Probiotics have the potential to positively impact gastrointestinal (GI) health, decrease adverse effects of antimicrobials on intestinal microbial communities, and improve our ability to prevent or treat colic and colitis. Additionally, probiotics improve our capacity to treat diseases outside of the GI tract, including allergies, neurologic disorders, and respiratory disease. This is because probiotics have the potential to modify the population of bacteria present within the GI tract, improve intestinal barrier function (i.e., “decrease gastrointestinal leakiness”), and decrease the presence of pathogenic bacteria like Salmonella. They may also interact with the immune system, which can have positive (or negative) effects beyond those that occur locally within the GI tract.

According to the World Health Organization and Food and Agricultural Organization, probiotics are defined as living microorganisms, which when delivered in adequate amounts, confer a health benefit to the host. Based on this definition, there are three criteria that products, marketed as probiotics, should meet: (1) they should contain live microorganisms, (2) the microorganisms should be delivered in adequate numbers, and (3) the microorganisms should do something beneficial for the horse. Below, each of these criteria are broken down and discussed in reference to the horse, and advice is provided on how to look for a product that meets as many of these criteria as possible. The first criterion is that the product contains live microorganisms. There are several potential challenges that microorganisms (typically, bacteria and/or yeast) in probiotics must overcome to remain viable in probiotics. Most consumers are looking for probiotics to improve “hind gut” or cecum and/or colonic health, thus microorganisms must survive transit through the horse’s acidic stomach and small intestine to reach the cecum and/or colon. Bacteria and yeast are not invincible and, thus, their viability can be adversely affected during transit. Additionally, they must survive many conditions that are out of the consumer’s control, such as manufacturing, shipment, and storage.

To improve probiotic viability, choose a product that lists both storage instructions and an expiration date. Then, follow the storage instructions appropriately. Many companies recommend storing their probiotics in a “cool and dry place.” While this is not specific, products likely should not experience the extremes of summer and winter in a barn.

Second, look for products that advertise “enteric protection” or a mechanism to help the microorganisms survive transit through the acidic stomach and small intestine to reach the cecum and colon. If you choose a product without enteric protection factors, then it is beneficial to administer the probiotic when hay (or other roughage) is fed.

The second criterion is that microorganisms should be available in adequate numbers. Unfortunately, we do not have a definitive answer for this topic in horses. In adult humans, it is recommended to use 1 x 106-108 colony forming units (CFU) of microorganisms at the time of consumption. It would seem intuitive that adult horses should receive at least this many, if not more, CFU per dose given the size of the horse and the horse’s GI tract in comparison to that of a human. However, currently, we do not know the optimal dose to administer a horse. Until an answer has been determined, consider choosing products that provide a total number of CFUs per microorganism and per dose for their product. A repeated concern for probiotics is that independent research has identified discrepancies between label claims and actual contents when certain products have been evaluated, raising concerns for quality control. Thus, finding a company that is willing to provide information on quality control and testing would also be desirable.

Finally, the product should provide a health benefit to the horse. In humans, a large body of research has been conducted to identify beneficial microorganisms, assess whether theoretical benefits translate to patient benefits and to evaluate potential adverse effects. Unfortunately, microorganisms that are beneficial for people may not provide benefit to horses given differences in diets, eating style (grazing) and GI physiology (i.e., hind gut fermentation).

Few studies have evaluated probiotics for potential beneficial and adverse effects in adult horses and foals. A handful of different organisms and products have been evaluated in adult horses and were found to have no clinical effect or potential positive effects, such as improved clearance of sand accumulation in the colon, decreased duration of diarrhea, and decreased shedding of Salmonella. Currently, there is not enough research to make blanket statements or recommend specific microorganisms for specific benefits.

The potential for adverse effects must be considered. In three studies utilizing neonatal foals, the use of probiotics was associated with an increased risk of diarrhea and need for veterinary care. Although it has not been documented in neonatal foals, probiotic microorganisms administered to human neonates have been identified in blood cultures, indicating bacteremia or fungemia. Finally, recent work has identified the presence of antimicrobial resistance genes in probiotics marketed for horses – whether these genes are being transferred to the horse’s normal intestinal microbiota and/or pathogenic bacteria, like Salmonella, has yet to be determined.

In conclusion, probiotics have the potential to improve not only the GI health of horses but their general health as well. Currently, many products are on the market which can be confusing to choose from. More research is needed to determine basic questions like which microorganisms should be administered to horses (adults and foals) and in what quantity to result in benefit. And, at least in foals, there does appear to be a real risk for negative side effects, so careful consideration should be given to this age group. In selecting a product, look for one that provides you with necessary information to make an informed choice, including a list of the product’s microorganism(s), statement of the quantity of microorganism per dose, storage instructions, and an expiration date and, ideally, a product that provides scientific references that support the health benefits of the product.

Editor’s note: This is an excerpt from Equine Disease Quarterly, Vol. 31, No. 3, funded by underwriters at Lloyd’s, London, brokers, and their Kentucky agents. It was written by Jamie Kopper (DVM, PhD, DACVIM, DACVECC) of Iowa State University.