Digestion.  It is a simple, yet, very complex process in the horse that is often overlooked by owners and veterinarians.  We often see it as just being a matter of putting food into the mouth, then the stomach, and all will be well in the end.  Although that concept is rather simple, the process is much more complex than we’d like to believe or even understand.  Gut health in your horse is not only impacted by the diet that you choose to feed them, but is also heavily influenced by other factors.  In the end, the level of health in your horse’s gut, digestive tract and microbiome, can easily influence the creation of many disease conditions, not to mention lameness ailments.  Do you care to dig deeper?

Horse Feces, Digestion, and Gut Health

Horse Feces, Digestion, and Gut Health

Digestion is the process of breaking down food stuff into smaller components, then absorbing the nutrients contained within to further the health of the organism.  A complex process involving enzymes, acids, and other proteins to accomplish such a feat.  In addition, the various sections of the intestinal tract, from the stomach to the small colon, each play a role in the nutrient and fluid extraction.  In a simplistic form, the foods are broken down higher up in the digestive tract, with most nutrients being absorbed in the small intestine, and then further processed with fermentation and fluid extraction in the large and small colon.  The remaining food stuff, after all processing, is eliminated in the form of feces.

In this series of events, there are a few things to keep in mind.

  1. Each section of the digestive tract must be working properly to perform its assigned task
  2. The food provided must be in optimal form for digestion and nutrient assimilation
  3. The digestive microbiome must be properly balanced and healthy in each section of the digestive tract to assist with the food processing

We, as owners and veterinarians, often realize the first point, but tend to not put as much emphasis on the other two.  This is often due to lack of understanding or having true knowledge of how those factors play into the role of digestion.

Digestion and the Gastrointestinal Microbiome in the Horse

As a veterinarian and researcher, the first two points mentioned above are obvious in most cases, and can stand by themselves as main contributors to gut health in the horse.  What is interesting is that the third point, being the microbiome of the digestive tract, is extremely important and is actually not only dictated by the first two points, but can also influence those points as well.  Essentially, the proper functioning of the digestive tract along with proper nutrition can either benefit or harm the microbiome.  Then again, the microbiome, if not in balance and healthy, can influence the function of the digestive tract and nutrient extraction from the diet being fed.  Taking this into consideration, in our equine patients, the second and third points are of high value in our therapy programs.

The digestive microbiome in the horse is really an organ system by itself and is composed of hundreds of bacteria, fungi, and protozoa that aid in digestion of food stuff, but also impact the health of the patient.  The types of organisms found within the digestive tract can vary dependent on the location, implying that the types of bacteria found in the stomach or small intestine are likely different than those found in the cecum or large colon.  There is a reason for this as there are different functions to those sections, thus different players are needed in the game.

These organisms, when in balance, create health and proper digestion for the horse.  When in balance, food is digested properly, nutrients are extracted, absorbed, and cellular function is supported.  When these organisms are out of balance, bad things can happen on many levels.  This can be seen in cases of:

  • Allergies
  • Uveitis
  • Body condition concerns
  • Gastric or hindgut ulcers
  • Irritable bowel disease
  • Diarrhea
  • Metabolic syndrome / laminitis
  • Behavioral problems
  • Many other conditions

Modern research in human medicine has pointed us in the direction in making the connection between the digestive microbiome and many health conditions.  In equine medicine and research, the data is lacking and lagging behind by many years, however, some data is coming out slowly.  Despite there being little data on the equine side, the connections have been properly demonstrated in human medicine, thus we would be incorrect in assuming the same does not hold true in our equine companions.

The Digestive Microbiome and Health in the Horse

The exact or precise mechanisms on how the microbiome can impact health in the horse are likely too numerous to mention, or in some cases, we simply do not know or understand.

There are however, a few mechanisms that we do understand.

  1. The microbiome when in balance supports proper food digestion and nutrient extraction which then impacts normal health, cell function, repair, immune health, and vitality.
  2. The microbiome when out of balance can create ill side effects due to noxious substances produced within the gastrointestinal tract, including abnormal fermentation byproducts.
  3. The microbiome when out of balance can instigate inflammatory and immune related changes on a gut level, which can then lead to systemic inflammatory and immune problems.
  4. The microbiome when out of balance, can contribute to increased permeability of the gastrointestinal lining, which then can lead to leakage of food stuff, bacteria, and toxic components (endotoxin) into the general circulation, contributing to more inflammation within the horse.

All 4 of these factors are vital to the health of the horse, with the 3 later factors often being closely linked in with chronic inflammatory conditions.  In many cases of health related problems in the horse, including laminitis and equine recurrent uveitis, there is an ongoing inflammatory event in the patient. The concept of the microbiome and leakage of endotoxin due to increased permeability, might just be the source we have been seeking in these chronic inflammatory equine patients.

Diagnosing the Imbalance in Gut Health in the Horse

Knowing or seeing something is one thing, proving it is another.  For many, hard proof is not needed, but more so a simple response to a therapy approach is all that is needed to prove a suspicion.  For others, hard facts are required.  Either way is okay, but the question remains as to whether if you will hold firm in your beliefs while a horse suffers due to this lack of hard evidence?  Equine medical research is far behind human medicine.  It may be a decade or two before the true connections are made and hard data is revealed.  Do you want to wait that long or do you start to use information from the human side?

In human medical research, the links between a digestive microbiome imbalance, termed a dysbiosis, has been suspected for many years.  More modern research is making connections, revealing obvious imbalances in the microbiome that are closely linked with the development of many health conditions from arthritis, to Alzheimer’s, to cancer.  In almost every health condition impacting the human, a link can be made with digestive health.  The question is why and what is the imbalance?

Most equine and human research data, in current times, is using DNA sequencing of fecal matter or food stuff retrieved from different sections of the digestive tract.  This DNA sequencing allows the researcher to get a better feel for the overall bacterial population and how well certain groups are represented.  Keep in mind that digestive microbiome consists of likely hundreds of different bacterial species, so it is not a matter of looking at just one type, but seeing the bigger population.  Unfortunately, this DNA sequencing is only available via research and is very expensive.  In the end, this methodology is seen as the gold standard means of evaluating the microbiome and diagnosing an imbalance or dysbiosis.

Is this really the only way?  If so, how are we to then make a diagnosis if we suspect there is a problem?

This may be partially why many veterinarians are not on board with the concept of a dysbiosis in the horse, mainly because testing is not possible.  If we can’t test, then there is no sense in looking?  If we can’t test, then there is no reason to truly understand because there is no perceived solution or treatment.  So, why bother?

Considering the vastness of the bacteria involved with the microbiome, it is next to impossible, nor desirable to pin the problem on any one type.  In human medical research, they have concluded that it is really about two large phyla of bacteria and the balance between these two that can dictate health in the patient.  These two groups are Bacteriodetes and Firmicutes.  They look for overall balance between the two groups, ideally having both well represented.  In many cases of disease, at least in human research, there is noted to be an over-growth or over-representation of bacteria in the phylum Firmicutes.

Interestingly enough, when you dig back into equine research, you can find data going back a decade or so that indicates that many horses with metabolic type of conditions, including pasture associated laminitis, have an overgrowth of Lactobacillus and Streptococcus bacteria, which are both lactic acid fermenting bacteria.  This information was known before the advent of DNA sequencing and was likely accomplished via routine culture plate technology using fecal samples.  The interesting part is that these two bacteria types belong to the Firmicutes phylum, so it is not far fetched to assume that horse health may also rely on a balance between Firmicutes and Bacteriodetes.  It may mirror human medicine and research.

Okay, so now that you know this, how do you determine if there is a problem in your horse?  The DNA sequencing is not readily available and costly even if available.  It is not a routine test that your veterinarian can order for your horse.  Given this, you have two options from my point of view.  The first option would be to almost assume the dysbiosis is present, if there is a health condition in your horse. The second option would be to pursue more focused fecal testing or cultures to see if you could demonstrate a higher level of bacteria present that belong to the Firmicutes phylum, when compared to a normal horse population.  I, personally, tend to do both in our patients.  For many I assume the condition is present and pursue treatment.  In others, I will perform fecal cultures to evaluate Lactobacillus and Streptococcus levels in a given fecal sample. It is a crude means of testing, and often perceived as rudimentary by many researchers, but simply put, it works and helps to demonstrate that a problem exists.

In our private research, culturing over 300 fecal samples, there is an obvious trend noted, with about 80% of all chronic inflammatory equine patients demonstrating overgrowth on culture plates.  These horses can be contending with allergies, recurrent uveitis, metabolic conditions, digestive complaints, or laminitis.  What is even more interesting is that with modification of the microbiome, taking steps to aid with balance, repeat fecal cultures in 2 weeks demonstrate improvement and additionally, the horses are improved clinically.

Now that you can demonstrate it, in a crude way, what do you do?

Fixing the Equine Digestive Microbiome

Seeing the problem and correcting it are two different things.

First, you need to understand that this microbiome is not just about food digestion, but these bacteria can produce byproducts (chemicals) that can either enhance health or mitigate it.  This can impact the horse not just on a digestive level, but also physical level, immune level, healing level, inflammatory, and even a cognitive level.  This microbiome is not just something that is obtained or lost, in the horse, but is an organ system that requires physical support to maintain.  It is also under the influence of many other factors, so often there is no one remedy.

Second, you need to realize that the dysbiosis is not about any one organism, but more so the problem is connected to many types.  Thus, the solution is not likely in a medication or antibiotic, targeting one specific type of bacteria.  If you were to do this, then likely the end result would be more dysbiosis or imbalance, as the medication chosen would impact other bacterial types additionally.  This approach has been noted with the creation of Virginiamycin many years ago, which was an antibiotic used in research to combat a perceived overgrowth of Lactobacillus and Streptococcus bacteria in horses with laminitis. On paper, the theory worked.  In reality, the medication created bacterial resistance and further dysbiosis in the horse.  Even in our clinical fecal cultures, I am not targeting those two organisms but am more so using their increased levels to indicate an overall pattern in that patient, which is a dysbiosis.

In order to resolve or correct that dysbiosis, one needs to look at all factors involved in the horse. With that being said, probiotics are not a given solution at least in my perspective, mainly because the types of bacteria used in those formulas are often related to the Firmicutes phylum.  Research does not support probiotic use in the horse, outside of some foal diarrhea studies, and worst case scenario may be that through their use you may be adding to the dysbiosis.  Plainly speaking, resolving the dysbiosis is more complicated that simply thinking a probiotic will remedy it.  Human microbiomes can and do differ from horses, based on diet and digestion.  Thus, the probiotic research does not always carry over well.

In my patients, I tend to look at a few factors in helping that patient:

  1. Current diet, including grains and forage source
  2. Stress levels (physical and emotional)
  3. Current inflammatory status
  4. Medications

These 4 factors are the main contributors to a dysbiosis in most equine patients.  When we address each of these properly, then the dysbiosis is almost self-correcting, without the need for medications or other interventions.  It is not a simple fix, meaning a one-step process, but more so a matter of taking a look at the bigger picture, determining the contributors, and modifying them to the best of our ability.  This then yields the most optimal results.

In another article, I will take a look at these 4 factors in more detail and how they can be modified to better promote health and balance for the microbiome in the horse.


Further Reading:


Author:  Tom Schell, D.V.M., CVCH, CHN







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