As medicine trends more than ever toward natural treatments, scientists are taking a closer look at the digestive system's impact on health, for humans and horses. The gut hosts an array of microbes that digest food to produce energy, vitamins, and other substances that benefit the body, while maintaining a balance between beneficial and harmful bacteria. This environment is called the microbiome.
Probiotics and prebiotics are mentioned often as a way to maintain a healthy microbiome. Probiotics are beneficial bacteria, and prebiotics are the “food” that nourishes these beneficial bacteria. Studies into the efficacy of probiotics in horses have returned variable results, with each study seeming to produce more questions than answers about this complex environment. Not only is the function of the microbiome different from species to species, but it also is highly individualized within members of a species. Moreover, certain probiotics appear to be more effective against particular conditions than others. In some situations, certain probiotics actually have been found to be detrimental.
“If there is efficacy, it is very dose-specific and the species of microbes you are putting in are very specific,” said Dr. Joe Pagan, founder and president of Kentucky Equine Research. “I get the impression that most of the commercially available products are not very specific.”
For this reason, Pagan does not advocate giving probiotics to a healthy horse or routinely administering them daily as a supplement. Rather, he believes they only should be used to address a specific issue, such as a horse whose microbiome has been compromised by antibiotics, and the species of microbe should be one that has shown efficacy in that particular situation.
“Giving a prebiotic every single day is a different matter,” he said. “Instead of feeding the live microbes to the horse, you're using nutrients that grow the beneficial microbes so they can outcompete the harmful microbes. Something as simple as beet pulp, I would consider a prebiotic.”
In a 2015 study at Texas A & M University, the late Dr. Josie Coverdale recognized two forms of prebiotics (fructooligosaccharides from barley and wheat and mannan-oligosaccharides from yeast) for their role in improving digestion and reducing disruption of the microbiome after an abrupt change in diet.
She concluded: “Potential use of prebiotics and probiotics to create greater stability in the equine microbiome impacts not only the digestibility of feed but also the health of the horse.”
But some studies of probiotics contradict each other.
Japanese researchers developed a product from Lactobacillus and bifidobacteria from the intestines of Thoroughbreds and then fed it to foals from birth to 20 weeks of age. They saw a 45 percent reduction in incidence of diarrhea between the treated group and the control group and a shorter duration in diarrhea that did occur.
At Guelph University in Ontario, healthy foals were placed on an unidentified probiotic for three weeks after birth. The researchers found there was no benefit, and there actually were adverse effects. “Foals treated with probiotics were more likely to develop diarrhea requiring veterinary intervention,” their report read. “Foals 8-15 days old having the highest probability of developing diarrhea.”
Other studies of probiotic use to prevent sand colic, treat antimicrobial‐associated enterocolitis, and combat salmonella did not return convincing results because of too few horses in the study or too many variables that may have affected the outcome.
Further complicating the issue is the fact commercial probiotic products are subject only to quality standards set by the individual manufacturer. The company's reputation is the only guarantee what is listed on the label is contained in the product and those ingredients are of high quality. But even quality products can break down when stored in an environment hostile to the live microbes. Microbes may not survive in a product shipped in hot, humid weather by the time it reaches the consumer, either at the tack shop or delivered to the farm.
“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,” concluded Dr. Scott Weese, internal medicine specialist at Guelph considered to be one of the foremost authorities in prebiotics/probiotics.
“The problem we have in the equine sector, when talking about probiotics, is that you have to start each sentence with 'theoretically' until we get really convincing data to warrant their continued use,” Pagan said. “It may be that there are instances where the routine chronic introduction of a specific probiotic may be beneficial, but more research is needed in horses.”
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