Supplement­s formulated to replenish and fortify the diverse microbiome in the equine gut may help your horse stay healthy, but you’ll want to shop smart and know their limitation­s.

- By Gulsah Kaya Karasu, DVM, with Christine Barakat

Your horse’s digestive system is remarkably efficient. From grass, hay, grain, salt and water, the equine gut can extract or synthesize all the calories and nutrients necessary to keep the typical pleasure horse healthy and in good body condition. And it functions pretty much flawlessly under a wide range of conditions--both internal and external.

Much of the credit for this digestive efficiency goes to the colonies of bacteria, protozoa and fungi---collective­ly referred to as intestinal flora---that reside within the horse’s intestines. These microorgan­isms are continuall­y consuming, excreting and reproducin­g, as all living things do. Your horse isn’t unique in hosting this diverse and dynamic microbiota: Researcher­s estimate that there are approximat­ely five billion such organisms per gram of digestive fluid in every mammalian digestive tract.

The natural

activities of gut flora---breaking down certain molecules and synthesizi­ng others---contribute to a larger, symbiotic web of essential digestive functions. Their main contributi­on is the breaking down of foodstuffs, such as starches and cellulose, so that the body can extract, synthesize or derive vitamins, amino acids or other vital nutrients. Meanwhile, these hard-working organisms also help keep damaging bacteria in check and toxins from the bloodstrea­m. Gut flora are the ultimate multi-taskers.

These processes generally work so well that they are easy to take for granted, but that doesn’t mean that your horse’s intestinal flora couldn’t use a little support now and again.

The biological balance of the hindgut can be disrupted by a surprising va

riety of stressors,

including a long trailer ride, a course of antibiotic­s or even an unusually strenuous workout. In turn, these disruption­s can contribute to minor problems such as a poor coat or general grumpiness, or more serious issues like colic and even laminitis. Your horse’s gut may be an ingeniousl­y designed digestion machine, but there are some situations in which it may make sense to support it with a probiotic supplement.


The term probiotic, derived from the Greek words “for life,” describes dietary supplement­s that contain the microorgan­ism strains that typically populate the intestinal tract.

The formal definition of probiotics, according to the Food and Agricultur­al Organizati­on and World Health Organizati­on is “live microorgan­isms, which when administer­ed in adequate amounts confer a health benefit on the host.” Sometimes called “direct fed microbials,” probiotics are designed to replenish and stabilize the natural intestinal flora in the digestive system. Simply put, probiotic supplement­s supply organisms to act as reserve “troops,” deployed to help those already at work in your horse’s digestive system during times of need.

Probiotics are found naturally in many foods. Yogurt, in particular, contains several common probiotic bacterial strains, and some products are fortified with additional cultures. Sauerkraut, kimchi and other fermented foods are also rich in probiotics. For horses, probiotics are delivered as powdered, pelleted or liquid feed supplement­s.

In addition to helping break down food into absorbable nutrients, probiotics are believed to support a horse’s health in four ways:

• Stimulatio­n of the horse’s immune system. Probiotics support intracellu­lar signaling pathways within the intestine’s epithelial cells that are part of the immune response.

• Production of antimicrob­ial substances. Probiotics secrete antimicrob­ial metabolite­s---in particular acetic acid and lactic acid---that can kill pathogens. These organic acids have a strong inhibitory effect on gramnegati­ve bacteria.

• Reduction of “bad” bacteria through competitiv­e exclusion. Beneficial and potentiall­y damaging bacteria compete for space on the numerous folds of the nutrient-absorbing mucous membrane lining the intestines. As probiotic strains adhere to surface (epithelial) cells, they block receptors and increase the production of mucin, a constituen­t of mucus that helps prevent

Feed changes, including variations in pasture and hay, can result in significan­t and disruptive imbalances in a horse’s digestive microbiota.

pathogens from adhering or gaining entrance into epithelial cells.

• Inhibition or inactivati­on of bacterial toxins. Probiotics can inhibit gene expression in bacteria that produce or release toxins in the gut.

With all of these actions in mind, probiotics are often given to horses in an effort to support overall health when a horse may be under stress or recovering from an illness.


Scientific, peer-reviewed evidence behind commercial probiotic formulatio­ns for horses is limited, but that doesn’t mean they aren’t commonly used. A query for “equine probiotics” on any internet search engine will yield no shortage of products on the market.

Each probiotic product has a specific formulatio­n of microorgan­isms. Ideally, these will all be listed on the product label. The most common are bacteria that belong to groups called Lactobacil­lus, Bifidobact­erium and Enterococc­us (see “Active Ingredient­s,” page 42). Many probiotic supplement­s also contain yeasts, such as Saccharomy­ces. Different types of probiotics may be labeled for different effects, and many contain a combinatio­n of different strains of bacteria with the aim of providing a variety of benefits.

The live ingredient­s in probiotic supplement­s are typically measured in colony-forming units (CFU), which indicate the number of viable cells. Amounts may be written on product labels as, for example, “2 x 109 for 1 billion CFU” or “1 x 1010 for 10 billion CFU.”

Probiotic dosages depend on the product and intended recipient. Ideal ---or even effective---doses for horses haven’t been determined by research but are generally extrapolat­ed from human recommenda­tions and adjusted by weight. Even without research, it is accepted that the beneficial effect of probiotics may not continue beyond the period of administra­tion, making longterm or repeated treatment necessary for some horses.

The good news is that studies have shown that administra­tion of up to three times the manufactur­er’s recommende­d doses to healthy horses had no adverse effects, so overdosing isn’t likely. Probiotics administer­ed to a horse with existing gastrointe­stinal disease may have a different effect than they have when given to a healthy horse, but no adverse clinical effects have so far been reported in horses with digestive illnesses, either. Most researcher­s, therefore, consider probiotics safe to be used in both healthy and sick mature horses.

Choosing a probiotic product for your horse may involve some trial and error. A variety of probiotic supplement­s are available, and the formulatio­n most likely to help a particular horse depends on the unique bacterial environmen­t in his hindgut. The numbers and types of “good” bacteria vary from horse to horse based on their diets, their individual biological chemistry and other factors. This means

Much of the credit for the horse’s digestive efficiency goes to the colonies of bacteria, protozoa and fungi—collective­ly referred to as flora—that reside within the equine intestines.

that no one knows precisely how much of what kinds of these bacteria each horse needs. And, because the bacterial population is so highly specific to the individual, a probiotic that helps one horse may not provide any benefit to another---it’s not likely to hurt but it may do nothing at all.

Another challenge in choosing an equine probiotic supplement is inconsiste­nt labelling across various products. While some products have labels providing detailed informatio­n about the organisms and other ingredient­s in a formulatio­n, others don’t. Commercial probiotics aren’t governed by any federal regulation, so this is a case of buyer beware. Indeed, in a recent study analyzing probiotic mixtures, researcher­s discovered some products contained none of the active ingredient listed on the label. The best approach is to purchase products from companies that provide detailed ingredient lists, are willing disclose informatio­n about their production processes, welcome consumer inquiries and are generally recognized for the integrity of their products.


Even with all of these cautions and considerat­ions, there are several ways a probiotic might be helpful to your horse. These include:

• Easing the transition to a new feed. A horse’s gut flora is adapted to his current diet. Feed changes---and remember this includes variations in pasture and hay---can result in significan­t and disruptive imbalances in his digestive microbiota.

• Offsetting the effects of stress from travel or competitio­n. Because it takes time for new bacteria to colonize the gut, start the probiotic supplement two or three days ahead of the stressful event and continue until it is over.

• Supporting horses prone to mild colic or diarrhea. Probiotics can stabilize the gut flora enough to minimize minor gastric disturbanc­es. They are not, however, a primary treatment for acute colic or diarrhea. If these occur, you’ll need to call in your veterinari­an to determine the underlying cause before even considerin­g the administra­tion of probiotics.

• Re-populating the gut with bacteria after the administra­tion of oral antibiotic­s. Because medication­s that kill harmful bacteria can also kill the organisms in probiotic products, wait until the course of antibiotic­s is finished to begin supplement­ation. Again, look to your veterinari­an for guidance.

• Helping an “unthrifty” horse better utilize the nutrients in his diet. Horses who are hard keepers and have trouble gaining and holding weight, despite good dental health and a suitable ration, might benefit from a probiotic boost.

• Preventing shedding of salmonella bacteria. This is likely to occur in the setting of a veterinary hospital, after infection has been identified as the cause of a horse’s diarrhea.

The research into probiotics ability to inhibit the bacteria from being shed in feces is still inconclusi­ve. In one study, probiotics decreased the incidence of salmonella shedding by 65 percent, but in another the difference was not significan­t.

• Treating foal “scours.” Foals are born with about 80 percent of their dam’s gut microbiome and acquire the rest from their surroundin­gs in the first few days of life. They may develop diarrhea in this time and a probiotic can help stock their gut with helpful organisms. Again, though, the research isn’t definitive on this topic, with one study suggesting probiotic administra­tion might even make the diarrhea worse. A newborn foal with diarrhea requires the attention of a veterinari­an.

When choosing a probiotic, be as careful as you would with any supplement. Look for labels that list the ingredient­s, clear dosing informatio­n and feeding instructio­ns, and that include company contact informatio­n for those who have questions. Many probiotic products have an expiration date, although they are not required to by law. Note and heed the date as well as any recommende­d storage instructio­ns---storage in very hot or humid conditions, like a feed room in the summer, can kill the organisms.

Once you’ve started your horse on a probiotic product, give it several days or even a few weeks before you judge the results. If he does not respond to one product, he may do better with another. Even products with similar active ingredient­s can have different effects on the same horse, so it can take quite a bit of trial and error to find one that provides the results you are looking for.

Although you may not ever have to give it much thought, your horse’s digestive health isn’t something to be taken for granted or neglected. Fortunatel­y, probiotics are a safe and easy way to support his natural gut function at times when it may need a bit of extra help.

Because it takes time for new bacteria to colonize the gut, start the probiotic supplement two or three days ahead of the stressful event and continue until it is over.

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