horse standing in stall
A horse’s immune system might not function optimally if his gut microbiome is unhealthy. | Thinkstock

The equine gut microbiome has been proven to influence many aspects of horses’ health and well-being, including behavior and gastrointestinal health. But why is the microbiome so powerful? Researchers say it’s because of its connections with the nervous and immune systems.

Equine Microbiome Basics

The equine intestinal microbiome refers to the genetic material of the gastrointestinal (GI) microbiota, the diverse population of microorganisms living in the GI tract, said Liz Schatz, DVM, senior specialist of veterinary affairs at SmartPak Equine, headquartered in Plymouth, Massachusetts, during her presentation at the 2024 virtual EquiSummit conference, held Dec. 3-4. These microorganisms include bacteria that break down fiber and protein and produce B vitamins and vitamin K; fungi that break down fiber; protozoa and archaea that produce methane gas; and viruses and parasites, she added.

“The microbiome is unique to each horse, like a fingerprint,” said Schatz. “And many variables affect that horse’s microbiome, including diet as well as age, weather, management, illness such as metabolic syndrome, stress, and medications. Unfortunately, there are pretty big gaps in our knowledge about the (equine) microbiome. But in human medicine, many diseases are associated with changes in the microbiome.”

The Microbiome and Whole-Horse Health

One of the key GI microbiome concepts to understand is the microbiome-gut-brain axis. “This means the nervous, immune, and endocrine systems are all interconnected with the intestinal microbiome, and that changes to the gastrointestinal tract can have far-reaching effects,” said Schatz.

For example, alterations in the microbiome can result in behavior changes, such as anxiety, spookiness, and even stereotypies (repetitive movements with no apparent purpose). Mental stress can also affect the microbiota, causing a vicious circle.

Implications for Equine Immunity

“Seventy percent of the body’s immune system is in the GI tract, specifically the lymphatic system, which is the immune system’s superhighway,” said Schatz. “Anything that alters the GI tract can affect the immune system.”

For example, horses with equine metabolic syndrome (EMS) have gut microbiomes that differ in microorganism diversity compared to healthy horses.

“This may contribute to the decreased immune response we see in horses with EMS, but much more research is needed,” said Schatz. Researchers working to identify different microbe types commonly found in the GI tract and better ways to obtain samples from horses have made several advancements in this field.

“We need to better understand the microbiome-gut-brain axis and better understand the implications for immunity in reference to specific diseases,” said Schatz. “Then we can focus on potentially improving treating certain conditions by supporting the microbiome.”

Maintaining Your Horse’s Microbiome

The veterinary community needs more targeted research to understand this large and powerful organ, which includes the three “otics”:

  • Prebiotics: ingredients or supplements that feed the beneficial bacteria in the GI tract;
  • Probiotics: live microorganisms that confer a health benefit to the host when administered at adequate concentrations; and
  • Postbiotics: deliver metabolic products of the microbiota, or the attempt to deliver preconverted metabolic compounds straight to the gut.

“More research is needed on these also, so the best way to support the microbiota right now is minimizing stress, maximizing turnout on pasture, offering good-quality grass/forage, and making dietary changes slowly for both grain and hay,” said Schatz.

Additionally, monitor horses for behavior changes, changes in appetite, and signs of gastric ulcers, colic, colitis/diarrhea, laminitis, and EMS. Schatz suggested working closely with your veterinarian to address these underlying health issues early in the course of disease and having a complete physical exam performed on your horse twice annually.

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