Written By Dr. Kris Hiney

This month we will explore the use of digestive aids in horses, in particular probiotic usage. Probiotics are increasingly used in human medicine, production animal species, and of course in horses.  More owners are looking for safe and effective alternatives to pharmacological methods for promoting the well-being of their horses.   In this article we will discuss what type of organisms fall under the probiotic umbrella, the form in which they may be fed, their effectiveness and when their use might be warranted.

In general, probiotics are live organisms which are fed with the intention of their survival within the gastrointestinal tract.  The original concept behind the use of probiotics was to provide a beneficial type of microorganism which can alter the fermentation process in the hindgut, or to shift the microbial population away from more negative types of organisms.  Typically these organisms promote digestion and alter the types of volatile fatty acids that are produced.  This was typically referred to as a competitive exclusion effect.  However, it is becoming more widely understand that probiotics may have farther reaching effects than just simply outnumbering undesirable bacteria.  This differs from when organisms are fed for their nutritive value, such as often done with yeasts.

Horse owners have many options when selecting probiotics, including powders, pills, pastes, feeds, live culture yogurt or even innoculations of fecal microflora from healthy horses.  The key feature for a probiotic to be effective is that it is able to survive exposure to acid, bile and enzymes in the foregut of the horse and reach the hindgut alive.   In addition it must remain viable during processing and storage of the product. Further, microorganisms must be present in sufficient quantities to have an effect.   From extrapolations in human studies, it is suggested that foals be provided with a minimum of 10 to 20 billion colony forming units or CFUs with some studies suggesting an increase of 10 fold in adults.  Therefore concentrated forms of probiotics are often the most effective, rather than just a feed with added probiotics which may contain insufficient organisms.  It is highly recommended that horse owners read product labels carefully in choosing a digestive aid for their horse to ensure the product contains living organisms at sufficient numbers.  Unfortunately many commercial products may not actually even contain the amount of microorganisms listed on the label.  In a study from 2002, products contained as little as 2% of the CFUs claimed on the label.    In addition, some claims may be misleading and actually only contain fermentation products, which are not live cultures and therefore not probiotics.

Beyond viability and amount of probiotics, the type of organism contained in the probiotic is key.  The most common classes of probiotics are the lactate utilizing bacteria including lactobacilli, bifidobacteria and enterococci. These bacteria are those that convert lactate to propionate in the gut which may help stabilize colonic pH.   Live yeast cultures have also been used, in particular Saccharomyces cervisiae.  This differs from the use of yeast products which may be fed in order to supply vitamins or protein from the process of digestion of the yeast itself.  When looking for a yeast supplement intended to be a probiotic, be sure that it actually contains live yeast   Most species of organisms in probiotics are not typically found inhabiting the gut of the horse. Thus they fail to form permanent stable colonies in the gut, and will no longer be present after administration has been ceased.  Therefore continual supplementation may be necessary depending on the desired outcome.

Live yeast and bacteria supplementation may have beneficial effects beyond that of just supplying a different microorganism with fermentative capabilities.  Some yeasts may release enzymes which digest the toxic by-products of pathogenic bacteria.  It is also believed that yeasts and lactate using bacteria may have immunostimulatory effects, stimulating the gut associated lymphoid tissue.  This enhances the immune system of the horse and may make them more capable of handling exposure to pathogens. Other pathogenic bacteria such as Salmonella and E coli may bind to the yeasts rather than the epithelial tissue of the gut, thus preventing their colonization. Supplementation of live yeast has also been shown to improve digestibility of fiber and increase the amount of lactobacillus in the hind gut which again may be protective against acidotic conditions in the hindgut.

Probiotics are frequently administered when there is believed to be a disruption in normal gut microflora, such as during bouts of diarrhea, following anti-biotic administration or other gastric upsets.  This can include any stressful period for the horse such as travel, new environments, or alteration in diets. Horses supplemented with yeast and subjected to transport had greater biological diversity of bacterial species in the hindgut, and an increase concentration of lactate using bacteria and cellulytic bacteria.  Thus these horses maintained a healthier hindgut population compared to non-supplemented controls.  Supplementation of live boulardii yeast, a sub species of Saccharomyces cervisiae resulted in a shortened period of diarrhea and a quicker return to normal feces in horses suffering from enterocolitis compared to a placebo group.  Horses in this study had a broad range of causative factors for the diarrhea.  Thus probiotic administration may be an additional therapeutic tool in managing colitis or diarrhea in horses.    Probiotics may also reduce the detrimental effects of a high starch diet on the microbial population.  Typically high starch diets promote the growth of amylotic bacteria and decrease the population of cellulytic bacteria, thus suppressing fiber fermentation.  In addition, the by-products of amylotic bacteria are responsible for lowering the pH of the hind gut.  If probiotics are used in conjunction with higher concentrate diets, the overall health of the gut may be improved.

So when is a probiotic right for you?  Certainly during periods of digestive upsets, probiotics can help return the microbiology of the gut of the horse to a healthier state.  They may also assist a horse during times of stress, not only preserving the health of the GI tract, but also the health of the horse itself.  Probiotics promote a stable pH in the gut and can assist in fermentation in the gut.  There a very few negative indicators for probiotic usage, rather just be sure that you choose an effective product.