Probiotics Explained

Although much still needs to be learned about supplements that contain “friendly” bacteria, their potential benefits for ill or stressed horses are widely accepted.

Credit: Celia Strain

 You’re preparing your horse for a 200-mile trailer ride to your new home. You know the trip will be stressful, but you plan to wrap his legs carefully, make sure he’s up to date on his shots and even give him a preventive dose of antiulcer medication. Is there anything else you can do to safeguard his health during the trip?

 The cut on your mare’s chest was deep and dirty, but your veterinarian arrived quickly. The wound is now stitched closed and your mare is on a course of antibiotics to combat infection. During the weeks of nursing care to come, can anything else be done to support her overall health as her body mends?

 Your schedule couldn’t be any busier this fall. Each week, it seems, you’ll be at a different show, clinic or event. Your horse’s new routine will be no routine at all. What can be done to help prevent his hectic, constantly shifting schedule from dragging him down?

In each of these scenarios, one relatively new option may be helpful in safeguarding the horse’s health: probiotics, dietary supplements that contain beneficial live microorganisms, usually bacteria, but also sometimes yeast and other fungi.

Credit: Simko/

Probiotics have been studied for more than a century, and their use is now common for food animals, such as cattle. For the past decade or so, veterinarians have been administering them to horses, too, to aid recovery from serious intestinal illnesses. Described in academia during the 1960s and ’70s as any organism or substance that benefited the intestinal microbial balance, the generally accepted meaning today is “a live microbial supplement which beneficially affects the host animal by improving its intestinal microbial balance.”

However, how probiotics can help horses is still under study. “We are honestly at our infancy of understanding how we can use probiotics to benefit the horse,” says Kelcey Swyers, head nutritionist at Ranch-Way Feeds in Fort Collins, Colorado, and a PhD candidate at Colorado State University. “While research does not yet have a solid recommendation for probiotic supplementation in the equine diet, it cannot be disputed that an owner might see a benefit from offering probiotics to their horses in a real-life setting.”

Billions of bacteria, protozoa and fungi, often referred to as the intestinal flora or microflora, live in the horse’s gut. These organisms break down foodstuffs to make nutrients available, prevent growth of harmful bacteria and produce vitamins and amino acids. Probiotics are administered “to manipulate the normal intestinal flora in such a way that is beneficial to the health of the horse,” says equine nutritionist Kelcey Swyers.

Probiotic bacteria colonize the mucous layer of the equine intestines.

Intestinal flora “The main goal of administering probiotics is to manipulate the normal intestinal flora in such a way that is beneficial to the health of the horse,” Swyers says. “With that said, researchers are looking for ways that we can use probiotics to improve the digestibility of feedstuffs, reduce the incidence of digestive upsets that could lead to colic or diarrhea, and act as a natural alternative to administering antibiotics, just to name a few areas of interest.”

The inside of a horse’s gut is home to colonies of bacteria, protozoa and fungi, often referred to en masse as the intestinal flora or microflora. “It has been estimated that there are approximately five billion organisms per gram of digestive fluid in the mammalian digestive tract,” Swyers says.

These tiny organisms are engaged in the usual activities of life: consuming one set of substances, excreting another and reproducing. Together they create a complex, symbiotic web. The excretions, called metabolites, produced by one organism may feed another, which in turn produces a third that may be a nutrient necessary to the horse’s life. For example, the complex molecules in starches and cellulose cannot be directly absorbed by the horse’s intestine—they must first be broken down by the flora into components that he can use.

Credit: Eric Erbe/USDA In addition to producing many of the vitamins, amino acids and other nutrients the horse needs, the intestinal flora help keep potentially damaging bacteria, such as Salmonella and Escherichia coli, under control.

In addition to producing many of the vitamins, amino acids and other nutrients the horse needs, the intestinal flora help keep potentially damaging bacteria, such as Salmonella and Escherichia coli, under control. They do this in several ways, including colonizing the gut wall—by attaching themselves to the epithelium0, they block spaces that might otherwise be occupied by invaders. Some of the bacteria also produce lactic acid, which maintains the local environment within a narrow zone of acidity that is comfortable for the beneficial bacteria but hostile to the pathogenic ones.

Maintaining a balance Intestinal microflora function in a delicate but constantly fluctuating balance. For example, if a horse who usually eats mostly hay gets more pasture grass one day, the organisms who thrive on that particular mix of foodstuffs thrive while their counterparts decline. Most of the time, these fluxes are harmless and self-correcting. 

At other times, however, the balance can be tipped in a direction that can cause harm to the host. The classic example is the horse who breaks into the feed room and gorges on grain: Suddenly, there is too much starch to be digested in the stomach and small intestine, and it passes into the hindgut. There it fuels accelerated population growth among certain microbes, which results in greater volumes of lactic acid and a lower pH, killing off other good bacteria in the vicinity. This can begin the cascade of events that leads to devastating digestive-related illnesses, including laminitis.


But there are many other factors that can alter the microbe levels in the gut. “Anything that changes the pH, interferes with gut motility or affects the fluid levels—all would change the balance,” says Kathleen Crandell, PhD, a nutritionist with Kentucky Equine Research. In addition to sudden changes in diet, specific events that can upset the microflora levels include the administration of oral antibiotics, dehydration, fever and ingesting feed tainted with molds or bacteria.

Even stresses, such as those associated with travel and competition, can have an effect. Going on the road tends to disrupt the horse’s routine: He may not drink as much, he may be getting different hay or grasses, his anxiety may keep him off his feed entirely for atime. Also, stress produces measurable changes in levels of hormones, body temperature, immune responses, heart rate and other functions, all of which may cause the populations of beneficial bacteria to drop.

These fluctuations, too, would likely self-correct in time, but meanwhile the horse might be somewhat “off,” perhaps enough to affect his performance, to inhibit weight gain, or to contribute to digestive upsets. “[Another] goal of giving probiotics to a horse is to add organisms—seed organisms—to form new colonies and rebalance the beneficial bacteria,” says Crandell. The thinking is that by helping to restore the populations of beneficial bacteria sooner, the probiotic supplement will offset any potential negative consequences of the disruption to the microflora.

Credit: Miloslav Kaleb/ Probiotics contain living organisms. The bacteria may be in a dried, dormant state, but they can still die if mishandled. Heat is the biggest threat: “Don’t let them sit on the dashboard of your truck on a 100 degree day,” says Kathleen Crandell, PhD, a nutritionist with Kentucky Equine Research. Even some tackrooms might get too hot—if you can, keep your probiotic refrigerated. Light can also be damaging. Keep the product in its original (opaque) container out of direct sunlight and even away from bright indoor lights. Finally, don’t forget the expiration date: Make sure you purchase the product well ahead of the “use by” recommendation.

Does your horse need probiotics?Although research into probiotics for horses is still ongoing, there is general agreement that they can be useful in the following ways: 

• To give young digestive systems ahead start. Foals acquire their intestinal flora from their environment—that’s one reason why they sometimes eat manure. A probiotic may help populate the young digestive system more quickly.

• To aid recovery from illness or infection in horses who have received oral antibiotics. Medications that kill harmful bacteria sometimes do collateral damage to beneficial organisms. After the course of antibiotics is finished, a probiotic may aid in faster recovery.

• To compensate for the effects of stress. Dietary changes, travel and/or competition can put stress on a horse’s system. To give the probiotics time to colonize, it is best to start administration two to three days ahead of the stressful event and continue until it is over.

• To reduce digestive upset in horses susceptible to chronic diarrhea and/or mild colics. Probiotics may stabilize the microflora and promote a healthier environment in the gut. They may also be a good preventive measure in horses who have experienced serious colics.

• To help old or unthrifty horses better utilize nutrients. A probiotic may improve the efficiency of digestion and aid nutrient absorption in aged horses and others who have trouble maintaining weight.

Consult with your veterinarian about giving your horse probiotics, just as you would when making any other dietary change. If your horse is not holding weight or has frequent mild colics or diarrhea, a probiotic may well be part of the solution, but you’ll also want to address any underlying medical issues.

On the other hand, says Crandell, giving a probiotic to a healthy stay-at home horse can put unnecessary stress on your wallet. “There’s no reason to give a probiotic if your horse is healthy, he’s not traveling, he’s holding weight, etc.,” she explains. “Sometimes horse owners will not see a benefit from offering probiotics, and this can simply be because their horse is already healthy on his normal diet, so no additional benefit can be realized.”

On the marketA wide variety of probiotic products for horses are now available. Some are pure cultures with only one or two bacterial species; others contain up to eight or more; many are mixed with other substances, including vitamins, yeasts, electrolytes, enzymes and prebiotics. 

Living microorganisms include any of several species of bacteria, often Lactobacillus spp. as well as Enterococcus spp., Bifidobacterium spp., Lactococcus lactis, etc. Sometimes bacterial species are listed individually on the label; other products will identify only the class, such as “mixed lactic acid bacteria.”

Yeasts (Saccharomyces cerevisiae, Aspergillus spp.) offer well-established dietary benefits, including improved performance during exercise and the production of better quality milk in mares; yeasts are often included in regular horse feeds.

Prebiotics are nutrients that aren’t necessarily absorbed or utilized by the horse but instead “feed” the beneficial bacteria. You’ll often find these identified on the label as some sort of oligosaccharide, but bacteria themselves can also have a prebiotic effect if they produce by products that in turn nourish other bacteria.

Your veterinarian or equine nutritionist may well have experience with specific formulations and can advise you about which ones might best suit your situation. For example, Crandell recommends that foals be given a product that also contains dried egg yolk antibodies and, in general, she believes that products with a wide variety of bacteria or a combination of probiotics and yeast are more likely to be effective. Once you’ve got that input, it’s a matter of choosing a product and keeping close watch on your horse to see how he does.

Swyers advises caution with products that might promise too much: “Probiotics are not considered a drug, so be leery of marketing claims that state a probiotic supplement will do specific things for your horse, such as‘reduce your feed costs, reduce the potential for your horse to get colic, or enhance the performance of your horse.’”

Credit: Michael Abbey/ (center) (bottom)

If your horse does not respond to one product, he may do better with another. Even if the bacterial species are the same, there may be other ingredients in the formulation that affect how the product works. But keep in mind that you may not see results right away: “The benefits of administering probiotics to horses can only be seen when given for a prolonged amount of time, and generally when administration of that supplement goes away, so do the benefits,” Swyers says. “So feed according to the manufacturer’s label, and feed for the whole amount of time that your horse could benefit from it.”

And, she says, “like anything, just because giving ‘some’ could be good, giving more is not necessarily better. No matter what supplement you might give to your horse, it is a good practice to always follow the manufacturer’s feeding or dosing instructions as indicated on the label. And, if there are any concerns, to consult a trusted veterinarian or nutritionist.”

The next few years ought to bring more answers about the best uses of probiotics in horses. In the meantime, if a probiotic is helping to keep your horse happier and healthier, there’s no reason not to keep offering it.




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