To garner the greatest benefit of probiotic supplements for horses, owners must understand what probiotics are, be aware of the scientific data surrounding their use, and administer enough to be effective, said Jamie Kopper, DVM, PhD, Dipl. ACVIM-LAIM, ACVECC-LA, assistant professor at Iowa State University, in Ames, during her presentation at the 2023 EquiSummit, a virtual equine nutrition conference, held Sept. 5-6.

What Is a Probiotic?

Both the Food and Agriculture Organization and World Health Organization define probiotics as, “Living microorganisms that confer a health benefit to the host when administered in adequate amounts.”

“This is a fantastic definition because it includes three critical criteria that all probiotics should meet: The microorganisms (either bacteria or yeast) need to be alive at the site of action, the large intestine, not just at the time of purchase; they should not cause harm to the horse; and they should be administered in an adequate amount,” said Kopper.

Selecting an effective product is no easy feat because many probiotic labels are either missing important information or do not describe the product properly. 

Examples of labeling issues include:

• Not containing the bacterial species claimed.
• Not listing the exact type of bacteria included. “Some products only stated that lactic– acid-producing bacteria were included but did not specify which ones,” said Kopper. “But not all lactic-acid-producing bacteria are good for horses.”
• Not indicating how much bacteria (measured in colony forming units, CFU) it includes.
• The label doesn’t include an expiration date for the live organisms.
• Not containing the bacteria claimed based on culturing the probiotics. For example, 10 out of 11 evaluated products also contained “bonus” microorganisms not listed on the label. It is unknown if these “bonus” microorganisms could be harmful contaminants, or intentionally added for the horses benefit, said Kopper.

The Probiotic Should Confer a Health Benefit to the Host.

The product’s benefits should be based off scientific studies; however, researchers and manufacturers have conducted very few probiotic studies in horses.

“Most studies in adult horses show that there was neither harm nor benefit of the product,” said Kopper. “Some found that probiotics may decrease the severity or duration of diarrhea without any adverse effects.”

The story in foals, however, is quite the opposite: “Different studies found that foals treated with a probiotic were more likely to develop diarrhea that required veterinary intervention than untreated foals or had worse diarrhea than untreated foals,” Kopper said.

“In humans 10⁹ CFU is the minimum dose believed to have positive effect,” explained Kopper. “We would expect horses to require at least this much, but the dose horses should receive is unknown.”

Take-Home Message

When choosing a probiotic for your horse, look for species names of microbes, not just terms like “probiotic blend” or “lactic-acid-producing bacteria.” The amount of each microbe in CFU should be listed on the label, along with storage instructions and an expiration date. Ideally, manufacturers would be able to provide quality control data to support the presence of live bacteria at time of purchase and have studies to show that microbes maintain viability in the intestinal tract.

It is dangerous to say, “I’m not sure if it works, but it sure can’t hurt,” said Kopper, especially given the unfavorable results shown in foals.

“Probiotics have so much potential to prevent and treat disease, but the truth is if we don’t have data to show that they help and we don’t have data to show it doesn’t hurt, all we can say at this time is, ‘I’m not sure if it currently works.’”