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What Are They?
Horses, like humans, naturally have bacteria and yeast in their digestive tracts that assist in digestion. During times of stress, illness or antibiotic use, these beneficial organisms are killed and the healthy digestive system disrupted. Probiotic is another name for a source of “good” microbes and Prebiotics are the substances that these organisms live on. These microbes assist the horse in breaking down and fermenting the horse’s food into usable energy sources in the hindgut- the large intestine and cecum. The organisms also create B vitamins and other nutrients, as well as keep unwanted microbes, such as Salmonella, from colonizing the GI tract.
These are live organisms that, when given in sufficient quantities, offer a health benefit to the host. Typical equine probiotics include bacterium Lactobacillus, Bifidobacterium, and yeast Saccharomyces boulardii.
These are substances not digested by the host that stimulate the growth or activity of digestive system organisms that are healthy for the host body. Most of these substances are carbohydrates, or long chains of sugar molecules. Examples include fructooligosaccharides (FOS), xylooligosaccharides (XOS), polydextrose, mannooligosaccharides (MOS) and galactooligosaccharides (GOS).
Benefits of Supplementation
Antibiotic administration, stress, transport, infection with Salmonella or Clostridium species, and abrupt changes in feeding can all disrupt the normal flora of the gastrointestinal tract. Owners often elect to use probiotics and prebiotics in horses that are off feed, geriatric, a “hard keeper”, or before transport or other stressful events. Horses may also benefit from these supplements when they have diarrhea or are undergoing antibiotic therapy.

While probiotics and prebiotics have the potential for beneficial effects, there is minimal scientific evidence to support their use and much of what is known about them is extrapolated from human studies, a species with a very different digestive system. Also, because these substances are not drugs, they do not go through the rigorous quality assurance/quality control techniques that are required of government-controlled drugs. This means that poor-quality products are available to consumers. While they are widely considered safe, they may not be as effective as they have shown to be in other species.


The equine digestive tract. Courtesy Dr. Reynolds