Gut Function and Probiotics: Could Your Horse Benefit?

 If you could see a microscopic view of your horse's gastrointestinal tract, you would discover a massive microbial population. The diverse microbial "ecosystem" maintained within the GI tract is known as the microbiome and it is unique to each individual horse.

"Our understanding of the equine microbiome has improved considerably over the past five to 10 years but certainly there is more that we don't know than what we do know," says Bill Gilsenan VMD, DACVIM, an internal medicine specialist at Rood and Riddle Equine Hospital in Lexington, Kentucky.

Included in the microbiome are a host of beneficial bacteria that are crucial to proper digestive function and also support the immune system. Think of these microorganisms as "good bugs."

Understanding gut function

Horses digest feed through fermentation in the hindgut and the good bacteria perform a crucial role in this process.

When the gut environment is consistent, digestion proceeds normally. However, a number of things can upset the status quo in the GI tract, such as a course of antibiotics or other medications, competition, travel, stress, or changes in feed.

"In addition to antimicrobials altering the microbiome, we have learned that stress, transport and other common medications can alter the microbiome," says Gilsenan.

Gilsenan points out that the critical factor in the equine GI tract is homeostasis, or consistency. When the internal ecosystem of bacteria is altered and becomes inconsistent, good bacteria can die off, releasing endotoxins and other toxic substances into the bloodstream. This allows opportunistic ("bad") bacteria—which are always present, but usually in small numbers—to reproduce and increase.

"I would emphasize the need for homeostasis, in that there are 'good' bacteria and 'bad' bacteria. If there's an alteration to the horse's GI tract/colon, then some of the good bacteria might die off. The good bacteria often function to use up some of the byproducts (such as lactic acid) of the 'bad' bacteria; if they are not there to do that, then the byproducts accumulate," Gilsenan explains.

"This alters the microenvironment more, which favors different, or bad, species of bacteria. As time goes on, the microbiome can veer off in a very different direction from 'normal,' which can cause inflammation of the GI tract, and consequent translocation of bacteria or bacterial toxins into the blood."

What exactly are probiotics?

Also known as "direct-fed microorganisms," probiotics are live bacteria, such as Lactobacillis, Entercoccus, Bifidobacterium, and more. Aspergillus, a fungus, and Saccharomyces, a yeast, also have some characteristics of probiotics and may be included in some supplements. 

Most probiotic supplements[LC1] are designed to be fed to the horse daily according to label directions. The idea is that introducing these bacteria may help maintain the population of good bacteria in the GI tract.

Depending on the product, a probiotic supplement may also contain prebiotics, substances that aren't digested by the horse but serve as a preferred food source for beneficial bacteria.

When to use probiotics

"Probiotics are commonly used in healthy and diseased horses. In our clinical practice, we use probiotics most commonly for horses being treated with antimicrobials, as logic would suggest this would have the greatest effect on a horse's GI microflora," says Gilsenan.

"The equine GI microbiome is exceedingly complex, so it is difficult to be certain that a manufactured product is going to effect or normalize abnormalities in the microflora," he notes.

"Our research for the use of probiotics in horses is limited," says Gilsenan, adding that while probiotics are considered safe to use, there are "knowledge gaps" and more scientific research is needed to prove their benefits.

Horses turned out on pasture with little to no work or stress probably aren't in need of probiotics. On the other hand, horses in training or competition, or on medication may very well benefit from a probiotic supplement, so ask your veterinarian for input.

"It would be best to discuss with a veterinarian whether probiotics are indicated for your horse and, if they are, which product is optimal," advises Gilsenan.

Maintaining the status quo

In addition to feeding a probiotic supplement, there are management practices you can follow to help maintain consistency in the horse's microbiome, including:

• Making any feed changes gradually
• Having forage (hay/pasture) consistently available
• Maintaining regular feeding intervals
• Avoiding large amounts (over 5 lbs.) of grain at one time
• Providing adequate daily exercise and turn-out
• Eliminating stress as much as possible
• Keeping a regular dental care schedule

"As we learn more about the microbiome, likely any management change will cause some impact on the GI flora, but we don't know at what magnitude that impact becomes relevant," notes Gilsenan.

It's worth noting that digestion proceeds at very different rates for grain and hay. For example, the long length of time it takes for hay to digest is why feeding more hay when it's cold actually helps warm the horse.

On the other hand, grain ferments rapidly in the gut.

"This can induce rapid and often very dangerous consequences in the GI tract if the diet is changed quickly," Gilsenan points out. "Forage is broken down much more slowly, so if hay changes it is a lot less likely to disrupt the GI tract." 

When you must make feed changes, do so gradually, ideally over a week's time, not all at once.