Solid research proving probiotics' worth to horses is missing, even though they have tremendous backing from anecdotal evidence.
Photo: Alexandra Beckstett, The Horse Managing Editor
Probiotics have been used by people for at least a century to promote good health. Hard scientific evidence has proven that probiotics can be beneficial to humans. As it so often does, the horse world saw this good thing and adapted it to equine uses. But there's one difference: Solid research proving probiotics' worth to horses is missing, even though they have tremendous backing from anecdotal evidence. Here, we sift some of the wheat from the chaff to help expose the facts, the hypotheses, and the potential myths surrounding probiotic use in horses.
It Starts With Microbes
To understand the basic intent of probiotic use, it helps to understand the role of beneficial microorganisms in the horse's digestive system. A variety of beneficial bacteria live throughout the equine gastrointestinal tract, with the highest numbers concentrated in the cecum and large intestine, explains Scott Weese, DVM, DVSc, Dipl. ACVIM, assistant professor of clinical studies at the University of Guelph's Ontario Veterinary College.
These microbes assist in the digestive process. In particular, they help the horse's gut break down the glucose bonds in insoluble carbohydrates, which come from the plants (hay, grass, grains) that horses eat. Without the microbes, your horse couldn't effectively digest these products.
Horses gradually accumulate these microbial allies as they grow, and by adulthood, a healthy horse possesses a complex, dynamic microbial population. Many factors can potentially alter the type and number of bacteria present in the horse's gut at any given time. These include stresses such as hauling and competing; changes in diet, management routine, or lodgings; gestation and milk production; disease; antibiotic use; dehydration; and even fluctuations in the weather. On the mild side, effects of bacterial population shifts can decrease the efficiency of your horse's digestive system. At worst, a significant disruption of the normal, "protective" microbial population could allow for development of colic or diarrhea caused by "bad" bacteria such as Salmonella.
Probiotics by Definition
This is where probiotics come in. In essence, they are intended to stabilize the internal microbial population during minor daily upsets as well as larger, occasional upheavals. The theory is that this will lead to better health and efficiency of the digestive system, as well as multiple other health benefits.
A good definition of probiotics, says Weese, "is that they're live microorganisms given orally that give a beneficial health effect in the horse. That's key, that it must do some good."
For a probiotic to be effective, a number of things must happen, Weese explains. First, the bacteria must be able to survive passage through the intestinal tract. And they must be able to restore critical microbial populations by going on to rapidly reproduce (or colonize) in the cecum and large intestine.
Of the thousands of microbes in the horse's system, Weese notes that, "in general, different species of Lactobacillus and Bifidobacterium are the two most common and have the best research behind them."
How Do They Work?
To this point, we've dealt in undebated fact. But from here on, the information on probiotics and horses begins to enter a land of conjecture and even controversy. It begins with the basic question of how probiotics actually work. As Weese succinctly puts it: "We don't really know."
Probably, he says, the mechanism varies from one species of microbe to another and from one host species to another. Thus far, researchers have focused on the following four possibilities as the most likely mechanisms of probiotic action:
- Stimulation of the immune system, improving the horse's overall immune response.
- Production of anti-microbial substances. For instance, some research has shown that lactic acid bacteria have anti-viral properties that inhibit harmful organisms, including E. coli, Staphylococcus, Clostridium, and Salmonella.
- Alteration of the gut pH, keeping it acidic and thus inhospitable to certain harmful bacteria, particularly those related to some types of diarrhea.
- Activation of the "competitive exclusion" principle, wherein the good bacteria essentially push out the bad bacteria. The idea is that if good microbes occupy a particular niche in the digestive system, then there's no room for the bad bacteria to colonize there, and not enough nutrients to let them survive.
Do They Work?
The search for concrete answers continues when you look at the benefits probiotics are said to have for horses. According to Weese, "No one has been able to show that they are effective in horses." Still, he adds, "Probiotics will probably be shown to be useful to horses, but probably only a few species (of bacteria)."
Extrapolating from research conducted in other species, he adds, probiotics could theoretically prove beneficial to horses with these particular health problems:
- Chronic diarrhea, since probiotics have been shown to be effective against chronic inflammatory bowel disease in other species;
- As an adjunct treatment in acute diarrhea, to help the colon return to a normal state;
- Foal diarrhea, since foal digestive systems don't yet have complete microbial populations;
Side effects of antibiotic use, particularly in decreasing the risk of antibiotic-
associated diarrhea; and
- Potentially the prevention of colic, particularly gas colic, which might be linked to disruption of internal flora.
Proponents of probiotic use point to even more potential uses, including reducing the incidence of developmental orthopedic disease (DOD), increasing the butter fat content in broodmares' milk, and weight maintenance or gain. Some advocates of probiotics encourage using these products anytime a horse is stressed, such as during care after surgery, for horses that are stall-bound due to illness or injury, those that are trailered long distances, or those being bred.
Some fans of probiotics also believe these products are beneficial to young foals and older horses that have less efficient digestive systems. However, Weese isn't convinced that old age has a negative impact on the intestinal flora, provided the horse doesn't experience any diet or management changes.
If you're interested in giving probiotics a try in your horse's health care program, Weese advises caution when purchasing commercial products. He points to five major areas of concern:
1) Non-regulation--Since probiotics are considered a nutraceutical (or sometimes a food supplement), they are not regulated as drugs, says Weese. As long as the manufacturer makes no specific claims on the label--such as stating that the product prevents foal diarrhea--the company doesn't have to prove the product's efficacy or safety.
2) Lack of testing--"Most probiotics commercially available have no testing behind them," says Weese. In particular, he adds, while some lab research has been conducted and anecdotes abound, few, if any, scientific studies have evaluated probiotics in the field. So there is no solid proof that the products actually work when used in live horses with real health issues.
3) Mislabeling--"In one study, most veterinary probiotics didn't have what [the label] said they contained," says Weese. "The majority were mislabeled, there were misspellings of the contents, and in one case, an organism was listed that doesn't exist."
4) Dosage--The first point here is that an effective dosage level for horses has not yet been determined. Still, Weese suspects that adult horses probably need 100 to 500 billion colony forming units (CFUs), and foals might need as many as 100 billion CFUs for the probiotics to have any effect. The second point is that some probiotic products have low concentrations of CFUs, making it more difficult to meet dosage recommendations. If the product has, for instance, a mere 10 million CFUs per gram, then you'll need to feed large quantities for your horse to receive the recommended dose.
5) Potential to harm--In general, says Weese, probiotics won't do any harm. "The FDA classifies most probiotics as 'generally regarded as safe (GRAS),' which is its mildest classification," he says. However, Weese adds, there's a caveat to that statement: In the case of probiotics, too much of a good thing might potentially cause harm. Furthermore, while one particular strain of a microbial species might be beneficial, a different strain of that same microbe can cause health problems.
At the University of Guelph, Weese and colleagues conducted a field trial of a probiotic they developed that showed beneficial effects in the lab. However, when given in high doses to a group of young foals, the group that received the probiotic had a higher incidence of diarrhea compared to a group that received a placebo.
With these cautions in mind, Weese also has some advice on selecting a probiotic product with the most potential to do good:
- Freeze-dried products are probably a better choice than liquid. A lot of liquids are not truly probiotics, but are actually microbial byproducts and probably have very low numbers of actual organisms, says Weese. The powders made from freeze-dried microbes tend to be more concentrated, meaning you'll need to give less quantity of the product to reach a potentially effective dosage.
- Capsules might be a better choice than loose powder. Probiotics can be damaged by exposure to sunlight or changing temperatures, and Weese speculates that the capsule shell might offer some protection, thus potentially extending the product's shelf life.
- Microbes proven in other species are probably your best bets until products proven to work in horses are available, says Weese. Such organisms include Lactobacillus acidophilus and Lactobacillus rhamnosus.
- Purchasing from a grocery store might save you money, and purchasing probiotics made by major nutraceutical companies might mean better quality control procedures behind the product.
In addition, Weese urges you to critically assess the label of any probiotic you consider purchasing. Make sure that contents are specifically listed--not simply called a "blend" or "mix," but with specific species and strains noted. In addition, the ingredients should be correctly spelled, and concentrations should be clearly stated.
"If they don't tell you what's in it or what the numbers are, or they can't spell it, I can't have good confidence in them," says Weese.
Also look for references to research and testing, in particular studies done on colonization and effectiveness of the product in horses. But don't expect to find too much, because according to Weese, no probiotic company has yet published their research findings in a major scientific journal. This is one area where public pressure could make all the difference.
"We need companies to show that (their products) have beneficial effects and that they are safe," says Weese. "We need well-designed efficacy and safety testing that is scientific, not anecdotal. And the only way we'll get that research is if people start demanding it. So if you're going to use them, start putting pressure on companies. Tell them you want good research showing that their products are safe and effective."
Meanwhile, if you opt to try probiotics with your horse, Weese recommends consulting with your veterinarian. Then keep an eye on your horse's overall health when you begin using a product. If you don't note a positive impact within a couple of weeks--or especially if any negative effects pop up, such as loss of appetite, diarrhea, or colic--discontinue use. Otherwise, you can continue with the product as needed.
About the Author
Sushil Dulai Wenholz is a free-lance writer based in Lakewood, Colo. Her work appears in a number of leading equine publications, and she has earned awards from the American Horse Publications and the Western Fairs Association.
POLL: University Equine Hospitals