gut microbiome
A prebiotic might be able to prevent leaky gut in horses. | Adobe stock

Leaky gut in horses has rapidly become a widely discussed topic, yet most of what veterinarians know about this condition has been extrapolated from other species. During her presentation at the virtual 2024 EquiSummit, held Dec. 3-4, Wendy Pearson, PhD, associate professor of equine physiology in the department of animal biosciences at the University of Guelph, in Canada, shared results from her research team’s recent study, showing leaky gut might be manageable with prebiotics.

Defining Leaky Gut in Horses

The lining of a horse’s gastrointestinal (GI) tract is a single layer of cells separating the lumen of the GI tract (where food is stored) from the horse’s bloodstream. In a healthy GI tract, cells on the lumen side engulf the nutrients and release them on the bloodstream side. Some small molecules can squeeze between the cells to reach the bloodstream.

“The tight junctions between the intestinal cells stop larger molecules like bacterial products such as lipopolysaccharide (LPS),” said Pearson. “Small molecules and solutes such as sodium ions can easily pass though the tight junctions.”

But when the cells lose those tight junctions, a leak pathway forms between the cells, allowing larger molecules, such as the bacterial products and toxins, to cross. Toxins then enter the circulation and can contribute to the development of low-grade inflammation. In other species this low-grade systemic inflammation has several consequences; it impairs skeletal muscle metabolism, affects metabolic function, and contributes to allergies or chronic inflammatory disorders such as arthritis, among other effects. In theory, this can also occur in horses, but no research exists to support this hypothesis.

Factors Contributing to Equine Leaky Gut

Researchers conducting work in other species have identified a few contributors to leaky gut, including:

  • Individual bouts of exercise. “When animals exercise, blood is diverted away from the GI tract and toward exercising muscles,” said Pearson. “As a result, the decreased blood flow to the intestinal cells causes damage, swelling, and inflammation, making them leaky.”
  • Stressful situations. Cortisol compromises the tight junctions and disturbs the intestinal microbiome—the microbes that reside in the GI tract, playing important roles in digestion, immunity, and nervous system function. Stressful situations could include transport and heat stress.
  • Diet. This can either positively or negatively affect leaky gut, depending on the diet composition and its effect on the microbiome.

Leaky Gut Research in Horses

In 2023 Pearson and her lead graduate student, Melissa McGilloway, published two studies designed to develop a model for studying leaky gut in horses. They chose trailering and exercise for the model because those activities mimic a typical day for many horses.

They transported 16 horses for one hour then immediately exercised them for 30 minutes. Before trailering they gave the horses oral iohexol (a contrast agent).

“Iohexol is a small molecule that does not fit through a healthy tight junction but will pass through the leak pathway in horses with leaky gut,” said Pearson. If the horses have leaky gut, the iohexol will pass from the lumen of the GI tract, through the leak pathway between the intestinal cells, and enter the bloodstream, where the researchers would be able to measure it.

They collected blood samples immediately before administering iohexol, after a one-hour trailer ride, after 30 minutes of exercise, then again intermittently for eight hours after exercise. The horses in the control group stayed at the barn. The researchers administered iohexol and took blood samples at the same time as they did for horses undergoing transport and exercise.

“We found that the blood levels of iohexol were significantly increased in the stressed horses for up to two hours after exercise, showing that leaky gut did develop,” said Pearson.

Can We Prevent Leaky Gut in Horses?

Based on research in humans, Pearson’s team tested a fermentation product of the mushroom, Aspergillus oryzae, designed to support the GI microbiome and limits stress-induced GI tract hyperpermeability.

They fed the horses this prebiotic for 28 days and then repeated the study.

“The horses that were transported and exercised had no spike in iohexhol, indicating that the leak pathway did not form in horses fed the fermentation product of A. oryzae,” Pearson said.

Take-Home Message

Pearson’s team has shown that normal, daily events such as transport and exercise can induce leaky gut in horses, but feeding the prebiotic A. oryzae blocked that hyperpermeability in this study group. More research is needed to fully understand the uses of this prebiotic.

Additional 2024 EquiSUMMIT Coverage: