HERDA: Not Just Skin Deep

Poco Bueno was a giant among horses. Built like a rock, he possessed power and speed. He was a champion in the show ring, then retired to stud. His greatness was carried on in succeeding generations of Quarter Horses.

Today, it is primarily through Poco Bueno's bloodline, say researchers at Mississippi State University and Cornell University, that the recessive gene that causes hyperelastosis cutis (HC) has passed. HC is a brutal affliction that carries with it a death sentence. Although affected horses can be made more comfortable and their lives prolonged, there is no cure. In some scientific circles, the disease is called hereditary equine regional dermal asthenia (HERDA).

HERDA is a term favored by researchers at the University of California, Davis (UC Davis), while researchers like Ann Rashmir, DVM, MS, Dipl. ACVS, associate professor of surgery and head of the Hyperelastosis Cutis Research Program at Mississippi State University, and Nena Winand, DVM, PhD, a geneticist and assistant professor in the Department of Molecular Medicine at Cornell University, opt for hyperelastosis cutis. Rashmir and Winand are collaborators on hyperelastosis cutis research.

"It doesn't matter what you call it," says Rashmir, "it's the same disease." For the purposes of this article, we'll use HC.

When a horse has HC, there is a lack of adhesion within the dermis, the deep layer of skin, due to a collagen defect. Think of it like glue holding the skin layers together, only with HC, the glue is inferior. Because the layers are not held firmly together, they separate. When the horse is ridden under saddle or suffers trauma to the skin, the outer layer often splits or separates from the deeper layer, or it can tear off completely. It rarely heals without disfiguring scars. New damaged areas arise continuously, sometimes even without obvious trauma.

There are cases where horses with HC have lived to a fairly old age, says Rashmir, but great care must be taken to prevent trauma that can rip the skin. Sunburn is also a concern. In dramatic cases, she says, the skin can split along the back and even roll down the sides, with the horse literally being skinned alive. Generally speaking, the average lifespan for an HC horse is two to four years.

Winand and Rashmir state that 95% of the horses they have identified with HC trace back to Poco Bueno through both their sires and dams, with a few tracing back to Poco Bueno's full brother Old Grand Dad. The other 5% trace back to their sire King and perhaps even beyond. The statistical evidence, Rashmir says, includes HC horses from around the country.

The American Quarter Horse association is concerned about HC, says Gary Griffith, executive director of registration for AQHA. He says the announcement by Rashmir and Winand, to his knowledge, is the first public revelation concerning the Poco Bueno bloodline being the primary reservoir for the HC gene. He said that AQHA is funding research at the UC Davis, which is attempting to identify the gene responsible for HC. Hopefully, he says, such research will provide a simple genetic test to identify HC carriers.

Pinpointing The Start

Where along the line did the mutation occur? Winand says: "As of this writing, pedigrees from approximately 100 well-documented cases have been examined. These show that 95% of the horses identified with HC trace back to Poco Bueno through both their sires and dams. The other 5% trace back to other horses in this sire line, including King, Zantanon, Little Joe, and Poco Bueno's full brother Old Grand Dad. While this suggests that Poco Bueno may have inherited the HC mutation from his sire, this may not be the case. With the available information, it is difficult to rule out the possibility of Miss Taylor, Poco Bueno's dam, transmitting the HC gene. It may never be possible to scientifically identify the origin for several reasons. We have no pedigrees to date that completely differentiate the lines of King versus Miss Taylor, and even if we did, it is difficult to verify the accuracy of pedigrees that far back."

Diagnosis of the disease is made via pedigree evaluation and clinical signs and can be confirmed with a skin biopsy. In a number of cases, the disease has surfaced when the horse is two years of age and goes into training. The weight, pressure, and movement of the saddle, compounded by the rider's weight, often are enough to cause the skin to separate, according to Rashmir.

There is some good news, the two researchers say. The genes that cause HC are recessive. This means that both sire and dam must possess the recessive gene in order for an offspring to possibly be afflicted with HC. In this way, the disease differs from hyperkalemic periodic paralysis (HYPP), which descended through the bloodline of the Quarter Horse stallion Impressive. HYPP is caused by a dominant gene that can be carried by only one of the parents and still cause the affliction.

The HYPP gene has been identified, and horses' DNA can be tested for it. Researchers around the country are working to identify the HC gene, but estimates are that they might be a couple of years from success.

It isn't just a matter of identifying the gene, says Winand. Once that has been accomplished, the researchers face the task of perfecting the test to avoid any chance of false positive or false negative results.

In the meantime, the first line of defense by horse owners is to avoid breeding a known carrier to a known carrier. A horse which is an HC gene carrier is never afflicted with the disease. However, when two carriers are mated, there is a 25% chance the offspring will have HC.

Under the genetic law of averages, here's how it works. When a normal horse (not a carrier) is crossed with a carrier, 50% of the offspring will be carriers and 50% will be normal. When a carrier is crossed with a carrier, 50% of the offspring will be carriers, 25% will be normal, and 25% will be afflicted (will develop clinical signs of HC). When an afflicted horse is crossed with a carrier, 50% of the offspring will be carriers and 50% will be afflicted. When an afflicted horse is crossed with a normal horse, 100% of the offspring will be carriers.

Carrier status is established when a stallion sires an HC-afflicted offspring or when a mare gives birth to an HC-afflicted foal. When properly diagnosed, it only takes one afflicted foal to establish carrier status, Rashmir and Winand say.

Some in the cutting horse industry have referred to HC as the Doc O'Lena affliction because a number of HC horses carried his bloodline. Doc O'Lena was a "prolific carrier," says Rashmir, but it went beyond that--back to his grandsire Poco Bueno.

It has been established, say Rashmir and Winand, that the recessive gene was passed on to Doc O'Lena through his dam, Poco Lena, rather than through his sire, Doc Bar. There is no evidence that Doc Bar was a carrier, they say. Poco Lena had only one other foal, Dry Doc, and he, too, was a carrier, according to the researchers.

One of the great sons of Poco Bueno was Poco Tivio, foaled in 1947. The researchers say he was a carrier of the HC gene. Poco Tivio became well known for his excellent producing daughters, many of which were bred to Doc Bar. Many of the offspring became cutting champions. Unfortunately, many of them also were HC gene carriers.

Cutting horse enthusiasts reasoned that if a little of the Poco Bueno bloodline was good, a lot might be better. Through the years there has been a good deal of inbreeding, which has allowed HC to flourish in the gene pool, and it now is manifesting itself with greater frequency.

A check of the 2004 Quarter Horse News Stallion Register, Rashmir says, reveals that out of the top 100 cutting horse stallions, lifetime, based on earnings of offspring, 14 are known HC carriers.

"That," says Winand, "is just the tip of the iceberg. Because of the popularity of sires that are (or were) carriers and the use of assisted reproductive technologies (i.e., shipped semen), it is likely that the HC gene is present in thousands of horses."

Winand says that another study, also based on statistics published in the 2004 Quarter Horse News Stallion Register, reveals that between 1998 and 2002, some 1,241 offspring of HC carrier stallions were sold at public auction for $26,749,650. One-half of those offspring, based on the genetic law of averages, she says, also are carriers.

Winand emphasized that other disciplines besides cutting are involved in these bloodlines--reining, working cow horse, and even pleasure riding.

"Breeders are going to have to take responsibility for their decisions and not breed known carrier to known carrier," says Rashmir.

Horse owners can send pedigrees for potential matings to Rashmir and she will seek to determine the probability of the offspring inheriting either the gene or the disease. A fee of $25, which goes directly into the HC Research Fund, is charged. The address is College of Veterinary Medicine, P.O. Box 6100, Mississippi State University, Mississippi State, MS 39762. (For more information, see article #5037 online.)


Research into hereditary equine regional dermal asthenia (HERDA; also known as hyperelastosis cutis, HC) is also proceeding at the University of California, Davis. The research is headed by Stephen White, DVM, Dipl. ACVD (dermatology), and has a four-part focus.

The first part collected data and skin samples from more than 50 horses examined at veterinary schools and by private practitioners. That phase has been completed and a paper on the results has been accepted for scientific publication.

"We mainly diagnose this disease by seeing clinical signs," White says. "But not seeing these signs does not rule out the disease."

Part two evaluated pedigrees of horses diagnosed with this disease to try and establish a mode of inheritance. The geneticists concluded that it is autosomal (inherited equally by males and females) recessive (both parents must have the gene and pass it to the foal in order for the foal to have it).

Part three is seeking to develop a test for this trait. "We will be able to definitively show if an animal is a carrier," says White. "We are still working on this phase; our geneticists tell us it will be two or three more years. Looking for differences in the genes is like looking for a needle in several haystacks. We are trying to find where these animals differ in their DNA from normal horses. If we can work backward from that, looking at blood from their parents, we hope to find the defect."

Part four of the research, which White is doing along with Ann Rashmir, DVM, MS, Dipl. ACVS, of Mississippi State University, involves looking at affected foals during their first year of life to see if there are differences in skin biopsies (or on electron microscopy of the biopsies) compared with normal, healthy foals.--Heather Smith Thomas

About the Author

Les Sellnow

Les Sellnow is a free-lance writer based near Riverton, Wyo. He specializes in articles on equine research, and operates a ranch where he raises horses and livestock. He has authored several fiction and non-fiction books, including Understanding Equine Lameness and Understanding The Young Horse, published by Eclipse Press and available at www.exclusivelyequine.com or by calling 800/582-5604.

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