has too many non-structural (soluble) carbohydrates (from grasses that are higher in sugars or fructans, or from grain) are ingested, the size of the meal or the amount of the carbohydrates may overwhelm the capacity of the foregut and cause undigested food to spill into the cecum and large intestine.  The hindgut is not well-equiped to deal with this, and the bacteria in the cecum may produce excess amounts of gas from digesting food components - particularly easily-digested carbohydrates - that should have been digested in the foregut.  The result of this gas can be pain and colic, and also the balance of the digestive microorganisms in the hindgut can also be disturbed with unpleasant results including diarrhea or colic.

The microbial population of the horse's hindgut is specific to the horse and its diet - this is the main reason changes in diet have to be made slowly and carefully - it can take the microbial population up to three weeks to adjust to a substantial change in diet.  And when a horse has been on antibiotics for any length of time, this may adversely affect the microbial population and feeding probiotics (essentially supplemental microorganisms like those that exist in yoghurt) may be necessary to begin to restore an appropriate microbial population.

There are a number of different microorganisms that participate in fermentation in the cecum - bacteria, some protozoa and even some fungi.  This population is also affected by the acidity/alkalinity of their environment - the cecum ideally maintains a relatively neutral pH, in contrast to the acidity of the stomach and the alkalinity of the small intestine.  Certain potentially harmful bacteria, such as salmonella and E. coli, are inhibited by the normal pH balance of the cecum.  Disruption of this balance often increases the acidity of the cecum, leading to the death of desirable bacteria and the proliferation of potentially harmful bacteria.  It is these changes, including the release of endotoxins by dying bacteria, that can lead directly to colic and complications such as laminitis.

Once again, making sure that the horse's diet is primarily composed of high-quality forages, is fed continuously or in frequent small meals, and has appropriately low levels of non-structural (easily digestible) carbohydrates, is the way to optimal equine digestive health.

Once the food exits the cecum, it passes into the large intestine, which will be the subject of the next post in this series.

8 comments:

  1. Good explanation as to why changes in feed and badly balanced feed programs can cause so much trouble.

    Again, thanks for a good post.

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  2. Loving these installments! Another question that I'm sure you'll get to: How long does it take food particles to travel the entire digestive system, start to finish?

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  3. All that footage of intestines! Can you believe it? It is truly amazing to me that digestion happens so well at all. I guess this is true of ANYintestine, but what' s not to love about the cecum? Weird, amazing organ! It is almost like world class ballet happening in the equine gut!!
    I haven't had my computer on in afew days so I have to go back a few posts and see what is gping on with Pie!

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  4. Another great overview of the equine digestive tract. It's so important to understand the hindgut especially, because so much goes wrong there when we feed lots of grains or only a few times a day as you mentioned. Besides the really serious issues like colic and laminitis, there are also problems like hindgut acidosis and colonic ulcers that result. Sometimes these are less obvious, but make a horse quite uncomfortable and unhealthy ... and can be the beginning stages of colic and laminitis. So much to know about the hindgut!

    @Karen - Food moves through the foregut in just a few hours but stays in the hindgut for up to 2-3 days. Most elements (starch, vitamins, proteins) are digested in the small intestine and thus are gone pretty quickly. But structural fibers (ie grass and hay) are fermented and digested in the hindgut and are what provide (or should anyway) most of the horse's energy.

    Another great installment in a great series!

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  5. "It is these changes, including the release of endotoxins by dying bacteria, that can lead directly to colic and complications such as laminitis."

    This is interesting to me because I don't understand how laminitis or founder are really created. I've read it and read it, but still don't get it.

    I've never had a horse get laminitis or founder, and I feed a lot of forage--almost entirely forage.

    Also, I'm wondering if we should grain our horses at all?? If we provide the vitamins and minerals that are deficient in whatever hay we provide them (in our area it's selenium)--do we need to give them grain if they have 24/7 forage?

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  6. *****My inability to understand the compications of laminitis or founder have nothing to do with your post today, I'm referring to other articles I've read trying to understand how to prevent it, and I never really have figured it out. I must be dense!! Lucky for me, it's never happened, and I'm wondering if it's because my horses are fed so much forage--maybe I've inadvertently fended off a host of problems??

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  7. Linda - forage comes first, along with clean water and access to salt. Many horses, particularly those not in heavy work, don't really need anything else. We feed all the horses in our barn a vitamin/mineral balancer pellet that is specially formulated for our area - our part of the country has low-selenium forages, as well as a bit of cocosoya oil. None of our horses are in heavy work, and only a few of them need any supplemental feed, and then we try to use one of the low-NSC feeds - there are several good ones on the market. Similarly, a horse in heavy work may well need extra calories and again a low-NSC feed is the best way to go from the point of digestive health.

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  8. Succeed Equine - good point about hind-gut acidosis and colonic ulcers.

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