Horse with rider.

Probiotic Use in Horses: What is the Evidence?

By Kelly S. Swanson, PhD, The Kraft Heinz Company Endowed Professor in Human Nutrition, University of Illinois at Urbana-Champaign, USA

Horses play a special role in many people’s lives, serving as a partner in leisure activities, therapy, various forms of work, and athletic competitions. Being large herbivores, they are adapted to a diet rich in grasses and other high-fiber forages. The complex community of microbes inhabiting the hindgut (cecum and colon) is necessary for the efficient breakdown of these fibers as well as maintaining the gastrointestinal health and overall health of horses. In recent years, a lot has been learned about the composition and activity of the gastrointestinal microbiota of horses and their role in health and disease (Kauter et al, 2019). There has also been interest in testing whether yeast- or bacteria-based probiotics may help manage equine health and disease.

Is there evidence supporting probiotic use in horses? The answer depends on the animal’s life stage, dietary and exercise strategy, and health status.

Probiotics for foals

A common target of probiotic use has been young growing foals. Similar to other host species, the gastrointestinal microbiota population of foals has a lower diversity and stability than that of adult horses (Earing et al., 2012; De La Torre et al., 2019). This instability makes foals more susceptible to pathogen-induced microbiota alterations, diarrhea, dehydration, and intestinal inflammation (Frederick et al., 2009; Schoster et al., 2017; Oliver-Espinosa, 2018). But probiotic use in foals has had both helpful and harmful outcomes. Positive results were obtained with a probiotic containing 5 Lactobacillus strains (L. salivarius YIT 0479, L. reuteri YIT 0480, L. crispatus YIT 0481, L. johnsonii YIT 0482, L. equi YIT 0483), which were shown to increase body weight and reduce diarrhea incidence in 3-4 week old foals (Yuyama et al., 2004). Similarly, a probiotic composed of 4 Lactobacillus strains (L. reuteri KK18, L. ruminis KK14, L. equi KK15, L. johnsonii KK21) and 1 Bifidobacterium strain (B. boum HU) was reported to reduce the incidence and duration of diarrhea in foals during their first 5 months of life (Tanabe et al., 2014). However, administration of a different probiotic (L. pentosus WE7) was associated with anorexia, development of diarrhea, and greater need for veterinary examination and treatment (Weese and Rousseau, 2005). Based on the evidence thus far, caution should be used when considering probiotic use in foals.

Probiotics for adult horses

Even though adult horses have a more stable and rich gastrointestinal microbiota than young animals, microbiota disruptions can occur with rapid changes in diet, transportation stress, the onset of gastrointestinal disease, or other diseases such as laminitis or grass sickness (Garrett et al., 2002; Costa et al., 2012; Moreau et al., 2014). Horses are susceptible to gastrointestinal disorders such as enterocolitis that may be due to antibiotic use, stressful conditions, or pathogen infection (e.g., Clostridioides difficile; Salmonella). Not all probiotic interventions have led to improvements, but there are examples of success. In one study, a Saccharomyces boulardii treatment reduced the severity and duration of illness in horses with acute enterocolitis (Desrochers et al., 2005). In another study, a probiotic mixture of 3 Lactobacillus strains (of the species L. plantarum, L. casei, L. acidophilus) and 1 Enterococcus strain (E. faecium) reduced the incidence of Salmonella shedding in horses admitted for routine medical and surgical treatments (Ward et al., 2004). Overall, there is weak evidence for probiotic use in horses with enterocolitis at this time.

In healthy adult horses, the reasons for using probiotics may differ depending on the fiber and starch content of the diet being fed. In horses fed a high-fiber diet composed of grasses and hay, live yeast cultures (Saccharomyces cerevisiae) have increased nutrient breakdown and energy extraction (Medina et al., 2002; Jouany et al., 2008; Garber et al., 2020). Such increased efficiency may be helpful for horses eating low-quality forages or performance animals that have higher energy requirements. To meet the energy needs of many high-energy or performance animals, grains that are rich in starch and have a higher energy content are often fed. A high-starch diet helps meet the energy requirement, but if not managed properly, it can exceed the capabilities of the horse’s small intestine, resulting in significant starch loads entering the hindgut. These starches are highly fermentable by hindgut microbiota, resulting in the rapid production of lactic acid and short-chain fatty acids. The accumulation of these acids can lead to hindgut acidosis and diseases such as colic or laminitis. Lactobacilli have been shown to modify equine microbiota populations, decreasing amylolytic bacteria and increasing lactic-acid utilizers, and ultimately attenuating starch breakdown and pH decline ex vivo (Harlow et al., 2017). Live yeast cultures have also been shown to help attenuate the hindgut lactic acid concentrations and maintain the hindgut pH of horses fed high-starch diets (Medina et al., 2002). These studies suggest that probiotics may be useful in increasing the digestive efficiency and/or maintaining the hindgut homeostasis of healthy adult horses.

Probiotics for horse athletic performance

Because probiotics have been used to support exercise performance in humans (Pyne et al., 2015), similar interventions have been tested in performance horses recently. In one study a probiotic mixture of 5 Lactobacillus strains (L. acidophilus DSM 32241, L. plantarum DSM 32244, L. casei DSM 32243, L. helveticus DSM 32242, L. brevis DSM 27961), 2 Bifidobacterium strains (B. lactis DSM 32246, B. lactis DSM 32247)), and 1 Streptococcus strain (S. thermophilus DSM 32245) reduced post-exercise blood lactate concentrations and modified blood and urinary metabolite profiles (Laghi et al., 2018). In another study, a probiotic mixture of 2 Lactobacillus strains (from the species L. plantarum and L. paracasei) increased blood oxygen saturation and reduced blood lactic acid concentrations (Zavistanaviciute et al., 2019). Because lactic acid production and accumulation results in fatigue and reduced performance, these studies suggest that probiotics may support athletic performance in horses. The results of these studies are promising, but more research is necessary.

State of the science

Data to support use of probiotics in horses is emerging, but the occurrence of harmful outcomes in at least one study reinforces the need for high quality studies that can precisely establish efficacious conditions and formulations for use. Similar to recommendations for other host species, equine probiotics should provide an effective dose, be designed for horses, target a specific life stage and condition, and be supported by evidence. It is important to remember that probiotic efficacy can depend on specific microbial strains, supplement form, storage conditions, and dosage  – see ISAPP’s infographic ‘What Qualifies as a Probiotic’ for more details on probiotics.

Kelly Swanson joined the ISAPP board of directors in June, 2020, providing valuable expertise in animal gut health and overall health. Swanson also chaired the 2019 ISAPP-led international consensus panel on the definition of synbiotics.