Throughout the 100 feet of intestines packed as winding passageways inside your horse’s abdomen, life runs abundant, in abundant forms. Entire communities of bacteria, fungi, and protozoa share this microcosm, known as the gut microbiome, where they interact with your horse—and with each other. Worthy of their own science fiction movie, these millions of independent, living organisms have an entire world of their own within the equine gut’s lining, with “good guys,” “bad guys,” and the ever-constant risk of some group taking up too much power and destroying their universe. It’s a fascinating but highly complex story—and it’s told by science.

As researchers move toward a better understanding of the equine gut microbiome, one scientist has sought to unite the more than 150 studies exploring this somewhat obscure part of the horse’s body. The objective of such a globally encompassing review was to “assimilate our current knowledge on equine microbiome studies,” and in particular to focus on the various effects of factors that could influence gastrointestinal microbiota in horses, said veterinarian and nutritionist Anna Garber, PhD, of the School of Veterinary Medicine at the University of Glasgow, in the U.K.

Understanding Good and Bad Gut Bacteria

Understanding the gut microbiome and how different factors positively or negatively affect it would be easier if we understood what truly constitutes “good” and “bad” bacteria. But it’s not that cut and dry, said Garber. Much depends on delicate balances as well as on context, making the evaluation of factors particularly challenging.

“It’s difficult to divide what is good and what is bad,” Garber said. “It’s not always clear what can be considered negative changes in the gut microbiome of the horse