Finc out what scientists are learn a oout the communiy of microoes inside your horse. By Elaine Pascoe

hen it comes to nutrition, nothing is too good for your horse. You give him the best hay available and a top-quality concentrate along with unlimited fresh, clean water. You study feed labels and nutrition tables. Maybe you add a supplement. But even if your horse

cleans his bucket and whiffs up every strand of hay in reach, he wont get the nutrition he needs without help.

That help comes from trillions of microbes in his digestive tract. A

horse's gut microbiome—the entire community of bacteria and other microscopic organisms that live in his intestine—plays critical roles_not only in digestion but also in his general health. Yet little is_known about it. Although research is revealing more about the organisms that make up this community (the gut microbiota, or gut flora), "Our understand-ing of that, and what the results actually mean, is still fairly basic," says

Even if your horse eats a top-quality concentrate and the best hay available, he won't

get the nutrition he needs

without help from the trillions

of microbes that live in his

digestive tract.

Scott Weese, DVM, a professor in the De-partment of Pathobiology at the University of Guelph's Ontario Veterinary College. Here, Dr. Weese helps explain what scien-tists are learning about the microbes, what happens when they are disrupted and how they can be kept in balance.

How Microbes Help It may seem strange, even creepy, to think that your horse plays host to hordes of

ABOVE: A vet listens for disruption of the microbial balance, which can cause colic and diarrhea.

LEFT: Although antibiotics are an essential defense against disease-causing microbes, they can be disruptive to the gut bacteria balance.

microbes; but so do you. In fact, all animals partner with microbes in this way. You have trillions in and on your body right now—so many, it's thought, that they outnumber

your body cells 10 to one. We're used to thinking of microbes as pathogens, agents of disease that should be wiped out. But most of these microbes are beneficial or at worst harmless.

Those microbes in your horse's in-testines, for example, are not hitching a free ride. Your horse would have trouble surviving without them. "Microbes play immense roles in various functions, includ-ing nutrition," Dr. Weese says. "The gut

microbiota also interacts with the immune system, interacts with the brain and can be associated with a wide range of diseases." In humans, changes in gut microbiota have been linked to allergy, immune-me-diated diseases, obesity, cancer, metabolic diseases such as diabetes, and many other conditions, he notes. A healthy commu-nity of microbes may help keep disease-causing bacteria at bay, stimulating the lin-ing of the intestines to produce antibodies against them.

Future research may show similar links in horses, but right now the best under-stood role involves nutrition. Microbes in the hindgut (the cecum and large intes-tine) help the horse digest fibrous plant materials like cellulose (the complex car-bohydrate that makes up plant cell walls) that you find mostly indigestible—materials that, since he's an herbivore, make up a large part of his diet. These materials pass through the horse's stomach and small intestine pretty much intact. When they enter the cecum, the microbes that live there set to work.

The microbes ferment the fibrous

54 PRACTICAL HORSEMAN • DECEMBER 2014

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material, breaking it down into sugar (glu-cose) and volatile fatty acids, which are the primary fuel for most body tissues. The process is slow. It takes up to three days for food to pass completely through the horse's digestive system, and much of that time is spent in the hindgut, which accounts for the bulk of the digestive tract. But the glucose and fatty acids produced by fermentation are absorbed through the intestine wall and carried in the blood-stream to tissues throughout the body, filling the lion's share of the horse's energy needs. Hindgut bacteria also help by producing essential amino acids (the building blocks of protein), B vitamins such as biotin and folate, and vitamin K.

There are also mi-crobes in the equine stomach and throughout the digestive tract, not just the hindgut. The roles they play in diges-tion and other body functions in horses are less well understood. And researchers are just beginning to learn how changes in gut microbiota affect the horse's health.

When the balance of microbes is disrupted, diarrhea and colic are the most obvious results but not the only ones. "Laminitis can also be associated with the gut microbiota," Dr. Weese says. "Given data from other species, metabolic syndrome and obesity may also have a gut component, but that's purely speculative at this point" Horses with equine metabolic syndrome develop insulin resistance and have difficulty me-tabolizing sugars. They need special diets; even small amounts of grain may be too much. Both obesity and insulin resistance increase the risk of laminitis, a devastating disease in which the bond between the hoof wall and the underlying bone weak-ens and may separate.

Dense and Diverse While a normal gut microbiome is clearly important to a horse's overall health, researchers are only beginning to get a pic-ture of what "normal" is, Dr. Weese says. Until recently most investigations involved culturing feces or intestinal contents in the lab to see what microbes grew. But that method missed microbes that can't be cultured or are unclassified, and there are lots of those.

Now researchers are using genetic se-quencing to characterize the microbiota.

They're finding that it's dense and diverse, comprising thousands of different spe-cies. The exact mix of microbes varies from horse to horse, but there are broad similarities. It's dominated by a large group of bacteria (a phylum, in the system sci-entists use to classify living things) called Firmicutes. 'This phylum includes a wide range of 'good' bugs, bugs that can cause disease in the right situation, and a huge pool of bacteria that we know very little about," Dr. Weese says. Many Firmicutes break down fiber, so it's not surprising that they thrive in the horse's large intestine. Other large groups of bacteria are well

represented in the gut, too. What roles do these different bacteria

play in the horse's health? "We know very little at this point," Dr. Weese says. "We know that some cause disease in certain situations. We are getting more evidence about bacteria that are more common in healthy horses and that are lost in disease like diarrhea and colic, but figuring out what that means is quite difficult"

Dr. Weese helped carry out a study at the University of Guelph that used DNA analysis to compare the microbiota of

healthy horses (assessed from fecal samples) to the microbiota of horses affected by colitis (severe diarrhea). The research-ers analyzed a total of 195,748 bacterial ge-netic sequences from six healthy horses and 10 horses with colitis and they found important differences between the sick and healthy groups: ■ Firmicutes were the major phylum in fe- cal samples from the six healthy horses, and Clostridium species (part of that phylum) were also well represented. Although some types of clostridial bacteria can cause intestinal disease, the types found in this

study seemed to be part of a core popula-tion that the healthy horses shared. ■ Microbes from a different phylum, Bac-teroidetes, dominated in the samples from horses with colitis. Bacteria in this phylum are thought to be the most abundant in healthy people, but they're a minority in healthy horses. The horses with colitis also had higher percentages bacteria from oth-er phyla, Proteobacteria and Fusobacteria, than the healthy horses did.

It's not clear what allowed these other bacteria to proliferate in the sick horses or if they have some still-unknown role in causing disease.

DECEMBER 2014 • PRACTICAL HORSEMAN

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Sudden changes in diet, such as when a horse overindulges in rich spring grass, can disrupt the gut microbiota.

Out of Balance It's clear that the balance of the microbe

community can change dramatically in

intestinal disorders like colitis. What else

might throw the balance off? Many fac-

tors, potentially; but most are not well

understood. Here are two:

■ Antibiotics can knock out the good

guys along with the bad. These drugs

are essential weapons in the fight against

disease-causing microbes, but a course of

antibiotic treatment can reduce the diver-

sity of the gut bacteria and thus disrupt

the balance among the various types. In

fact, diarrhea is a common side effect of

treatment with these drugs. Dr. Weese

is studying changes in the microbiota in

horses treated with various antibiotics

with an eye to understanding the effects

of different drugs. The answers may help

veterinarians design targeted treatments

that cause fewer harmful changes to the

balance of gut bacteria.

■ Sudden changes in diet can disrupt the

gut microbiota. When a horse gets into

the feed room and gorges on grain or

overindulges in rich spring grass, he gets a

sudden influx of sugars and easily digest-

ible starch. Microbes that thrive on those

materials flourish, the acidity of the gut

changes, and microbes that ferment com-

plex carbs and fibers start to die off. Those

changes may set the stage for colic, colitis

or laminitis.

Less is known about how different

daily diets influence the gut microbes

over time. A recent study used DNA se-

quencing to compare the effects of three

different diets—forage only, forage with a

starch-rich grain concentrate, and forage

with a low-starch, oil-rich concentrate.

(The research was carried out at Michigan

State University in collaboration with re-

searchers at Aberystwyth University and

the Waltham Centre for Pet Nutrition

in the United Kingdom.) There was little

56 PRACTICAL HORSEMAN • DECEMBER 2014

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@ Lori Faith / Photography By Faith

One study found that elderly horses (aged 19-28) had less bacterial diversity overall than mature horses (aged 5-12).

difference in the overall diversity of bac-teria in horses on different diets or in the relative levels of different bacterial phyla-Firmicutes dominated, followed by Bacte-roidetes and other groups. But there were differences in the core bacteria (those

present at significant levels in all horses) given the three diets. Horses getting just forage had the largest and most diverse core group. The core in the oil group was smaller, and the core in the starch group was smaller still. What does that mean

for the horse? The researchers speculated that a small core group of bacteria might make horses more susceptible to metabolic problems. But in the study horses stayed healthy on all three diets, so it's hard

to say. Age may also play a role. The Michigan

study found that elderly horses (aged 19-28) had less bacterial diversity overall than mature horses (aged 5-12)-but again, all the horses stayed healthy. Season, the environment and geographic location may affect the microbe population. Research in humans and other species suggests that exercise, stress and other factors may, too. "We just don't know whether they do that in the horse, or more accurately, whether they do anything of clinical relevance," Dr. Weese says.

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Help for a Heathy Gut Richness and diversity—meaning lots of microbes, lots of different kinds—are asso-ciated with a healthy gut. So what can you do to help your horse maintain a rich and

diverse community of microbes? That's still not clear, Dr. Weese says—although there's no lack of supplements that claim to help. Most of these products fall into two groups:

Probiotics. Probiotic products contain live beneficial bacteria and they're given with the idea that these bacteria will colonize the gut and improve the mi-

crobial balance. However, Dr. Weese says, "Probiotics can be marketed without any efficacy testing and even safety testing." Unlike drugs, supplements aren't required to go through clinical trials to show they're effective. Thus, there's little research to show which ingredients are helpful or in what quantities.

In several studies evaluating these products, Dr. Weese has found that many are labeled inaccurately and do not contain the claimed levels of viable organ-isms—bacteria that could actually develop and function in the horse's gut. There is little data showing products colonize in the intestinal tract.

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It's also not clear whether the microbes listed on the labels of these products are the ones that would be most helpful to the horse if they reached his hindgut alive. For example, Lactobacillus species make up a large portion of many probi-otic products. Many people know these bacteria as helpful members of the human gut microbe crew, and they survive fairly well in commercial probiotics. But it's not known whether Lactobacillus species are as important in the horse as they are in people. "We can find various Lactobacillus species in the gut but at rather low levels in the colon, the location we're most wor-ried about in terms of colic, diarrhea and laminitis," Dr. Weese says.

What organisms should be in a probi-otic mix for horses? "We don't know," he says. That doesn't mean that some prod-ucts won't help some horses with some conditions, he adds, "But we don't know the 'somes.'"

Prebiotics: These products are thought to help keep the hindgut microbiota stable and healthy. Instead of live organisms, they contain nondigestible carbohydrates (mostly oligosaccharides, linked sugar mol-ecules that are common in plant fibers) that feed the beneficial bacteria in the digestive tract, stimulating their growth or activity. Some research suggests that pre-biotics can lessen the impact of changes in diet.

"Prebiotics might be useful, but data are lacking," Dr. Weese says. "We know that prebiotics can alter the gut flora, but we need to know if that's good, and part of the issue is still the lack of clear under-standing what 'good' means."

As that understanding grows, it will become easier to see how various supple- ments affect the microbes. Meanwhile, your best bet is to follow these guidelines: ■ Feed high-quality forage. Make good hay and pasture the basis of your horse's

diet and feed concentrates only to meet energy needs that forage can't fill. ■ Maximize turnout. The more he's out and moving around, the better his diges-tive system will function. ■ Avoid sudden changes in feed. Make changes in feed and hay gradually, mixing in a little more of the new each day. ■ Use antibiotics only when necessary. If you give these drugs preventively or when they're not really needed, you may upset the microbe balance and do more harm than good. ■ Before adding a supplement, discuss the change with your vet. Choose products from established makers.

"Horses are meant to eat a lot of fiber, eat slowly much of the day and move around. The more we manipulate their diet, change their eating patterns and change their movement, the more we are potentially impacting the microbiota," Dr. Weese says. 2

62 PRACTICAL HORSEMAN • DECEMBER 2014

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