Feeding the Horse: An Introduction to Probiotics
By Miranda Stripe
Though resilient, horses have a major Achilles heel — the GI tract. As horse owners, we aim to nurture their delicate gut with various supplements. Recent trends focus on the horse’s gut microbes, specifically probiotics. But, with hundreds of products on the market, how do we choose?
It narrows down to what we want to accomplish by feeding a probiotic to our horses. To determine this, we must first understand what a probiotic is and how it can affect our horse.
What is a probiotic?
Most of us have heard of probiotics, yet their true meaning may be unclear. Probiotics (AKA “direct-fed microbials”) are live strains of microbes. The Food and Agricultural Organization (FAO) and World Health Organization (WHO) specify probiotics as microbes that provide benefits beyond their nutritional profile (e.g., carbs, vitamins, etc.) when given in “adequate” concentrations. To truly understand this definition, we need to examine probiotics on a molecular level.
First off, we must distinguish probiotics from prebiotics — they are not the same. While probiotics refer to live microbes, prebiotics are microbial fermentation products. However, the two are similar in that both must provide benefits to the host — the horse.
Prebiotics
Prebiotics are fibers, primarily oligosaccharides, that benefit the host by feeding the microbes already present in the gut. In other words, prebiotics help grow and support microbes that help break down food and provide nutrients to the horse.
Probiotics
Probiotics introduce live microbes to provide benefits through one or more of the following basic mechanisms:
1. Host immune response
Tightening of cells in the intestinal wall to prevent pathogens (“harmful” microbes) from getting into the body and infecting the host
· Production of antibodies — proteins made by host cells to target and suppress growth of pathogens
· Stimulation of anti-inflammatory pathways
These responses can occur even if bacteria within the probiotic are no longer alive. The presence of specific antigens (i.e., pieces or products of microbes) alone can stimulate the immune system.
2. Microbe-microbe interactions
· Production of antimicrobial compounds harmful to pathogens
· Competition for space on the intestinal wall
· Inhibition of pathogenic toxins that aggravate and damage the intestine
So, by feeding a probiotic, we are adding new organisms to the gut to fortify our horse’s immunity and improve gut health. Whether this is actually accomplished is debatable, and active research aims to establish the effects of probiotics on horses.
The gut microbiome
Before considering which microbes to feed our horse, we must appreciate the diversity of microorganisms that already occupy the gut. The gut microbiome refers to the collection of microbes within the entire GI tract. However, when comparing different segments of the GI tract, we will find that not every “bug” lives in every segment. Where each microbe resides depends on the environment and function of the GI tract.
Stomach: The highly acidic stomach acts to chemically and mechanically breakdown food. Lactobacillus are the predominant bacteria in the stomach, particularly in the upper, non-glandular region.
Small Intestine: The stomach empties into the small intestine, which breaks down starches, proteins, and fats through enzymatic action. The first two segments of the small intestine, the duodenum and jejunum, are largely responsible for digesting and absorbing food. Yet, because of their location in the GI tract, little is known about their native microbes. The ileum comprises the last segment of the small intestine, absorbing some nutrients and harboring mostly Firmicutes and Proteobacteria.
Hindgut: In the horse, the ileum connects to the cecum — the large fermentation vat that classifies horses as “hindgut fermenters.” The other component of the hindgut is the colon, which contains the bulk of gut microbes in horses and other species. The fiber-fermenting microbes of the hindgut, particularly Firmicutes and Bacteroidetes, produce lactic acid and volatile fatty acids. These acids are absorbed through the colon wall, providing the main source of energy to the horse.
Even more variable than the microbes of different GI segments is the microbiome between individuals. Genetics, obesity, diet, and health status contribute to such variance. For example, cultures of fecal samples from healthy individuals yielded Actinobacteria, Spirochetes, and Clostridiales, while feces from horses with colitis-induced diarrhea contained large amounts of Fusobacteria.
Though there is high species variance among microbes between individuals, their microbiome is similar in terms of functional diversity.✝ In other words, microbes inhabiting one horse’s colon may differ drastically in terms of species or strain, but the overall colon microbiome of each horse performs similar metabolic functions.
Research has gained some insight on the equine gut microbiome, but much is still unknown. More studies and better methods are needed in order to better define which microbes live where and what exactly their roles are relative to their benefits to the horse.
Microbes on the market
Current veterinary probiotics in the United States are nutraceuticals (‘supplements’) versus pharmaceuticals (‘drugs’). In other words, probiotics do not undergo a screening process by the Food and Drug Administration, so quality control falls on the manufacturing companies. Europe holds stricter standards by classifying probiotics as “zootechnical additives,” which must be approved before reaching the market.
Of the microbes currently on feed store shelves, lactic-acid producing bacteria like Lactobacillus, Bifidobacterium, and Enterococcus dominate the market. The yeasts Aspergillus and Saccharomyces, particularly S. cerevisiae, are also common inclusions in probiotic supplements and grain products. However, of these probiotics available, Saccharomyces is the only one with any substantial research to support its use.
In addition to the type of microbe used, probiotic formulations also vary in delivery method:
· Pure microbe, no protection
· Microencapsulation
· Freeze drying
· Time-delivered release
Unprotected organisms likely can reach the stomach, where they die or potentially grow if adapted to low pH environments. The protective mechanisms listed aim to bypass the acidic stomach. However, some methods are susceptible to gastric acid or can kill microbes. Thus, when considering a probiotic, we should ensure the formulation can deliver the microbes to their desired destination in the GI tract.
The balance of microbes within the horse depends on the individual and is vital to maintaining gut health. Now that we know the concept behind probiotics, the basic functions of the gut, and the very general (yet relatively unknown) gut microbiome of the horse, we can start thinking about which — if any — probiotics to use. Next time, we will explore specific microbial species used in horses and their potential effects on the gut microbiome.
✝This is a finding in humans, which can likely be extrapolated to the equine GIT. However, as noted above, more research is needed to better understand the horse’s gut microbiome.
About the Author
Miranda Stripe is currently a UC Davis veterinary student (class of 2022), striving to become an equine veterinarian. She has a particular interest in nutrition, sport’s medicine, and client education. While in school, she works as a part-time vet assistant at a small animal emergency practice, VCA Bradshaw Animal Hospital, and as an equine sport massage therapist through my own business, In Sync Equine Sport Massage Therapy.
This blog post is sponsored by Fluxergy. Be one of the first clinics to test for respiratory and enteric pathogens stall-side. Visit fluxergy.com to learn more.
