Fat Horses, Thrifty Hogs and Foundered Mares

By Madison Seamans MS DVM & Deb Haines

It’s spring! We know this because the irrigation water is flowing, there’s bugs on the
windshield, the air smells like skunk, (I don’t know why skunks feel the need to perfume the air
more this time of year, whether it is to let the girl skunks know there’s a new stud skunk in town,
or just to aggravate the rest of us—who knows what goes through the mind of a skunk), the grass
is green—and there’s lots of it! This makes our horses very happy. After living on dry hay all
winter, a mouthful of fresh greens must seem like a gourmet salad. This is great for most horses,
but not all of them. For some horses a few days on lush, green pasture is like a diabetic working
at a candy store—the consequences can be disastrous.

We used to call these horses “easy keepers”. You know the kind: just show them some hay
string and they gain ten pounds. This type of horse has a thick “crest” on the top of the neck, and
often a deep crease down the center of their back you could hide a cheese burger in. If it were
just a case of obesity, I guess we could live with that, after all, fat is a pretty color on a show
horse. But it is not that simple. A nasty little problem called laminitis is part of this scenario.
The thyroid gland, found at the base of the throat in all mammals, has been studied for a long
time. However, exactly one hundred years ago, Kendall was able to identify one of the
hormones secreted by this gland and he named it “thyroxin”. This has been the focus of many
research projects over the years, and the horse has recently been included.

It has long been known that hypothyroid people, those with low thyroid hormone production,
tend to be more than just “pleasantly plump”. Researchers in equine medicine started evaluating
thyroid hormone levels many years ago, and their measurement became a part of the diagnostic
workup of the “fat horse that likes to founder” syndrome. Unfortunately, many of these tests
came back “normal”. So the scientific community was elated with this revelation because they
had a new disease they could name! For a while they called this “euthyroid sic syndrome”. That
means the hormone level is “normal”, but the animal (or person) is still sick. Yippee! We don’t
know what’s wrong, but at least we have a name for it!

This has become a common theme in this age of technology: our ability to measure
something is often greater than our ability to understand it! Some very good research has been
published over the last ten years which has helped clarify a few aspects of a rather complicated
system. Thyroid hormone, while a part of this metabolic puzzle, is apparently not the only key.
The subjects formerly known as “easy keepers” (horses and humans) are now referred to as
“metabolic syndrome” or “insulin resistant”. Equine metabolic syndrome (EMS) is the term
most recognized for this condition in the horse.

Some wild hogs on an island off the coast of Georgia have provided an unlikely source of this
understanding. Apparently, somebody forgot about a few hogs left on this island a couple of
hundred years ago, but somehow they survived without the benefit of human management. These
hogs were quite unique. In the wild, they did quite well, but when they became “domesticated”
and fed like regular hogs on their way to becoming breakfast, they got morbidly obese.
Subsequently, they developed cardiovascular disease similar to that in humans. Some of these
hogs even foundered! (And you thought that was just a horse disease?) The reason for this is that
this group of hogs had a “thrifty” gene that allowed them to store fat during the lean times when
vegetation was scarce. Unfortunately for the hogs, this gene didn’t get turned off when they
were well-fed, so they developed “it-ain’t-my-fault-they-built-a-burger-joint-across-the-streetfrom-my-house” weight problem, also known as metabolic syndrome.

It truly isn’t a behavioral “fault”, but some horses are apparently “blessed” with thrifty genes.
This, in turn, influences the metabolic function of insulin and glucose to store calories in a very
efficient way. These excess calories get stored in fat tissue; sometimes a mountain of it. (Figure
1)

Figure 1. Does this halter make me look fat? Note the “not muscle” lumps over the croup. Horses add weight here last, lose it here first.
It gets more complicated. The thrifty genetics, while it may serve to enhance the survival of
the species in some instances, can cause problems. In fact, the fat, or adipose tissue, the proper
anatomical term, creates hormone-like substances called “adipokines”. The first of these was
Leptin. Discovered in 1992, leptin and hormones like it, initiated a whole new way of studying
metabolism. It turns out that there are hundreds of these compounds and many of them are
involved in the regulation of blood flow. Normal tissue function requires strict cardiovascular
regulation, and “moderation” is the key. The body is looking for “Goldie Locks” levels: not too
much, not too little, but just right! It appears that one of the major problems with obesity is not
the failure of the heart to pump blood through a lot of extra horse or person —the original
theory–but rather something different. The adipokines produced by too much fat are poisoning
the lining of the blood vessels. This, in turn, leads to cardiovascular disease. If the horse lived to
be forty or fifty years old, we would be seeing a lot of equine heart attacks. In hooved animals
(equids, bovids and…piguids, what’s the designation for pigs?) however, the detrimental effects
of adipokines are commonly found in the vessels of the feet. This causes inflammation of the
soft tissue, or laminae, the supporting structure of the foot, thus the term “laminitis”. If left
untreated, the damaged tissues can no longer support the coffin bone in the foot and it sinks or
rotates into an abnormal position. This produces severe pain initially and a “dish” shape to the
dorsal hoof wall as the disease progresses over time. (Figure 2)

Figure 2: The dish shape of the dorsal hoof wall due damage to the laminae: laminitis.

The terms “laminitis” and “founder” are used interchangeably, but not correctly so. Laminitis
is the inflammatory disease, while “founder” is a shift in the coffin bone to an abnormal position
inside the hoof capsule. (Figure 3)

Figure 3: The coffin bone has “rotated”. The dorsal hoof wall and the coffin bone are not parallel. (Arrows) Management of EMS “easy keepers” should be directed at preventing the devastating pain of
laminitis. Obviously, this is not a problem in most horses. However, if we identify these
subjects, there are management techniques that can help most of them. The focus here is to limit
sugar, or non-structural carbohydrate, intake
1). Keep them off the lush, green pastures they would love to enjoy, we can prevent some of the
problems associated with this metabolic anomaly.
2).Supplemental thyroxin: thyroid hormone powder for about 10 months.
3).A “grazing muzzle” (a misleading name, a horse sure can’t graze much with one of these
things strapped to their face) allows the horse to be out, moving around with his friends, but will
limit sugar intake.
4). There are several “low carb” products available to aid in weight control of “thrifty” horses.
(Purina Well Solve is a good one). But a starvation diet is never a good idea. A potentially fatal
condition called ketoacidosis can result if we force weight loss too quickly. We didn’t get this
way overnight, it will take some time to get back to normal.
5). Soak the hay for 12 hours. Put a flake of hay, the variety matters little, in a bucket of water
every night for the next morning feeding. Pull the wet hay out, feed it, dump the water and
repeat the process for the evening feeding. This will reduce the sugar content by about 1/3.
6). Proper foot care, including radiographs (X-rays) is an essential part of maintaining these
patients. Many will do well bare-footed, but some will require a shoeing package to give comfort
and stop restricted blood flow.
7). Stall confinement is probably not a good idea. Allowing the horse to move, rather than
forcing the horse to move is important to re-establish blood flow which will aid in the healing of
diseased hoof tissue.
8). Anti-inflammatory medications are used as needed for pain, but the horse should be weaned
off of them as soon as possible. (Bute, Banamine, Previcox)
As in most disease states, prevention is the best course, so these measures should be
implemented before the horse develops problems. Obviously, sometimes this is not possible, but
early intervention will provide a good quality of life for many EMS patients.