Getting to know the horse’s microbiota
The horse is a non-ruminant herbivore. Herbivore means that horses live on a diet of plant material. Non-ruminant means that horses do not have multi-compartmented stomachs as cattle do. Instead, the horse has a simple stomach that works much like a human’s.
Wouldn’t it be interesting to take a tour of the horse’s gut and find out what happens inside? Who are the key players that are responsible for maintaining a healthy gut environment that ensures good nourishment, energy, stamina, and immunity for this large animal. Imagine you have shrunk in size and have boarded a tiny spaceship that will enter the interesting world of the horse gut. As you travel through the different compartments of the horse’s gut, let us introduce you to the characters and events happening inside. So buckle up and let’s begin this exciting tour!
How is the feed processed in the different compartments of the equine gut?
Let’s start with the stomach. This is where the horse’s feed lands after being partially digested by some salivary enzymes and moving through the oesophagus. The horse’s stomach is 9-15 liters in volume and makes up for 10% of its digestive capacity. Here, whatever the horse eats gets mixed with pepsin (an enzyme to digest proteins) and hydrochloric acid to help break down solid particles. Here you will see four major types of secretory cells:
- Mucous cells: secrete an alkaline mucus that protects the epithelium against shear stress and acid
- Parietal cells: secrete hydrochloric acid
- Chief cells: secrete pepsin
- G cells: secrete the hormone gastrin
Pepsin and stomach acid initiates the digestion and degradation of lipids (fats) and proteins (amino acids).
This partly digested matter passes from the stomach into the small intestine. The small intestine is approximately 28% of the horses’s digestive tract, is 15 – 22 m long and has a volume of 55-70 litres. This is the major site of digestion in the modern performance horse. In the small intestine, the digestive process (enzymatic breakdown of proteins, fats, starches and sugars) continues.
There are many components to this digestive process. Pancreatic enzymes will help digest the food; carbohydrates digest sugars and starches; proteases break proteins down into amino acids; lipases and bile from the liver is added to emulsify (break into smaller units) fats and to suspend the fat in water. Bile constantly flows into the small intestine from the liver because the horse does not have a gall bladder in which to store it. The pancreatic juice also contains some alkali and bicarbonates, which buffer the acidic material leaving the stomach, and help maintain an optimal environment for the functioning of the digestive enzymes.
After the feed has been digested, it is absorbed through the walls of the small intestine and carried off by the blood stream to whatever cells need the nutrients. Nearly 30-60% of carbohydrate digestion and absorption and almost all amino acid absorption occur in the small intestine. Fat soluble vitamins A, D E and K are absorbed in the small intestine as well as some minerals such as calcium and some phosphorous.
There is a wide variety of cells you will see inside the small intestine. The enterocytes are absorptive cells responsible for absorbing nutrients from the feed. The goblet cells secrete mucus to promote movement and effective diffusion of gut contents. The enteroendocrine cells secrete hormones to regulate secretion in the gastrointestinal tract (GI tract). The tuft cells orchestrate immunity against the parasites.
Peyer's patches are small masses of lymphatic tissue found throughout the ileum region of the small intestine. They form an important part of the immune system by monitoring intestinal bacterial populations and preventing the growth of pathogenic bacteria in the intestines. M cells are found in these Peyer’s patches. They are specialized cells that form a pocket-like structure where immune cells gather and neutralize invading pathogens.
Villi are small, finger-like projections that protrude from the epithelial lining of the intestinal wall. They increase the internal surface area of the intestinal walls which increases the surface area available for nutrient absorption. Lacteal lymph vessel is one of many central intestinal capillaries in the villi of the small intestine.
The duodenum is the first part you would enter the small intestine through. This part is covered by specialized glands called “Brunner’s glands” which secrete a protective alkaline mucus that neutralizes the acidic contents from the stomach entering the intestine. The jejunum is the longest and most ‘typical’ region of the small intestine. You will see very long villi in this region as you pass. Ileum is the last part of the intestine where goblet cells and Peyer’s patches are more abundant.
The hindgut or large intestine, consists of the caecum, large (or ascending colon, small colon, rectum and anus. Here is where a bulk of the digestive more is done. The hindgut comprises 62% of the entire gut. It is approximately 7 meters in length and has a volume of 140-150L. Digestion in the hindgut is largely microbial rather than enzymatic. Digestion is performed by billions of symbiotic bacteria which efficiently break down plant fibres and undigested starches into simpler compounds called volatile fatty acids which can be absorbed through the gut wall.
The caecum is a microbial inoculation vat. Feed remains in the caecum for about seven hours, allowing bacteria time to start breaking it down using the fermentation process. These microbes produce vitamin K, B-complex vitamins, proteins, and fatty acids. The vitamins and fatty acids are absorbed, but little if any protein is absorbed.
As you move further to the large colon, you will see that the region has a ‘sacculated’ appearance – that resembles a series of pouches. This design facilitates the digestion of large quantities of fibrous materials but it can also increase the risk of developing colic. Microbial digestion (fermentation) continues here, and most of the nutrients made through microbial digestion are absorbed here,, as well as B group vitamins produced by the bacteria, and some trace minerals and phosphorous.
Next you will travel to the small colon whose diameter is much less compared to the large colon. By now the vast majority of the nutrients have been digested, and what is left cannot be digested or used by the horse. The main function of the small colon is to reclaim excess moisture and return it to the body. This results in fecal balls being formed. These fecal balls, which are the mostly indigestible portion of what was fed some 36-72 hours ago are then passed to the rectum and expelled as manure through the anus.
Who are the bad guys?
You will see during this long journey many specialized immune cells, patrolling the various niches of the horse’s gut to stop any foreigners. Who are these foreigners and how are they dealt with? A term commonly used for the foreigners is antigens. Simply put, an antigen is any foreign substance that stimulates an antibody response. Antibodies are specific proteins that are produced by the horse’s body to engage in combat against specific antigens and eliminate them. Antigens include bacteria, viruses, and parasites. Bacterial pathogens such as E. coli and Salmonella commonly invade the horse’s gut. Clostridioides difficile, Clostridium perfringens and Neorickettsia risticii (Potomac horse fever) are also fairly common. Fusobacteria, although rare in healthy horses, seem to be significantly enriched in cases of diarrhea and colitis. Crytposporidium parasites also cause infections in horses worldwide. Diarrhoeal disease and abdominal pain are very common with this infection. Viruses such as rotaviruses and coronaviruses are another bane for the gastric and respiratory functions of the horse. Production of toxins by pathogens can promote cellular damage through disruption of intracellular protein interactions, leading to increased cellular permeability, and ultimately trigger cell death. As the immune cells of the horse are recruited to the sites of infection and cell damage a cascade of inflammatory events is triggered which damages the gut lining affecting absorption capacity.
Molds and fungal infections caused by species such as Aspergillus can also cause trouble to the equine athlete. Mycotoxins are harmful secondary compounds produced by molds that are found in the soil and vegetable matter including grains, forages and feed. They can be formed in the field both before and during harvest, and can continue to be formed under suboptimal storage conditions after harvest. These toxins can cause a wide range of clinical signs in horses, including respiratory, gastrointestinal, neurologic, and reproductive problems–even death.
Who are the watchful sentinels?
As you travel through the gut, you will notice the continuous battle on-going between these pathogens and the horse’s immune system. The immune cells and antibodies they make trap and eliminate any foreign material and invaders that succeed in breaching the outer defences. Some of the many vanguards you will see are: neutrophils, eosinophils, basophils, macrophages, dendritic cells, and T cells. These different cells scout out antigens and ‘eat’ them or neutralize them. They also release important signals, called cytokines, to attract more protective cells like themselves when any foreign invaders are sighted. Neutrophils are part of the horse’s security that can attack invaders quickly but is incapable of sustained attack. This is where you will see the macrophages swooping in to continuously ‘eat’ any foreign material. This dual attack ensures that none of the invaders survive. The eosinophils are meant to ‘eat’ parasitic larvae that can colonize the horse’s gut. The primary work of the basophils is to provoke an inflammatory response where antigens are deposited. Dendritic cells ‘digest’ the foreign invaders and present the antigen signature to other cells of the horse’s immune system alerting them of the attack. You will see many of these guard lying in wait in the Peyer’s patches of the horse’s gut, armed and ready to eliminate the next threat that comes along. These vigilant cells are also equipped to neutralize the toxins released by the invading pathogens, e.g. mycotoxins.
Another important component in the aftermath of the destruction of the invaders is battle–memory. T-cells play a major role here as they ‘retain the memory’ of the attack and the invaders. Once they recognize the invaders the T cells will quickly multiply creating an army of identical cells to fight the infection. They are specialized defenders that can remount an effective attack when the same invaders are spotted again.
What is the role of good bacteria?
Not only the immune cells, but there are also many beneficial microbes residing in the horse’s gut, that are guarding the lining of the gut and preventing pathogens from binding to cells on the intestinal walls and colonizing them. Horses can host up to 1015 microbial cells with the majority of microbes residing in the colon, especially within the comparatively enlarged caecum. Distinct microbial communities are observed in each major compartment of the equine gut i.e. the upper gut (stomach, jejunum, and ileum) and the lower gut (cecum and the colon). These microbes are the sentinels that perform important functions such as nutrient assimilation, prevention of colonization of the gut lining by pathogens, and immune response support. The gut microbiota also neutralizes drugs and carcinogens and modulates intestinal motility. A host of different ‘good’ bacteria, fungi and yeast such as Lactobacilli, Bifidobacteria, Enterococci and Saccharomyces are continuously working inside the gut lumen making it free of pathogenic invasion, neutralizing toxins secreted by these pathogens, and assimilating and generating nutrients such as vitamin B and K.
In fact, scientific evidence points towards a strong correlation between maintaining a healthy gut microbiota in the horse and its overall health and performance. To efficiently digest all the dietary fiber that it consumes as part of its feed, the horse’s gut microbes must be in “balance”, which means that proper bacteria in the proper percentages must be populating the hindgut, and they must be healthy. The bacterial health of the colon is dependent upon many factors including pH, food for the bacteria, and colonic motility or movement. When this environment for the bacteria is not correct, the helpful or ‘good’ bacteria may be low in numbers, have depressed activity, or be dead. Once any of these upsets to the bacteria occur, the environment may become conducive to the growth of unwanted and even toxic strains of bacteria such as ones that were referred to as foreign invaders above.
What causes an imbalance in the gut?
Any sudden change in diet can compromise and change the microbial population in the horse’s hindgut, potentially resulting in colic and at least a reduced digestive efficiency of the diet. For instance, high starch diets can result in rapid proliferation of lactic acid bacteria leading to acidosis and laminitis. Thus, more sugar and rich starches in the horse’s diet change the microbial environment of the gut. Keeping the microflora happy can be difficult if a horse is under stress, travelling large distances, suffered illness or injury, weaned foal, or a high-performance horse being fed large amounts of grain.
Antibiotics are also a particular cause for concern. One of the problems with antibiotics is that they are not very specific or targeted, which means they cannot just affect one strain of bacteria, and they do not just go to work at the site of infection. When a horse is placed on antibiotics, changes in the balance of the hindgut microflora is inevitable, due to the death of good and bad bacteria in the “fermentation vat.” This loss of the beneficial microbes following antibiotic therapy provides an opportunity for harmful microbes to colonize the gut and cause diarrhoea.
One of the major sources of digestive problems in horses results from worm infection, particularly parasitic helminths which inhabit the caecum and colon. Worm infections can lead to poor digestion, malabsorption of nutrients, and damage the gut wall.
Thus, treating your horse for worms becomes important but, de-worming drugs can cause imbalances in the gut bacteria which in turn can create its own problems. Dysbiosis or overgrowth of bad bacteria results in various problems such as inflammation and damage of the gut lining leading to diarrhoea, reduced ability to digest fibre and a predisposition to laminitis, toxin release and blood poisoning, excess gas which can lead to colic, and a grumpy disposition and aggression.
What can be done to restore balance?
All is not lost if the horse has a balanced diet supplemented with good bacteria (probiotics), itself promotes the growth of existing good bacteria in the gut (prebiotics), builds immunity, and provides energy to the immune cell arsenal of the horse’s gut. Replenishing the gut microbiome after and during treatment with antibiotics, deworming agents etc. is important to ensuring that complications such as colitis do not occur. The probiotics mentioned above, belonging to the Lactobacillus, Bifidobacterium, Enterococcus, Streptococcus, and Saccharomyces species, are all important forerunners in maintaining and restoring the integrity and health of the equine gut.
Who can help the troops?
There are also various helpers which can ensure that these good bacteria get enough nutrition to continue colonizing the horse gut. These prebiotics are abundantly found in various fungi such as Turkey tail, Lion’s mane, Chaga, and Reishi. They contain what are called fungal polysaccharides (long sugars) which serve as critical nutrients for the probiotic ‘troops’. Some of these fungi naturally have antifungal and antibacterial properties which further help in the battle against invading pathogens. They also have chemicals that stimulate the immune machinery to launch a double-pronged attack on the pathogens. Another important helper is bovine colostrum (BC). BC is the first milk produced by cows after calving and contains numerous beneficial substances for the immunity and development of the newborn calf. It is a very rich source of immunoglobulins (Igs). The immunoglobulins, are special proteins that can specifically bind virtually any bacteria, virus, protein, carbohydrate or cell that the body recognizes as “foreign.” Once the immunoglobulins attach themselves to the foreign substance (called opsonization), scavenger cells of the immune system (macrophages) can attack and destroy or neutralize it. For instance, IgA is an immunoglobulin found in colostrum which serves as a first line of defense, protects mucosal surfaces, and prevents the attachment of pathogens. BC has been shown to kill Salmonella, along with other disease-causing bacteria such as Campylobacter, E. coli, Helicobacter pylori (the main cause of stomach ulcers), Listeria, and multiple Streptococcus species and Staphylococcus. BC also has other inhibitors of microbial growth that specifically attack pathogens trying to disturb the gut lining of the horse.
BC also provides proline-rich polypeptides (PRPs). PRPs are short chains of amino-acids, the building blocks of proteins. PRPs are signaling molecules that have the unique ability to modulate the immune system, turning it up when needed to fight an infection, or turning it down when it is overactive, i.e. in autoimmune disorders. Bovine colostrum can also stimulate the growth of intestinal cells, strengthen the gut wall, and prevent intestinal permeability, a condition that causes particles from the horse gut to leak to the rest of its body. Fenugreek is another important herbal helper that has protective and immune stimulating properties to aid in the smooth functioning of the equine gut. What’s more, it is a flavor loved by horses. It has demulcent and nutritive properties which are very useful during convalescence from an illness or injury, and it is a great prebiotic ingredient too. Yeast hydrolysate is another helper for the troops. Yeast hydrolysate is obtained by hydrolysing yeast cells with acid, enzymes or other means of hydrolysis. It is a concentrated source of vitamins, minerals and digested nucleic acids (nucleotides). Nucleotides are a class of molecules that are linked together to form DNA and RNA. Several studies show that nucleotides in the horse’s diet can improve intestinal function, immune response, composition of beneficial gut microbiota, liver function and morphology, as well as growth performance. Although the equine gastrointestinal tract is covered in mucus, the acids and enzymes nevertheless take a toll on the enterocytes (absorptive cells) lining the gut wall. Nucleotides can increase the maturity and growth of normal enterocytes improving nutrient uptake and pathogen defense.
To conclude this informative trip you took along the horse’s gut, the equine gastrointestinal tract functions well under normal and constant conditions supported by the able and strong troops – various stomach and intestinal cells, microbial cells, and immune cells. They all work in consort to ensure smooth functioning of the digestive tract. However, as all horse people know, the equine gut is extremely sensitive and easy to upset. Sudden dietary changes, antibiotic therapy, travel stress etc. can significantly change the smoothly orchestrated functioning of the gut and problems like colic and ulcers can set in. You now know how important it is to safeguard and maintain the health and numbers of the various vanguards of the horse gut. It is imperative that we treat the horse gut with respect and supplement its forage with probiotics and nutritional boosters such as colostrum and fungal polysaccharides to help maintain stability and overall health.
