The Fatal Flaw, Part One

Greatness flowed through the stallion's veins. His blood made history, it trickled down through the generations and pooled in the pedigrees of modern quarter horses. He is credited as the lynchpin of performance horses: the cutters, cow horses, reiners and ropers.

At least...that's the theory.

A problem, ripped wide open
Noel Nelson had no idea what was wrong with her 4-month-old stud colt. The little guy had shimmied under a pole, scraping his back and integrity along the way. The experience amounted to little more than lost hair and a learned lesson.

Within hours, however, the colt developed a nasty-looking sore on his back. It was protruded and rounded, a shape Nelson thought was odd considering the narrow path the pole had pushed along the horse's back.

"We didn't think anything of it," Nelson said. "All colts go out and play, kick each other, bite each other and scrape hide off."

One month later and the wound had not improved and in fact it had grown worse. The skin around the sore pulled apart, leaving a gaping wound that exposed muscle. At a weekend cutting, Nelson described the horse's injury to a friend. That friend thought the symptoms matched those found in hyperelastosis cutis.

Hyperelastosis cutis?

As soon as she returned home, Nelson signed on to the Internet and learned about the genetic disease that has come to be known as HERDA (hereditary equine regional dermal asthenia) or hyperelastosis cutis. Two days later, the skin along the colt's back ripped apart.

Immediately, the Nelsons rushed the colt to the local vet. Without an answer, the vet referred the horse to nearby Oregon State University.

"They hadn't seen anything like this, ever," Nelson said. But the OSU staff did take several skin biopsies and administered cream to the horse's ghastly sore. The biopsies bore the unpleasant truth, the colt definitely suffered from HERDA.

"There really wasn't anything they could do for him," Nelson recalled sadly, adding that it was only a matter of days before the colt died.

HERDA, the fatal flaw
HERDA is a genetic disease characterized by abnormal skin along the back that tears easily and heals into disfiguring scars. Symptoms typically don't appear until the horse is subjected to pressure or injury on his back, neck or hips. These horses cannot be ridden because the saddle literally tears off the skin, a horrible discovery usually postponed until the horse's initial experiences under saddle.

Researchers speculate that HERDA results from the matching of two recessive genes, one each from the horse's sire and dam. According to the same researchers, all HERDA horses are related to one another and are the result of selective inbreeding. With very rare exceptions, HERDA is a disease found primarily in quarter horses and of those, cutting-bred horses make up the vast majority.

While some experts are willing to name the name of the common ancestor, others opt to wait until the DNA marker has been identified and located, a process that should be complete within three years. The common ancestor could actually be more than one horse although most of the HERDA horses studied so far descend from a single stallion.

It is not known how long HERDA has been around. Cases have been documented since the early 1970s. Over the years, the occasional horse was brought to vet clinics to be treated for open, oozing wounds that did not mend properly. But because of its undetectable, recessive characteristics, the disease was not correctly diagnosed. For years these equine patients went one of two directions: they healed, scarred severely and went to the breeding shed or pasture; or they were euthanized. Very few, if any, were ridden for long.

Records for these horses were not kept until Dr. Tony Stannard, DVM, a professor at University of California at Davis (UCD) noticed a common strand among the horses and started keeping track of the symptoms and pedigrees. Originally, the disease was referred to as hyperelastosis cutis, a term that has since been replaced by HERDA.

"The skin is not hyper-elastic," explained Dannika Bannasch, DVM, PhD, an assistant professor at UCD. Bannasch is recognized as leading geneticist and she oversees the DNA testing that UCD does for the AQHA.

Actually, HERDA affected skin is hyper-extensible. It's easy to pull out and then it does not immediately snap back into place. In most cases, those horses are characterized by unusual-feeling skin that covers the neck, back, hips and perhaps lower legs. The horse's skin appears normal until something pulls or pushes on the affected area.

"You know what a Shar-Pei dog feels like?" asked one trainer who went on to describe the HERDA-positive horse he'd tried to ride. "It was just like that. You could take the skin and wiggle it."

An owner of another horse noticed loose skin on her weanling filly, similar to that shown on this horse, but didn't think there was a problem. "We noticed her skin 'moving,' and we didn't think a thing about it," the owner said. "We thought, 'Oh she's just a baby and she'll grow into her skin.'"

HERDA symptoms typically surface when the horse is around 2 years old.

"That's when horses go into training and people try to put saddles on them and the saddles just start tearing the skin," said Ann Rashmir, DVM, associate professor at Mississippi State University. Dr. Rashmir oversees the MSU Hyperelastosis Research Program.

As the saddle presses down and shifts on an affected horse, it causes the outer skin along the back to rub on the underlying layers. The result is a raised area or several raised areas that become sore and can even pull apart.

Younger horses are typically diagnosed with HERDA after suffering an injury that scrapes, punctures or tears the affected skin. These wounds cannot be sewn together-think of trying to stitch together two foam pads and expecting them to hold under stress.

"The healing is dramatically impaired compared to that on a normal horse," said Melinda Poole DVM, clinical instructor at MSU. "It heals in an awful disfiguration."

Sometimes the disease is revealed through normal activity. For instance, a horse might roll in the pasture and cause the skin along his back to separate.

"The majority of these horses don't live past age four," Dr. Rashmir said about HERDA-affected horses. "By that time the owner is either frustrated or tired and they realize they can't use them any more."

A few HERDA-affected horses progress into old age, living as pasture pets. Great care must be taken to protect these animals from injury and sunburn. But even under the best conditions, some horses die naturally from the disease.

"Affected horses can actually 'de-glove,'" Rashmir said, explaining that the skin will spilt at the back and peel down the sides of the horse.

The problem is more than skin deep.

Just as it is for humans and other mammals, a horse's skin, the largest organ of the body, is a multi-layered protective covering. The epidermis is the outer layer, the thick middle layer is the dermis and the deepest layer is the sub cutis. Connective tissues bind the three skin layers together. Collagen is the most abundant protein the body and when it attaches with elastin, the combination forms a strong connective material.

The dermis has a great deal of this collagen connective tissue, and the breakdown caused by HERDA occurs in the dermis layer. Invisible to the naked eye, it is easily detected with an electron microscope. Normal collagen fibers are uniform and packed tightly. Collagen cells in a HERDA-affected horse are visible as damaged and disorganized. They vary in size and in many cases are loosely positioned, accompanied by an accumulation of granular material. The weak collagen/elastin fiber cannot stand up to external and stress, so the layers separate and the epidermis rubs against the sub cutis. Sometimes blood flows into the gap, producing painful hematomas. As the skin continues to move and stretch, it's held in place only by the small blood vessels that weave their way between the skin layers and muscle. Eventually, the skin gives way and tears.

"Think of it like a bridge," Dr. Rashmir suggested as a way to visualize the skin-collagen relationship. "The concrete is the skin and the rebar is the collagen fibers. If you build a concrete bridge without rebar for support, it will be OK for a while. But when an earthquake happens, there is no rebar to keep the bridge from falling apart. Even the little earthquakes in a horse's life, like putting a saddle on, result in wear and tear on the skin."

So far, every horse diagnosed with HERDA has shown damage to parts of his neck, back, hips and sometimes the lower legs. One could speculate that the disease is most prevalent in areas where the skin is the thickest, but what about the skin covering the lower legs? There are many more questions about the disorder.

"None of them have it in the cinch area and yet there is some trauma there too, every time you get on them," said Stephen White, DVM, a professor at UCD. White has examined dozens of HERDA-affected horses and is in charge of HERDA research at Davis University's Center for Equine Health.

White has seen HERDA-affected horses that have sores near their tail-head as well as ones with damage on the lower legs. White has not observed abnormal functions in the internal organs of HERDA-affected horses and gelding the stallions has not caused undue problems.

When asked why HERDA is manifested commonly on certain areas of the horse White answered, "One answer opens up 20 questions."

A wrinkle in the genetics
"What we have found is that all the affected horses are related to each other and that they all have in-breeding loops," Bannasch said about HERDA-affected horses. "Those are some of the signs that identify it as a simple recessive disorder."

It is a theory that works out on paper but has yet to be proven, a process that ends with completed breeding trials or when the DNA marker is identified and the genetic mapping completed.

The experts studying the inheritability of HERDA have identified similar markers in other research projects. Dr. Bannasch is credited with identifying the lethal white gene that results in the solid white, blue-eyed foals produced by certain paint and quarter horses. The lethal white condition results from a simple recessive gene and the trends for that disease appear very similar to those for HERDA.

"All of the affected horses are in-bred," Bannasch reiterated. "We haven't seen any evidence that it is a dominant trait, like HYPP, where you only need one line of descent from a horse that has it. In this case, it looks like you have to have it (the HERDA gene) on both sides of the pedigree."

HYPP (hyperkalemic periodic paralysis disease) as been proven to affect the descendents of Impressive, a legendary producer of top halter quarter horses. It was proven that HYPP results from the passing of a dominant gene, meaning that the carrier offspring exhibit symptoms. That is not the case with HERDA, a recessive disease that remains silent in heterozygous horses. Only those HERDA-affected horses that carry double recessive genes outwardly show signs of the disorder.

In most cases, HYPP carriers can be managed through medication and kept normal enough to use, ride and show. Plus, HYPP carriers show signs of the disease while HERDA carriers seem completely normal.

"I think hyperelastosis (HERDA) is more devastating than HYPP," Dr. Poole said. "There is no prevention and there's definitely no cure. You're literally playing Russian roulette because if your horse is affected, that's it, you're done."

"In the last eight or nine years it has become more common," White said, citing the increased number of HERDA-affected horses on record. "That's probably because it's been carried forward by successive generations without showing these signs. Now, their great grand offspring are being bred to each other and there's just a higher incidence."

Some breeders scoff at the notion. Even though some of the HERDA-affected horses have been by top sires and out of the industry's best mares, the number of diseased animals documented is relatively small.

But genetic protocols provide the means to dramatically increase the incidence: embryo transfer, super ovulation drugs, and collected and divided semen enable humans to successfully propagate several offspring from a couple of breedings. If a stallion is popular, he might breed as many as 200 mares in just one season. And the mare could produce two, three or more viable embryos and return to the show pen a month later.

Concentration of bloodlines has become the norm in the cutting horse industry and the HERDA gene has been quietly passing from one generation to the next. If a sire that carried the recessive trait is bred to a normal mare, 50 percent of the resulting offspring are carriers. The other 50 percent are normal. The HERDA gene remains dormant, as long as the carrying parent is paired to with normal mates.

Such is not the case when one carrier is crossed with another carrier.

"If you have two carriers and breed them together," Dr. White explained, "you're always going to have: a 25 percent chance of producing an affected horse; a 25 percent chance of having a normal looking, and genetic normal horse; and a 50 percent chance of having a normal looking carrier."

How does one know the percentages apply?

"It's a very, very basic principle of genetics," Dr. Bannasch answered. "The stallion has two copies of the gene, one normal and one mutant, and the mare has two copies of the gene, one normal and one mutant. The stallion has some sperm that has the normal copy and some sperm with the mutant gene, and so will the mare. It's just a matter of numbers."

When a stallion or mare owner learns that his horse has produced a HERDA-affected horse, it's natural to chalk the incidence up to unlucky chance, the rare strike of lightning. After all, if the stallion owner breeds 100 mares in a year, 25 of them don't come up with lesions and wrinkled skin. "He does not know that every mare he's breeding to is a carrier," Bannasch said. "Just because the mare is related to the sire, does not mean she is a carrier--a lot of them are not going to be carriers. From a stallion owner's perspective, it's going to look like there are many less than 25 percent affected-there should be less than 25 percent affected."

According to statistics proven through extensive research, established results and mathematical equations, when both parents are carriers there is only a one-in-four chance of producing a normal offspring. If an affected parent is bred to a carrier, the result is a 50-50 split between normal and affected offspring-no chance for a normal colt.

The autosomal recessive gene passes on in predictable manner, equally to males and females. The experts support the numbers with scientific evidence, although the process does take a while document in horses.

"If you had litters of foals, it would be a lot easier," Dr Bannasch added with a laugh. Overall, the incidence of HERDA-affected horses is low. For some, it blurs into the background of hock problems, stifle difficulties, small bones, height, stretched suspensories and a list of other deficiencies that plague cutting-bred horses. But HERDA is disfiguring, painful, costly and often deadly.

As HERDA carriers are allowed to reproduce, reproduce and reproduce again, they pass on HERDA genetics to new generations. As those second-generation horses perform and win, they'll be bred back to the best of the best, which are most likely relatives.

"We've had affected babies out there for over 20 years now, but they happened once in a blue moon," Dr. Rashmir said. "Those babies were not that frequent because we didn't have that many related cousins breeding. Now, we do."

Continue to Part II.

For more information on the research of HERDA click here.