Probiotics in Horses

I’ve written frequently about probiotics, microorganisms given as supplements to hopefully benefit health.  Often claimed as an “alternative” therapy, probiotics are really no different from any therapy in science-based medicine. The theory behind their use is certainly consistent with established scientific knowledge. Microorganisms are undoubtedly important in health and disease for humans and most other animals. And there is significant in vitro and lab animal research to suggest some value to the use of probiotics. Finally, there is some limited clinical evidence for the safety and efficacy of some probiotics for some health issues in human and veterinary patients.

However, there are also problems with probiotics as a medical therapy.  The natural microbial ecosystem of every individual is different, and the flora of different species is often quite dissimilar. We know very little about the type and role of most of the hundreds of species of microorganisms that live in and on most animals. Whether a particular probiotic organism helps, harms, or does nothing at all in a given patient depends on many factors, most of which we are still quite ignorant about. So our ability to utilize the potential of probiotic therapy is rudimentary for now, and it will only improve as we do the laborious work of building our knowledge and understanding.

Probiotic therapy is also hampered by the lack of regulatory oversight, especially here in the U.S. Several research studies have shown that the majority of probiotic products on the market are mislabeled and often contain nothing resembling what is on the label. A ridiculous proportion of these products contain no living organisms at all, and so are in no real sense even probiotics. The dramatic claims often made for such products are almost never justified given the limited evidence and abysmal quality control for most probiotic products.

This month’s issue of the Journal of Veterinary Internal Medicine has an excellent article on the subject of probiotics in veterinary medicine. Though this article focuses on probiotics for horses, a species I do not treat in practice, in touches on many important issues concerning probiotics that are also relevant to probiotics for dogs, cats, and other veterinary species.

Schoster, A. Weese, J.S. and Guardabassi. L. Probiotic Use in Horses – What is the Evidence for Their Clinical Efficacy? J Vet Intern Med 2014;28:1640–1652

The authors give a nice overview about the kinds of organisms that are most appropriate for use as probiotics. The usual criteria include the ability to survive passage through the acid environment of the stomach and commercial processing, adhere to cells in the GI tract, and have a measurable, beneficial effect on the patient. The available evidence they cite suggests that a probiotic organism need not necessarily come from the species in which it is to be used, though there is some debate about this.

The authors also mention a criterion often overlooked, especially by those who claim probiotics are inherently safe because they are “natural.” Microorganisms can share genetic material, and one type of gene sharing that can create significant harm is the transfer of antiobiotc resistance genes. If a probiotic organism has resistance to antibiotics and conveys this to another organism in the gut that can cause disease, this will make treatment of that disease more difficult if it develops. In Europe, regulators require probiotic product be tested for such antibiotic resistance, but this is not the case in the U.S.

The authors also cover the primary mechanisms of action proposed for probiotics: immune system modulation, production of antimicrobial substances, out-competing potentially harmful microorganisms in the gut ecosystem, and inhibition or inactivation of toxins produced by other organisms. The diversity of effects, and the differences between organisms, illustrate the complexity of such biological therapies, and why simple, general rules about “good” and “bad” organisms aren’t very reliable.

The authors discuss the issues of poor quality control and inadequate regulatory oversight, which I have already mentioned.

The article also covers the always-important question of safety. Few adverse effects have been reported in people or animals taking probiotics, but it is unclear what the real risks are at this point. For one thing, many probiotic products, as I have already mentioned, are mislabeled. If they don’t contain any appreciable quantity of live organisms, they of course aren’t likely to have any harmful effects, but they aren’t likely to have any positive effects either. For this reason, the absence of reports of harm may reflect the absence of any effect at all for many probiotic products. At least some negative effects have been reported, in humans and in other animals, so while these products are probably pretty low risk, there are likely some circumstances and individuals in which they can cause harm, and more research is needed o understand the risks.

Lastly, the article covers the clinical studies of probiotics in horses to date. Only eight studies are cited, with a mix of positive and negative results. Some studies showed beneficial effects, but many showed either no difference from placebo or effects on some outcome measures and not others. And at least one reported a negative effect. The overall evidence suggests some possible value to probiotic therapy, but at this time the risks and benefits are largely unknown. The authors conclusions are these:

Although probiotics have shown promise in the treatment of selected diseases in humans, the evidence that they can be used to control diseases in horses so far is weak.

Based on lack of regulation regarding quality control of commercial products, use of over-the-counter products is questionable, particularly in the absence of scientific information on safety and clinical efficacy.

Despite all of these limitations, probiotics generally are regarded as safe, cost effective and easy to administer. Therefore, additional research is warranted to test possible applications in equine veterinary practice.

Very similar conclusions likely apply to the use of probiotics in small animals. They are probably low risk and are relatively easy to use and inexpensive. In most cases, there is little evidence that they have significant benefits, but their use for some conditions, such as diarrhea, is reasonable given the limited but suggestive evidence available so far.

Unfortunately, most products are probably unreliable in quality, so those used should be ones from reputable companies that have demonstrated reasonable labeling accuracy (for example, Iams’ Prostora and Purina’s Fortiflora).

More research is certainly needed, and I am hopeful that as it is done more and better evidence will be available to support specific uses of particular probiotic organisms for particular problems. At this time, however, excessively broad or confident claims are, as always, unjustified, and we need to be clear with our clients that there is a lot of uncertainty and guesswork in our use of probiotics.

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