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Probiotics; Are they necessary?

Probiotics are common in today’s equine industry and human health.  They are heavily marketed for supporting gastrointestinal health on many levels.  Are they truly beneficial to the horse, or is all hype?There are many types of probiotics promoted in various products, but unfortunately, they are all lumped together and promoted as being beneficial, which can further muddy the waters. So how do you know if you need to use them and if so, what types are most beneficial? The answer to this question lies not only within research but also through clinical experience.  

To open the discussion, we must first apply some basic definitions:

1. Probiotics:  Live microorganisms when administered in adequate amounts provide health benefits to the host

2. Prebiotics: A chemical or food ingredient which can promote the growth of microorganisms, which can be anywhere including the gastrointestinal tract.

Probiotics Bacteria Used in the Horse

Probiotics Bacteria Used In The Horse

The two main groups or classifications of prebiotics include specific sugar molecules and fiber, which not only provide energy sources for the bacteria but can also assist in creating a more favorable environment for their growth.  Specific oligosaccharides and inulin are the two main accepted prebiotics by most authorities.  In other cases, we also have nutrients provided through various foods that may also serve as direct substrates for bacterial growth.  Natural sources of prebiotics include chicory root, artichoke, dandelion, onion, garlic, oats and bananas.  The use of prebiotics in the diet have been found to be beneficial in numerous human health conditions helping to reduce risk of hypertension, inflammatory bowel disease, immune related conditions and even colorectal cancer. 1

Probiotics in the Horse

Probiotics are numerous and involve many different strains of bacteria, some of which have research merit, while others do not. In the equine industry, a quick search of indicates 23 articles with the terms “horse probiotics” ranging from 1997-2014.  This number of articles is not high as compared to the human search equivalent which yielded over 8500 articles.  In actuality, much of the supporting research regarding probiotic usage is geared towards humans and not so much towards horses or even pets.  Given this, the use of probiotis in horses specifically is more extrapolated from human data.

Probiotic usage has been shown to alter populations of bacterial populations within the gastrointestinal tract, with research studies showing increased populations of Lactobacillus or Bifidobacteria, which are commonly used in probiotics. In reality, the increased level of those specific bacteria actually equates to altered populations and not necessarily improved ‘bacterial balance’ as proposed by probiotic supplement manufacturers.  There is actually little to no scientific data which demonstrates what a healthy bacterial population should be, so given the changes in these levels due to probiotic usage, we are actually uncertain as to what this really means.  Evaluating the studies that exist, probiotic usage may actually promote a change back towards a positive balance when a challenge is presented to the body, such as antibiotic usage or other stressful event, but in regards to supporting overall bacterial balance back towards normal, it is uncertain.  When it comes to specific species of probiotics, we have to keep in mind that there are many strains within that species and just because one study shows one strain to be effective or beneficial at one dose, does not mean that other strains of that same species will as well.1

The Horse Digestive Tract and Microbiome

The horse’s intestinal tract is home to an abundance of diverse bacterial, fungal and protozoal species.  It is true that often a single pathogen such as Clostridium or Salmonella species can create upset and clinical disease, but in fact, most clinical health problems are actually associated with an imbalance or dysbiosis overall. This dysbiosis in horses has been connected with a range of clinical problems from colitis to grass sickness, metabolic problems and laminitis.  It has been noted that the bacterial flora for each horse is unique and can vary, but often there are predominant species that are present including Firmicutes, Bacteriodetes, Proteobacteria, Verrucomicrobia, Actinobacteria and Spirochaetes. It is interesting to note that some species will change in their populations between healthy and diseased horses, for instance Clostridial species, which tends to be more prevalent in healthy versus diseased patients, which indicates a level of importance of this species for overall health. Lactobacillus species, interestingly, did not show a change in levels between healthy and diseased equine patients. However, in some studies evaluating metabolic and laminitic patients, Lactobacillus species tended to show higher than normal levels.  This is a noteworthy point as Lactobacillus species are one of the main probiotics promoted for use in horses.  Here is a study that we conducted at Nouvelle Research on Lactobacillus overgrowth in the horse feces. Given the information that levels of this species did not change between health and disease, and may actually be more prevalent in metabolic and laminitic patients, the question has to be raised regarding whether or not supplementation is worthwhile.  Even more, one could raise the question, given the high use of the Lactobacillus species in equine feeds and supplements, that maybe we might be contributing to a problem more so than resolving one? 2,3

The bacterial species used in most probiotics in the equine industry actually make up less than 1% of the entire microbial population within the gastrointestinal tract.  Species including Lactobacillus, Bifidobacteria and Enterococci are all common sources of probiotics for the horse, which actually make up a small percentage of overall bacteria within the hindgut. Given this low level population normally, it makes more sense to focus on those bacteria that are highly represented including Firmicutes and even Clostridial species, however data is lacking.2

Probiotics; Purpose and Desires For Efficacy

When it comes to probiotic usage, there is much concern not only over viability of that bacteria to survive the journey to its intended location, but also whether or not that bacterial species can colonize and further develop.  This can all be dependent and variable based on the strain of the bacteria and even the host species, being human or animal.  One area of concern is antibiotic resistance, which is genetically created within the probiotic bacterial species chosen.  Obviously, if an antibiotic resistant strain is selected and colonizes the host, then this could infer potential health problems down the road. In most cases with probiotics, we are more focused on colonization above survivability, meaning we not only want them to survive but to colonize and repopulate the area.  In several equine studies, colonization has proven difficult and detection of the chosen probiotic species in the feces of horses is only present during the course of administration, which infers poor colonization and a need for long term consistent administration.2

The main goal of probiotics usage is to infer health or an improved state of health upon the host.  The ultimate question is what strain or strains of bacteria are beneficial, at what dose and what administration frequency?  Probiotics infer their health benefits to the host by improving digestion potentially and preventing overgrowth of harmful or pathogenic bacterial strains which may infer disease.  They do this by 4 main mechanisms:2

  1. Modulation of the immune response
  2. Antimicrobial production
  3. Competitive exclusion
  4. Inhibition or neutralization of bacterial toxins.

Overall, we know that bacterial balance within the digestive tract is critical for health on many levels, however, each species (human or animal) is different and evidence for success of one strain in humans does not equate to same results in horses.  This can also vary further when evaluating an adult horse as compared to a foal, as microbial populations within the gut differ.  In the case of the horse, research data is weak and lacking.  What data is available is hard to decipher regarding overall efficacy as bacterial species vary as does the research model.

One area that has been evaluated in horses is acute enterocolitis or diarrhea, mainly using strains of Saccharomyces species of yeast, which demonstrated overall no distinct difference between treated and non-treated groups in regards to outcome, diarrhea recurrence or hospitalization stay. In one clinical study, the use of specific Lactobacillus species did reduce fecal shedding of Salmonella in clinically affected horses by 65% compared to 25% in the non-treatment group.  When stepping back and evaluating gastrointestinal disease, especially diarrhea, we come to realize that many bacterial populations are potentiall impacted and the success through supplementing one specific species is unlikely to yield significant results.2

The probiotic species, Saccharomyces, has demonstrated some clinical results in regards to improved cellulose digestion and even stabilization of the microflora in the horse when challenged with stress.3

Conclusion on Probiotics and the Horse

The bacterial population of the digestive tract within the horse is a complex entity, involved potentially in many facets of health and even lameness.  Disturbances or dysbiosis has been demonstrated in a variety of conditions ranging from colitis to metabolic syndrome, which helps us to see what drastic changes can infer regarding health.  Equine probiotics currently on the market promote the use of Lactobacillus and other species, which are often noted to be of low levels normally in the healthy horse. In actuality, most strains of bacteria used in equine probiotics have little research data to support their use and more often are extrapolated from human research.  The problem here is that there are variables from human to animals, and thus, the data is not completely reliable, no to mention that most cases of dysbiosis involved many bacterial species, not just one.

When it comes to probiotic usage in my personal patients, I have not found significant value in regards to bacterial based formulas. The one probiotic that we have used and still use, is Saccharomyces cerevisiae which is yeast based and commonly associated with Brewer’s yeast. We have used Saccharomyces in cases of overall poor digestion, hindgut concerns and even cases of poor hoof condition with good results as research does indicate its use improves cellulose digestion. In most of other cases, we were using the bacterial based probiotic formulas to help counter gastrointestinal disturbances including diarrhea, either as a primary problem or post antibiotic therapy.  Again, in our patients, there was often no diffference in patient outcome between those that received the probiotics and those that did not, other than cost to the owner.  I do believe that bacterial dysbiosis exists, but that the current approach to therapy or resolution to the problem is not effective.  Could this be due to improper bacterial strains used, dose used or even therapy frequency…it is uncertain.  The one therapy that does seem to work in many patients, is fecal transfunation, which is the transfer of fecal contents by nasogastric tube from a healthy horse to a clinically ill patient. This type of therapy has been utilized for decades, not only in horses but in cattle and even humans.  I believe that the main reason for efficacy is the supplying of a more broad spectrum species of bacteria, more so than relying on just one or two strains, believing that they will remedy the situation.

Bacterial dysbiosis is complex in regards to how and why it happens.  It is something that is difficult to completely measure and compare from one patient to the next.  Horses in today’s society do have a high prevalence of ulcers and hindgut concerns, which can impart gastrointestinal symptoms such as diarrhea or may actually related to clinical problems outside of the GI tract including metabolic problems and laminitis.  I think that the first thing in these patients is to admit that there is a problem in this region, either through clinical testing or personal experience.  Once we admit there is a problem, then instead of trying to supplement with a probiotic, maybe we need to dig deeper and discover why the bacterial population may be out of balance.  Is there stress that needs to be controlled, which may influencing the problem or is there a dietary relation in which may be promoting certain strains to proliferate while impairing others?  I believe that if we step back and really dig deep, the results can be much better for the patient if we look for causes and resolve them to the best of our ability, rather than rely on unsubstantiated probiotics.  One area of concern is leaky gut in the horse and how this influences health and soundness. Given the high use of certain bacterial strains in the equine industry, not just present in probiotic supplements, but also in hoof supplements, stomach supplements and even feeds…we may be potentially overdosing on bacterial populations that really only represent a small percentage in the normal equine gut, which then may actually contribute to bacterial overgrowth or dysbiosis.

This is a complicated topic and I hope this information helps.

Tom Schell, D.V.M.

Nouvelle Research, Inc.



1. Sanders, M. Probiotics; definition, sources, selection and uses. Clinical Infectious Disease; 2008; 46:S58-61

2. Schoster, A et al. Probiotic use in horses; what is the evidence for their clinical efficacy? J Vet Internal Med. 2014;28:1640-1652

3. Schell, T. Fecal Microflora and Dysbiosis: contribution to metabolic syndrome, inflammation and leaky gut. JAHVMA, 2015.


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