Case Report: Hereditary Equine Regional Dermal Asthenia (HERDA)

When her mare is diagnosed with a devastating genetic skin condition, a dedicated owner quickly learns to handle new medical and emotional realities.

Although she can never been ridden because of her condition, Penelope is learning to drive and successfully showing in hand. | Photo © Robin Davison

Penelope represented a new beginning for Robin Davison. After retiring her mare because of an injury, Davison had embarked on a search for a replacement. Eventually, a video of a 2-year-old bald-faced, blue-eyed filly made its way from Florida to Colorado, and it was compelling enough for Davison and her trainer to travel east for a closer look.

“As soon as I put my hands on her, I knew she was my next horse,” recalls Davison. Penelope had been ridden only 10 times, but she was sweet and willing, and Davison had visions of bonding with the pretty filly as they trained for competition.

But their relationship took a very different route.

Penelope was delivered to Davison’s boarding barn in December 2009. But the excitement of her arrival turned quickly to concern when the filly came out of the trailer obviously lame. Her right hind fetlock was swollen and had several small cuts. Davison called the veterinary school at Colorado State University (CSU), and an ambulatory clinician soon arrived, examined the leg, took radiographs and pronounced the bones and tendons to be intact. Perhaps the filly had gotten cast in a stall or kicked while in the trailer.

In any case, the injury would most likely heal with a few days of rest and hand-walking, Davison was told.

By chance, the same veterinarian from CSU came to the farm the next day to see a different horse. He offered to take a follow-up look at Penelope and was surprised to find the fetlock was larger than the day before.

“He figured it was an infection and put her on antibiotics for five days, but the swelling persisted,” says Davison. After yet another veterinary visit, more radiographs, an ultrasound, two penicillin shots and four weeks of hand-walking, Penelope was sound, but the swelling had extended up to her hock, and the cuts didn’t appear to be healing.

“I spent most of that winter hand-walking both my horses and nursing Penelope’s leg,” says Davison. “We’d cold hose it, and wrap and unwrap. Just when we thought it was under control, it would swell up again.” Eventually, with the help of a steroid, the leg appeared to have healed, and in February the filly was finally started in training under saddle.

Davison took Penelope to the Rocky Mountain Horse Expo in March, where the filly was used in a natural horsemanship demonstration. She proved to be a quick and willing learner, and the day went well, but something the trainer said afterward left Davison with a feeling of dread. “She told me that there was something not quite right with Penelope,” says Davison, “that she was very sensitive over her topline and touchy. I wasn’t sure what to make of that at the time, but my gut sank with the idea that there might be something seriously wrong with my new horse.”

A few weeks later, 45 days after her training had begun, Penelope began to develop saddle sores. “They were strange ruffled patches of hair first, then they turned into flaky dry skin,” says Davison. “Someone suggested it might be a fungal infection, so we tried a Nolvasan bath, but they just got bigger.” Because the skin looked “loose” in the area, someone else suggested the filly might be dehydrated, but her capillary refill time was fine and she was drinking plenty.

Davison called veterinarian Amy Jergens, DVM, DACVS, who noticed that Penelope’s saddle was too big, which might have been the cause of the sores. She recommended two weeks of rest and a new, better-fitting saddle. But the wounds did not heal as expected.

“She actually had three sores—one on each side of her withers and a third farther back on the right side of her spine, near the flank,” says Davison. They never healed.

Meanwhile, Davison had called in a massage therapist, who reported that Penelope’s skin, particularly along her topline, didn’t seem to “snap” back as quickly as it should. Davison recalled the observation the clinician had made at the expo and, with a sense of dread, called Jergens to make an appointment for the following day.

A heart-dropping diagnosis

Jergens took a close look at Penelope’s sores and recommended sending a sample of her hair to the University of California–Davis. “I asked her why and [she said] ‘Let’s just wait until we get the results,’ in that way that makes you worry more,” says Davison.

With prompting, Jergens explained she thought Penelope might have hereditary equine regional dermal asthenia (HERDA), also called hyperelastosis cutis. It’s an inherited genetic disorder that affects the development of collagen, the long, fibrous protein molecules that give strength to connective tissues, and interferes with how it folds and interlinks with other tissues.

As a result, horses with HERDA have weakened connective tissue. The disease often first becomes apparent in the skin of the back, after the horse begins training under saddle. The fragile skin tears and sloughs off easily and is slow to heal, and the scars that do form are lumpy and break open easily. HERDA horses tend to be sensitive along their backs, perhaps because nerves are pinched in malformed skin. But HERDA also affects connective tissue throughout the body, and these horses often have orthopedic issues related to the breakdown of tendons and ligaments; their heart valves are thin, and the structures of the inner eye can be weak.

Penelope seemed healthy, apart from her sores and the “touchiness” along her topline, but there was more evidence to consider. HERDA is found only in Quarter Horses and in breeds, such as Paints and Appaloosas, that carry Quarter Horse bloodlines, and it is particularly prevalent in some popular lines of cutting horses. Davison needed only a few minutes of online research to discover that her filly’s pedigree was riddled with the names of horses associated with HERDA.

Because the gene that causes HERDA is recessive, a foal needs to receive a copy of it from both the sire and dam in order to develop the disease. A foal who receives only one copy of the gene would be clinically normal but would be a carrier, capable of passing the defect on to the next generation.

Davison did not know the HERDA status of Penelope’s sire and dam, but she could infer that answer from her Internet research: “We started looking up some of the foals of both her sire and dam, and many of them were listing HERDA
test results.”

While she waited for the results on Penelope, Davison did enough research to be fairly certain what she was dealing with. “I moved Penelope from the large boarding barn to a friend’s farm so she could be turned out alone and get constant attention,” she says.

Still, when she received the confirmation that Penelope had HERDA, Davison was devastated: “I felt like I couldn’t breathe. And the more I researched, the more panicked I got. Not only was it clear that I’d never be able to ride her, I was reading that most of these horses are put down by the time they are 4 years old because of ongoing infections and the incredible amount of attention they require.”

Finding a way to live

Within days Davison contacted two leading experts on HERDA, Ann Rashmir-Raven, DVM, who was then at Mississippi State University, and Nena Winand, DVM, PhD, of Cornell University. “I learned that horses are affected to different extents,” she says—some cases are far less severe than others.

As Rashmir-Raven explains, “Some horses are so mildly affected they are ridden and compete—I’ve seen approximately 20 affected horses that have made money or had performance points. I’ve also seen cases that are so extreme, the skin spontaneously sloughs off or peels away. I also suspect that many of the most severely affected foals are born septic and die without a diagnosis.”

Davison was heartened to hear that Penelope’s case was apparently moderate, as the two experts assessed her from a distance, based on her owner’s description. Although it was unlikely she’d ever be able to carry a rider, she might have a chance to experience some normalcy in her life.

“That gave me hope,” says Davison, “and I started to move forward with groundwork and even some limited ground driving. I was determined to let her be a normal horse for as long as possible, even if it meant I could never ride her. Doctors Winand and Rashmir-Raven were extremely patient in answering all my questions on how to care for Penelope and how I might proceed.”

Davison developed a management routine designed to protect her filly’s fragile skin as much as possible, from abrasion as well as from extreme weather, including sunshine, which is damaging to horses with HERDA. “She’s on a thick bed of shavings with a private run,” she says. “The turnout pasture is treeless with a smooth, clean fence line to keep her from injuring herself.”

After the hottest, sunniest time of day, Penelope is turned out with a gentle equine companion. “I know there are HERDA owners who do not want to turn their horses out because of a fear they’ll be injured,” says Davison, “but I think it’s important to allow her to be a horse sometimes.”

Little research has been done on nutrient requirements in horses with HERDA. But in addition to hay and a bit of grain, Penelope receives a pound of flaxseed daily along with supplemental vitamins C and E, the amino acid lysine and MSM.

Davison’s grooming routine isn’t as much about appearances as it is monitoring the condition of her filly’s skin: “I typically only hose Penelope carefully, with just a bit of pressure. Other-wise, I’ll just use a very soft brush on her.” Any injury or lesion needs to be detected early, monitored and treated to prevent infection if necessary. Despite the precautions, just three months after the diagnosis Penelope experienced her first “wave of lesions,” as owners of HERDA horses describe sudden outbreaks of skin wounds. “She had between eight and 10, ranging in size from one to seven inches across,” says Davison. “These were probably triggered by a sudden heat wave; the more I researched, the more I learned that environmental factors can lead to lesions just as easily as trauma.” The largest of the lesions had to be drained by Jergens and tended to several times a day, but it eventually healed.

Davison kept up with her ground-work and occasional ground driving, using a surcingle thickly padded with fleece. By autumn Penelope was doing so well that a friend suggested Davison enter the showmanship class in a local show. She and Penelope placed, and suddenly the dream of a show-ring partnership didn’t seem so distant. They attended a few more events that fall, including a Halloween fun show and an in-hand trail obstacle clinic. At each appearance, Davison began to share more and more of Penelope’s story with fellow horse owners.

A new purpose
In the spring of 2011, with the help of family and friends, Davison raised enough money to bring Rashmir-Raven to the Rocky Mountain Horse Expo to speak about HERDA. Penelope would be her demonstration horse.

“It was great to finally meet Penelope,” says Rashmir-Raven. “She’s a very sweet horse.” After seeing the filly in person, Rashmir-Raven confirmed that her case of HERDA was indeed moderate.

It may at first be good news that a horse isn’t so severely affected that euthanasia is the only humane option. But a moderate case of HERDA may actually entail even more difficult decisions. Most horses with HERDA, like Penelope, are not able to be used in a traditional manner, says Rashmir-Raven, but neither are they debilitated enough to make euthanasia an easy choice.

“That’s the real heartbreak of these cases,” she says. “Owners have purchased these horses with specific goals and dreams in their minds. When it becomes clear that dream is impossible, the owner is faced with supporting this animal—possibly for years—which is an exhausting and expensive proposition. I know HERDA owners who spend more on veterinary bills every month than on board. We all like to say that we’d do anything for our horses, for as long as necessary, but when you’re in that position, the reality can be very stark.”

Rashmir-Raven says that many owners opt to euthanatize HERDA horses within a year of the diagnosis, a gut-wrenching decision that she won’t second-guess: “When a horse has 70 feet of dead intestine, the decision to put him down is obvious. But when a horse seems ‘fine’ yet is facing 10 or more years of open wounds, sometimes with significant pain—I’ve seen several horses whose entire skin hurts—and eventual arthritis or recurrent corneal ulcers, when do you make that call? There’s no easy, or correct, answer.”

The positive response from the horse community after Rashmir-Raven’s presentation led Davison to begin to imagine a new common goal to pursue with her horse: Educating other owners about HERDA.

In the months since, Davison has launched a website,, to share information about Penelope and HERDA, and the pair have continued to show. Penelope has also been used as a 4-H project horse. “I’ve decided to raise awareness of this disease while finding as many things to do with Penelope as her condition permits,” says Davison.

This fall brought a new, troubling change in Penelope’s condition. She’s developed significant lameness in both hind limbs and her right front. “She doesn’t seem to be in serious pain but we are managing these symptoms with low doses of bute and Reiki treatment,” says Davison, who remains committed to Penelope’s care. “I’m not sure what the future holds, but we’re doing the best we can with what we have each day.”




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