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Probiotics: What, Why & How to Select?

Updated: Mar 23, 2023

Horses have a vast microbe population in their hindgut, consisting of bacteria, protozoa and fungi. With regards to bacteria, the majority are meant to break down fiber, and convert fibrous carbohydrates (structural carbs) to volatile fatty acids for energy: Clostridiaceae, Fibrobacter and Spirochaetaceae. There is also a smaller population of bacteria which can hydrolyze rapidly fermentable carbs, such as starches, to lactate and propionate: Bacillus, Lactobacillus and Streptococcus (Durham, 2009).

The microbiome of a horse varies based on the individual, with large variations observed between horses living in similar management and diet conditions, however a greater abundance of the Clostridiaceae class has been noted in healthy horses, when compared to diseased horses (Gotic et al, 2017).

Probiotics are commonly used with the intention of supporting digestive health and reducing occurrence of GI upset and gut dysbiosis (Chaucheyreas-Durand and Durand, 2010). GI upset and gut dysbiosis can result from high grain diets, periods without feed, poor forage quality, stress, antibiotic use and dietary changes (Durham, 2009).

Gut Dysbiosis [definition]: "change in composition found relative to the composition found in the healthy individual”, and plays a role in various types of disease, can contribute to colic and laminitis (Collinet et al, 2021; Cipriano-Salazar et al, 2019).

The most common probiotic strains used in supplements are lactic acid producing bacteria: lactobacilli, bifidobacteria and enteroccoci. Yeast-based strains such as Saccharomyces cerevisae and its subtype, Saccharomyces boulardii, are also common (Gotic et al, 2017). Interestingly, while Clostridiaceae class abundance has been associated with gut health, no difference has been observed in Lactobacillales class abundance between healthy and diseased horses (Gotic et al, 2017).

A probiotic is defined as a “live microorganism which can offer a health benefit to the host”. To do this, a probiotic must reach the intestinal tract, and thus must:

  • Survive acidic conditions of the stomach

  • Resist bile digestion

  • Adhere to intestinal epithelial cells

From there, there are several proposed benefits of probiotics (Schoster, Weese and Guardabassi, 2014; Schoster, 2018):

  • Improvement of nutrient absorption

  • Improvement in gut health

  • Prevent colonization by pathogenic strains

Evidence supporting the mechanism behind these benefits are mixed, and most come from in vitro studies, with some evidence in humans and lab animals, so more research is necessary!

EFFICACY (A Summary)

There is contradictory evidence of probiotic efficacy in horses - more research is required. Efficacy likely depends not only on strain, but also dose and manufacturing. The best evaluation of efficacy is in vivo studies, which is how the following probiotics were compared.

In vivo: research evaluation took place with a living animal

In vitro: occurs in a lab, usually in petri dishes or alike, using cell culturing

Ex vivo: occurs in a lab, with tissue samples from a live animal

Yeast-based strains: S. cerevisae and S. boulardii

Yeast-based strains have been shown to survive digestion and reach the cecum (Schoster, 2018), which likely contributes to why S. cerevisae has the greatest body of in vivo equine evidence. This probiotic strain has been found to increase the relative abundance of fibrolytic bacteria, improve intestinal health, and improve feed utilization, and pairs well with other digestive health feed additives to support GI health. While less evidence is available for S. boulardii, studies have found that it can improve intestinal health and reduce diarrhea.

L. acidophilus

This bacteria strain has been evaluated in horses numerous times, most commonly in combination with other strains. Studies have found that probiotics containing L. acidophilus may help to reduce salmonella shedding, increase sand output in horses fed on sand, improve mucosa stability and reduce blood lactate in exercising horses.

B. lactis

There are a handful of studies which have investigated B. lactis supplementation in horses. When combined with other probiotic strains, this bacteria strain may help support gut health during antibiotic treatment, and reduce blood lactating following a workout in exercising horses. More research is required.

L. casei, E. faecium, L. plantarum, L. salivarious

These bacteria strains have only been investigated in horses a few times, but when included in a multi-strain probiotic, have been reported to improve mucosal integrity, improve some exercise parameters, and reduce Salmonella shedding. These strains may have useful benefits for horses who are used in sport, travel often or undergoing antibiotic treatment, but require further investigation.

L. paracasei, L. brevis, L. helveticus, Streptococcus thermophilus, L. sporogenes

These strains have preliminary data suggesting some benefit when included in multi-strain probiotic & digestive health supplements, with reduced blood lactate and increased blood oxygen saturation, but more research is necessary.

L. lactis, L. reuteri, L. buchneri, P. acidilactici, S. faecium, B. subtilis, B. licheniformis

These strains have promising potential from in vitro and ex vivo studies, but lack efficacy evidence in horses. More research is required to determine their application for horses.



Probiotic strain is extremely important, as beneficial properties are not demonstrated by all probiotic microorganisms, and benefits can vary between (Gotic et al, 2017):

  • Different strains of the same bacteria species

  • Same bacteria in different animal species (ie, might be proven to be beneficial in dogs)

  • Same animal species at different life stages (ie; may be beneficial for adult horses but not foals).

As lactobacilli can transfer antibiotic-resistant genes to and from other bacteria, careful evaluation of the probiotic being used within a formulation is also critical (Schoster, Weese and Guardabassi, 2014). Since probiotic bacteria are already present in the horse’s microbiome, probiotic supplements have a natural image, which further drives their popularity for supplementation ((Chaucheyreas-Durand and Durand, 2010). However, natural does not equal safe, nor does it guarantee a health benefit.

Only a few specific strains of bacteria have been evaluated and found to elicit positive health benefits, so selection and evaluation based on strain, not class, is necessary (Weese, 2001). For instance, one Lactobacillus strain was found to be associated with diarrhea development to the point where veterinary attention was required, despite very promising beneficial effects observed in vitro (Weese and Rousseau, 2005). Specific bacteria which has not yet been evaluated for horses include (Schoster, Weese and Guardabassi, 2014): Bacteroides, Escherischia coli, Nitrobacter, Nitrosomonas, Rhodobacter, Fusobacterium, Butyrivibrio, Clostridium and Eubacterium.

As such, selecting a probiotic which contains strains that have already been evaluated in horses is a good strategy to help you achieve the intended benefit.


Studies reporting beneficial effects of probiotic supplementation have used a range of doses. Probiotic supplements use the unit “CFU”, or Colony Forming Units, to describe the number of living microorganisms within the product. While the average total dose reported to elicit benefits in horses is 180 billion CFU/day, studies have observed benefits at lower doses (10 billion CFU/day) as well!.

That said, studies using doses as high as 113 billion CFU/day also have reported no effect of supplementation (Gobosso et al, 2018), so dose response of probiotics warrants more investigation. However, manufacturing losses might mean that the effective dose is not being supplied to the horse, and can make effectiveness of the supplement unpredictable (Gotic et al, 2017).

Manufacturing Losses

As with all equine supplements, more quality control and efficacy testing is required, and most probiotics have not been subjected to clinical studies in horses (Gotic et al, 2017). A study assessing bacterial contents and label accuracy of commercial probiotic supplements reported that of 25 products assessed, only 27% met the label claim, and 32% had misspelled organism names (Weese and Martin, 2011). As such, manufacturing losses might mean that the effective dose is not being supplied to the horse, and can make effectiveness of the supplement unpredictable (Gotic et al, 2017). Berreta et al (2019) found that probiotic strains such as L. acidophilus, S. cerevisiae, L. plantarum, L. lactis, L. sporogenes might be sensitive to manufacturing processes, while E. faecium and L. casei may be more resilient.

Without research specifying how the evaluated product was manufactured, it can be difficult to compare probiotic products. Due to bacteria’s sensitivity to manufacturing, products that have been tested in horses are also great choices, as their benefits result from the dose provided in the product following manufacturing and ingestion by the horse!


The most effective way to maintain proper GI health and microbiome balance is to feed high fiber, low starch diets, while allowing normal feeding behaviour such as slow and frequent consumption of forage. However, as many common practices and activities such as competing can increase risk of GI upset, use of supplements which support digestive health may be of value.

Probiotic Selection Checklist:

  • Contain bacteria strains that are safe for the host with no undesirable health effects (have been evaluated in horses).

  • Have enough CFU present to be effective (suggested minimum 10 billion CFU/d).

  • Can survive manufacturing and storage, to deliver a product that can survive transit to the intestinal tract to elicit the health benefit (in vivo efficacy testing).

As always, we would love to help if you have any questions! For help learning how to select a supplement or compare between supplements, check out our product breakdown services or contact us today at



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