Published November, 2014

Imagine miles and miles of beautiful, green, lush rolling pasture stretching to the horizon and surrounded by pristine white fences and glossy-coated horses grazing. It's hardly believable that the verdant grass these horses graze could lead to the debilitating disease called laminitis.

However, scientists have tied pasture-associated laminitis, or PAL, to horses' intake of nonstructural carbohydrates (sugars and starch), particularly fructans (long chains of fructose), in grasses. University of Kentucky (UK) researchers recently took some early steps toward finding a solution to this problem in a relatively common plant: hops (Humulus lupus).

When fructans reach the hindgut they cause certain strains of bacteria that produce lactic acid to proliferate, resulting in a drop in pH and a more acidic environment. Often referred to as hindgut acidosis, the pH decrease causes the intestinal cells to become more permeable, allowing bacterial metabolites to enter the bloodstream and ultimately leading to laminitis.

Similarities exist between equine hindgut acidosis and rumen acidosis in cattle. When a large amount of nonstructural carbohydrates reach the rumen, the main fermentation portion of the digestive tract, a bacterial strain known asStreptococcus bovis, produces lactic acid and decreases the rumen's pH. Researchers have also discovered that while the ionophore monensin is toxic to horses, it is highly effective in treating rumen acidosis in cattle. Ionophores draw cations, such as potassium, out of cells, ultimately preventing further bacterial growth. With this in mind, a team from UK and the USDA Agricultural Research Service set out to evaluate whether a natural, nontoxic alternative to monensin—hops extract—could mitigate changes (such as lactic acid bacteria proliferation and the drop in pH) associated with fructan fermentation and laminitis development.

In their in vitro (in an artificial environment) study the researchers collected fecal samples from three Thoroughbred mares, aged 19 to 23 years, and isolated from them the bacterial cells. “This is the GI microbiologist's version of tissue culture,” explained Michael Flythe, PhD, research microbiologist for the USDA Agricultural Research Service.

The researchers used inulin from chicory as a model fructan, as previous research has shown that administering this carbohydrate to horses will also cause laminitis. First, the team added inulin to each horse's bacterial cell suspension to determine its effect on pH. Next, they added hops extract to determine how it would affect lactic acid production.

As anticipated, adding inulin decreased the cell suspension's pH. When inulin was added with hops extract, the pH immediately decreased and then stabilized over a 12-hour period. However, hops extract additions at concentrations greater than or equal to 9 and 18 parts per million (ppm) mitigated pH decline.

The researchers then sampled, diluted, and incubated inulin-fermenting bacteria on growth media for 24 to 48 hours. After they isolated the bacteria, they performed genetic testing that classified the bacteria as S. bovis, which has previously been linked to laminitis development. The team also determined that by adding 18 to 45 ppm extract to the fecal cell suspensions containing inulin, they could inhibit S. bovis' lactic acid production by 90% and 99%, respectively.

So what does all this mean?

“This was a laboratory study that was performed to determine if hops extract could be used to control bacterial proliferation associated with fructan fermentation," explained Brittany Harlow, MS, a graduate research assistant at the University of Kentucky. "We found that in our in vitro model, hops extract at very low concentrations was extremely effective at mitigating the growth of these microorganisms (specifically S. bovis), lactic acid production, and consequent pH decline.

She said these results are exciting because they suggest that veterinarians could potentially use hops extract to control the microorganism-mediated problems linked to health conditions associated with carbohydrate overload.

Harlow cautioned, however, that this study was only conducted in the laboratory with equine fecal bacteria, and the safety and efficacy of the hops extract in horses has not been tested.

“For that reason, we don't recommend that hops extract be given to horses yet," she said. "Hopefully, we will have the opportunity to test the safety and efficacy of hops extract using horses in the future.”

The study, "Inhibition of fructan-fermenting equine faecal bacteria andStreptococcus bovis by hops (Humulus lupulus L.) β-acid," was published in theJournal of Applied Microbiology. 

Kristen M. Janicki, MS, PAS