The digestive tract of the horse is vast, encompassing about 100 ft in length from the stomach to the rectum. Each section of the gastrointestinal tract has its own job, and each section is influenced by health and balance in the other. In most cases, the digestive tract health in the horse only becomes a concern in cases of ulcers, colic, or diarrhea, but there could be problems lingering deeper outside of these areas. Could the digestive tract in the horse actually impact overall health and even soundness? Could your horse with ongoing joint, tendon, allergy, laminitis, or metabolic concerns actually have a gut problems and dysbiosis beyond your recognition clinically? Are you properly managing your horse’s gut health or is this potentially an illusion, or false sense of security? The answers lie in a deeper understanding of the problem. Let’s lift the covers on this topic and dig deeper.
In past articles, we have discussed the importance of gastrointestinal health in the horse on many levels. In a specific article, we discuss leaky gut syndrome and how this inter-plays with health and even soundness, but yet the problems goes much deeper. This is also a topic that I go into deeply in our book Seeing the Whole Horse. I tend to harp on this topic as it is very important not just in promoting health but aiding in recovery from lameness conditions. It is important to grasp even one aspect of the problem because the more you can understand as an owner, the more you can implement to aid in recovery and/or prevention.
As horse owners and even veterinarians, we often just look at the gastrointestinal tract when it rears it’s ugly head in disgust. This would be times of colic, especially recurrent colic, ulcers, diarrhea, and other ongoing conditions including inflammatory bowel disease (IBD). Although the gut is directly involved in these conditions, the clinical manifestations we are seeing are the end result of an ongoing problem. Essentially, by the time you see that ulcer, colic, or IBD horse, problems have likely been present for months if not years. The clinical manifestation is essentially like a pot boiling over. The problem has peaked and the end result is clinical disease.
Gastrointestinal Dysbiosis of the Microbiome in the Horse
Dysbiosis. What exactly does that term mean? By definition, dysbiosis refers to an imbalance or maladaptation of bacteria within the body, often the gastrointestinal tract but it can be in other areas additionally. Looking at the gastrointestinal tract of the horse, there are likely thousands of bacteria, yeast, and protozoal organisms that call it home. These organisms in a state of health are in balance or symbiosis. When the general population shifts from one extreme to the other, disease is created as is dysbiosis.
As a whole, we as horse owners and veterinarians don’t pay these bacteria much attention. We assume that all is well in the world of the gut and don’t give it another thought. The only time we may raise an eyebrow is in cases of Potomac Horse Fever, Clostridiosis, or Salmonella, in which case the horse can become quite sick, often with diarrhea. Even in those cases, we tend to think that the horse has contracted the illness, the bacterial infection, but in reality, many cases are actually due to dysbiosis.
In the equine gut, there are many species of bacteria. Clostridia and Salmonella are normal inhabitants, but at low levels. When a horse is stressed, due to illness or otherwise, that stress along with other factors including diet and medications like antibiotics, can kill off other bacteria and then allow others to increase in numbers. Thus, a horse that is stressed and hospitalized for laminitis, as an example, can often develop a Salmonella infection and diarrhea, often becoming toxic and critically ill. Likely, the horse did not contract the Salmonella in the hospital, but more so, the stress and medications allowed for a shift in the normal bacterial population. This shift favored growth of Salmonella or Clostridia.
The normal balance of the gastrointestinal bacteria, or microbiome, is extremely important not just in digestion but in overall health and even soundness. A shift one direction or the other can potentially contribute even to early or advancing osteoarthritis, not to mention insulin resistance, allergies, laminitis, and a host of other conditions. This shift could also negatively impact performance, stamina, and weaken hoof and tendon structures making them more prone to injury. All of these negative effects are a result of inflammatory changes produced due to the dysbiosis and also alteration in digestion and end products produced, including volatile fatty acids.
Factors Contributing to Dysbiosis in the Horse
There are many factors that contribute to dysbiosis of the gastrointestinal tract in the horse. Often, there are so many that it can become a challenge in regards to management.
Here is a list of the main ones that I consider:
- Diet (including forages, pastures, and supplements)
- Stress (mental and physical)
- Pain (which goes back to stress)
This may seem like a short list, easy to manage, but it is not. However, it is not impossible either. Just more complicated in some cases more than another. Each of these factors is not an isolated entity, but more so each factor can and does influence the other. So, how do these factors influence GI health?
Diet and Dysbiosis in the Horse
Bacteria in the gastrointestinal tract are dependent on food sources for their own viability. Some bacteria thrive off of some food sources, while others thrive off of another. This is where the term prebiotic comes into play. A prebiotic is a nondigestible food source that promotes the growth of beneficial organisms in the gastrointestinal tract. In most cases, prebiotics are non-digestible starches or fibers and are found in certain foods that are consumed. Depending on the diet provided to the horse, you can shift that bacterial microbiome one direction or another.
As an example, let’s assume you have a horse that just consumed 25 lbs of grain. This is termed a grain overload by most veterinarians. This is a serious, potentially life threatening situation. Why? The reason is that the grains are often high in carbohydrates, and when consumed in large volumes the sugar intake can be extreme, flooding the hindgut with digestible starches and sugars. These digestible sugars are favored by one general population of bacteria, termed lactic acid producers, which ferment those sugars as a food source. Given that they have food, their numbers multiply and things change. As they increase in numbers and ferment, the acidity or pH of the hindgut changes. This change then is not favorable for most ‘good’ bacteria, so they die off or decrease in number. This then allows the ‘bad’ bacteria to further increase in number due to lack of competitive inhibition. As we progress further, the environment becomes more acidic, which causes damage to the GI lining creating ulcers and acidosis. The immune system begins to respond as it views the problem like an infection, which produces inflammation. As inflammatory changes occur, you then develop damage to the gut lining further, leaky gut, which is due to break down of tight cell junctions. Due to the ‘bad’ bacteria overgrowing and a leaky situation, the bacteria then begin to cross over into the bloodstream. One of particular importance here are gram negative bacteria, including E. coli, which contain a LPS tail (Lipopolysaccharide). This LPS tail when in the blood stream then triggers a systemic or whole-body inflammatory response, which is termed endotoxemia. Endotoxemia then contributes to further organ damage, fever, changes in circulation, laminitis, shock, and in many cases, death.
Sounds like a pretty bad situation, right? It is and can be life threatening in a short period of time. These horses can have acute laminitis, rotating through the sole of the hoof within a matter of days, all as a result of the toxic and inflammatory events that have unfolded. This is just one extreme example of dysbiosis, but demonstrates how the diet can really factor in and play a role.
Now, let’s take it down a notch. The grain overload situation is extreme but is occurring in the average horse on a daily basis on a much lower level. Many horse owners in the competitive world are feeding a lot of grain, often beyond the normal level of carbohydrates than the GI tract can handle. So, on a low level, you may have this ‘leaky gut’ syndrome, dysbiosis, and even LPS leakage on a daily basis. This then triggers ongoing inflammation in that horse and may actually be one primary reason as to why you may struggle to find solutions for that lameness or health issue.
Aside from generalized inflammation and LPS leakage, the dysbiosis also leads to digestive concerns which means that the nutrients in the diet may not be absorbed as well as you’d like. Many see a perceived nutrient deficiency in their horse and feel the need to supplement specifics, such as vitamin E, selenium, magnesium, or even b-vitamins, but this may not be the solution. You may be dumping synthetics into a leaky container that is not capable of using them. In fact, in many cases, the over-use of synthetic nutrients may actually further contribute to the problem.
Stress and Influence on Dysbiosis in the Horse
Stress is a major problem for the horse, just as it is in humans. Stress can be mental or physical. The stress of trailering, training, and competition can be both mental and physical. The stress of recovering from a tendon, joint, or hoof problem can also be physical and mental. Pain creates stress and stress creates pain, so they are in the same category. Either way you cut it or dice it, stress is present in almost every horse. Even in that horse that is out on 10 acres with not a care in the world. In their case, too much grass exposure is like a kid in a candy store. The lush pastures lead to excessive carbohydrate intake in a short period, but also lead to less activity due to not needing to move much to eat. It all creates stress on the body to some degree.
The stress response is vast in the body leading to alterations of the gastrointestinal tract, mental health, and even the immune response. This is not a hard concept to wrap our heads around. Think of the last time you were really anxious or angry about something. Likely, you felt it in your belly to a degree by cramping, gas, bloating, ulcers, loss of appetite, or even diarrhea. That’s the ‘gut-brain’ axis at work. The neural connections of the brain and central nervous system alter the function of the enteric or ‘gut’ nervous system. Alter it even for a short period of time and you can create dysbiosis. Short term conditions often rectify and rebalance after a minimal period of time once the original stressor is resolved. However, the longer that stress response, the more damage not just in the GI tract, but the entire body.
The biggest impact on the GI tract is through the stress response altering motility or movement of the intestines. This is the transit time and is the time it takes for food to pass from one end to another in the GI tract. When the horse is stressed or you are stressed, this transit time can be altered. Sped up in cases of diarrhea, reduced in cases of constipation. As the transit time is altered, bacterial populations are also altered due to change in environment and also increased or reduced exposure to food stuff. As bacterial populations shift, more fermentation may take place, creating more gas. Populations can further shift to where diarrhea is present with increased transit times. Reduced transit times in the horse can be seen physically in the appearance of the feces as well. Hard, crumbly fecal balls are a sign of low level dehydration, which can be due to diet or slowed fecal transit time resulting in increased absorption of fluids. In some cases, the fecal balls may have a shine to them, which is actually mucous accumulation. This again is a result of a slowed fecal transit time.
Genetics and Influence on Dysbiosis in the Horse
Genetics are a complex topic and right now, there is no definitive data out there that clearly connects specific genes as a cause of dysbiosis. More so, what is noted is that with the act of dysbiosis, you can have altered gene expression as a byproduct of the inflammatory events in the body.
As an example, a horse may have a family history of insulin resistance and genetically more prone to that condition phenotypically due to weight gain, but this is not necessarily a cause. Genetics are often blamed for many health conditions, when in fact those genes can be regulated tightly by other factors in the body. That horse with a family inheritance of a predisposition to insulin resistance may be perfectly sound and healthy for many years, but then something changes. Maybe it was a high grain intake, overexposure to a lush pasture, a stressful event or injury, or even retention of fetal membranes post foaling. That event then led to a dysbiosis in the gastrointestinal tract, which through a sequence of events and inflammation, activated the genes associated with insulin resistance. This is essentially going into the world of epigenetics and how the environment alters gene expression. Despite, it does show us how we can have two horses that consume too much grain or retain membranes or otherwise, but both do not develop insulin resistance or even laminitis. Likely there is a genetic component, a switch, which was then activated as a result. Therefore, you should not just use the excuse that your horse is insulin resistant or foundered because his or her parents were that way too. That may be true, but it’s just possible that an event or sequence of events led to the activation of those genes. Likely what you are doing for that horse, in regards to feed, pasture, and other stress factors, is the same as what was done for his or her parents. Thus, genes were triggered in both groups.
Concluding and Further Thoughts
The more research that I conduct and review, along with more interaction with equine patients on consultations, the problems become very self evident. In our facility, I do routinely culture horse feces whether if they are a patient or even one of our rehab horses. The more I culture, the more self evident the problem is that a dysbiosis exists. The reality is that even in our facility, my abilities to fully evaluate are limited.
What I do see is that no matter what the problem is in the patient, as I achieve or restore balance to the microbiome in the horse, the more easily the problem is to manage. Pain becomes more controlled, tissues mend more readily, insulin levels balance out, allergies are more controlled, and even many laminitic horses become more comfortable.
I am sure everyone wants to know what that ‘silver bullet’ fix is to the problem, but in all honesty, there is no one single ‘fix’. There is no cure, but more so it boils down to management of many factors and using what is available to us to modify and support the bacterial population. That involves diet, stress modification, pain control, proper foot trimming and balance, and even exercise. Then we have specific supplements that are being used and explored to further impact and stabilize the bacterial population.
Many of our current Cur-OST Equine formulas do assist with management of dysbiosis, often beyond my original expectation. I actually think that in many horses, the formulas are exerting their positive effects strictly through modification of the bacterial population. When we make changes to the diet, such as cleaning up the diet, or feeding higher quality hays and increasing pasture time or socialization, you are impacting the gut microbiome positively. Of course, you can make the wrong changes and shift things the other direction as well.
In future articles, I will discuss how alterations in the microbiome influence health and inflammation, the exact impact of diet and stress on the microbiome, and areas we are exploring in our research to stabilize the situation. Some of them are pretty simplistic while others are more revolutionary.
Author: Tom Schell, D.V.M., CVCH, CVH