July 2021 Polo Player's Edition- Belly Buster

By Heather Smith Thomas

The horse’s gastro-intestinal tract is a complex and amazing organ. It performs many functions which include processing and absorbing all the fluids and nutrients that feed and sustain the body, as well as playing a huge role in the body’s immune system. One of the most important functions of the GI tract is to act as a barrier between the external and internal environment—keeping the “good” stuff inside the body and the “bad bugs” out. Even though feed and water ingested by the horse may contain harmful pathogens, the barrier provided by the GI tract is usually sufficient.

Dr. Michael Lindinger, president of the Nutraceutical Alliance (since 2012), and former professor at the University of Guelph for 25 years, has been involved in animal health research for a long time, looking at problems like leaky gut syndrome. He is currently working on nutraceutical product development.

“Leaky gut is merely a syndrome, which means there can be a lot of different causes that can contribute to making the gut leaky,” he explains.

“To put this in a human context, when we look at Crohn’s disease or irritable bowel syndrome or even just acute or chronic diarrhea, these are all examples of leaky gut syndrome. This just means the intestinal wall becomes leaky; fluids are not adequately absorbed into the body from the intestine, and fluids from the body can leak into the intestine—and toxins within the intestine can enter the body and cause many problems. Those leaks can be ulcers (such as gastric ulcers in the stomach, or duodenal ulcers, or colonic ulcers) but often if it’s something like diarrhea or irritable bowel syndrome, it’s actually due to a spreading apart of the cells that make up the intestinal walls,” he says.

The intestinal wall is a very thin, single-cell layer held together very tightly by tight junctions between the cells and other protein elements. The wall is thin because this makes it easier for the body to absorb fluid and nutrients; they only have to pass through that one-cell layer, being transported through and between the cells by very specialized systems.

When that thin wall becomes leaky, pathogens can go from the gut into the body, cause local inflammation and immune responses, and be picked up by the bloodstream where they can act on many tissues. “This is why we humans feel very sick and experience a very bad immune response when that happens. A horse is eating from the natural environment (uncooked food, unlike what we humans are eating) and always exposed to pathogens. These are usually contained within the gut however and don’t get into the body. The healthy gut has mechanisms for detoxifying most of the pathogens. The stomach, for instance, is very acidic and this kills many pathogens,” says Lindinger.

There are also beneficial microbes in the intestine that attack and kill many of the pathogenic bacteria, viruses and fungi that are taken in with the feed.

“A healthy GI tract helps maintain the gut in a non-leaky state and also helps fight off the pathogens that are taken in on a regular basis,” he says.

Pathogenic bacteria and yeast may proliferate within the GI tract under certain conditions, however, and produce toxins that can damage the cells, compromising the tight junctions between those cells. These toxins also cause inflammation of the intestinal wall and increased permeability, and then they can leak through the wall.

“Stress and poor water quality are probably the two biggest factors that can produce ulcers and leaky gut syndrome,” he says.

Abrupt changes in diet, especially sudden increases in grain or fat, can also negatively disrupt the GI tract microbiome (the normal population of beneficial microbes). Changes in diet should always be gradual over a period of at least a week.

There are a number of stressors that can damage the GI tract barrier. Leaky gut syndrome is often the result of two or more stressors acting together, according to Lindinger. For example, performance horses in training and competing in strenuous athletic careers, like polo, consume high starch diets (which can disrupt the normal digestive process) and also experience stresses of rigorous exercise, training and handling. This, compounded with the stress of stall confinement—a very unnatural environment for horses--can negatively impact the GI tract barrier. Transport is also a stress for horses.

“Excessive stress, even for short periods, can result in a loss of homeostasis and subsequent damage to the gut lining. Stress impacts hundreds of different signaling molecules and hormones, such as cortisol and adrenaline. In turn, these molecules effect numerous tissues and organs including the GI tract and brain. Some stressors lead to increased stomach acid secretion which can alter nutrient composition and the intestinal microbiome of beneficial bacteria. Ultimately, stress results in lowgrade inflammation that is often associated with increased intestinal permeability and leaky gut,” he says.

“Most competitive horses get minimal turnout time and that alone is highly stressful. Many trainers also withhold feed (especially hay) before a performance, race or competition and some even withhold water, and this is also very stressful and detrimental to the gut, and the horse’s health and behavior,” he says.

The most common sign of this problem is diarrhea or loose stools. “If this lasts more than half a day, it should be cause for concern and you should probably call your vet, but you can also start giving the horse some probiotics and prebiotics if you have some on hand,” says Lindinger.

Sometimes a horse with leaky gut will be constipated rather than have diarrhea, and show signs of impaction colic. “This is one of the odd things; leaky gut can cause diarrhea or constipation. The latter may be even more difficult to notice, especially if the horse is out on pasture,” he says. If the horse is stabled regularly, or in a small pen where you can readily see the manure, then you would notice. If there are no bowel movements for half a day, this would be cause for concern.

The horse’s attitude may also change. Anyone who knows the horse can readily tell when that horse is off. “The affected horse may lose appetite and is not interested in food, and may become irritable. Other signs may include reduced performance, fatigue, or even laminitis or skin allergies, due to disruption of the immune system,” he says.

The best prevention is simply keeping the horse healthy, providing clean feed and water, avoiding excessive stress, etc. Things that can be done to reduce stress and promote a healthy gut include keeping the horse in as natural environment as possible, and providing good hay and high quality, clean water. “The water should be free of chlorine; it shouldn’t be chlorinated tap water. Uncontaminated ground water or something from a running stream is better because chlorine can disrupt the healthy bacteria in the gut,” he says.

Changes in feed should always be gradual and it helps to make these changes in association with dietary sources of butyrate and an effective probiotic. “Many horsemen now are also using probiotics and other products that are helpful in maintaining or restoring intestinal integrity when it has been compromised,” says Lindinger.

If antibiotics must be given to a horse, people need to realize that even though the horse may need these drugs to combat a disease or infection, there are also some negative effects, especially with prolonged use. Antimicrobials do what they are intended to do—kill microbes systemically or in the targeted local tissues—but they also impact the microbiome of the GI tract, resulting in death of both beneficial and pathogenic microbes, altering the microbial population.

“When systemic antimicrobials are used, it is important to supplement the diet with butyrate, to maintain barrier function, and with probiotics to replace the loss of beneficial GI tract microbes,” Lindinger says.

“Probably the most important probiotics that are known to be safe are the yeasts (Saccharomyces cerevisiae or boulardii); there is a lot of research on these. There are many different strains and some are better than others, though all of them seem to be beneficial,” he says.

“There are also a number of beneficial Lactobacillus and Bacillus bacteria for which there is some evidence-based research supporting their effects. The proven probiotic yeasts (which are fungi) and bacteria have beneficial effects on the horse’s microbiome, which is the entire living community inside the intestinal system. We are trying to maintain a high proportion of beneficial resident microbes compared to the number of pathogenic microbes,” he explains.

There are always some nasty pathogens in the GI tract, and they are necessary—to keep stimulating the body to maintain a highly-functioning immune system—but everything needs to be in balance. “Studies on disease-free mice (raised in an environment with no pathogens) show that when animals are completely pathogen-free they don’t thrive as well as a normal animal that has everything in healthy balance,” he says. Without some “bad” bacteria to stimulate immune response, the animal has reduced intestinal function and immunity and may become more vulnerable to infection or disease.

Providing probiotics is a very natural way to finetune and restore the balance between the beneficial and detrimental microbiome throughout the GI tract. “We see the start of a significant microbiome in the ilium and definitely through the cecum and large intestine. All the way through the cecum and large intestine there is a lot of microbial activity,” says Lindinger. These beneficial “gut bugs” are crucial to digestion of forages as well as to the health of the gut.

“In horses and ruminants these microbes break down the otherwise non-digestible starches and fiber that come in through the diet. These microbes use non-digestible starches as substrate and do the actual digestion, producing a lot of beneficial molecules in that process,” he says.

“The beneficial bacteria and yeasts can also provide substrate for other beneficial bacteria (playing a prebiotic role) and some can actually produce butyrate, which is a very important molecule involved in maintaining the tight junctions between each of the intestinal epithelial cells. Studies in many species of animals (but not yet in horses because they are more difficult to study) have shown that when we provide something like butyrate, we see an almost immediate tightening of these junctions and the gut becomes less leaky,” he says.

In the hindgut, the “good” bacteria also produce volatile fatty acids that serve as nutrients that are taken in through the intestinal wall and utilized by all the cells in the body. “Throughout the intestinal system some bacteria also help the intestine’s epithelial cells produce and maintain a mucus layer which serves as the first line of defense inside the intestine itself.” The mucus lies over the top of the epithelial cells and acts as a protective shield and buffer against irritants and harmful substances and pathogens.

“The mucus also possesses bactericides to kill the pathogens but not the beneficial bacteria. Thus it provides both a physical and chemical barrier against pathogens, yet allows the diffusion of nutrient molecules to get into and go through the epithelial cells,” he says. Anything that disrupts and disturbs that mucus layer can put the horse at risk for leaky gut syndrome.

“While some pathogens are able to directly disturb the mucus layer; mainly it is mechanical disturbances from ingested foreign materials such as sand, wood fragments or splinters, straw, etc. that erodes the mucous barrier. Abrasive feeds or foreign material in the gut can actually pierce the intestinal wall occasionally but this wall is capable of very rapid healing and repair. There is always an ongoing cycle of breaking down and repairing,” says Lindinger. Even gastric ulcers can rapidly come and go as the stomach wall heals itself.

“In the intestinal system this healing is even faster, with replacement of damaged cells and repair of leaks within 3 to 6 hours, with turnover of cells in a matter of 2 to 3 hours. It is a very dynamic system, but energetically expensive to maintain. It is only one cell layer thick, so it is easy to patch and repair as well as damage,” he says.

Use of non-steroidal anti-inflammatory medications should be minimized. He says, “All of the NSAIDs are known to have detrimental effects on horses, causing pockets or areas of insufficient blood flow within the GI tract, creating dead zones. Then we get an area of inflammation and leaky gut, colic and possibly ulcers. There is a very strong association between use of NSAIDs (when used for longer than 3 days) and the incidence of colonic ulcers.”