Hereditary Equine Regional Dermal Asthenia
Inherited Quarter Horse Disease Traces To Poco Bueno
by: Les Sellnow
Poco Bueno has been identified as the sireline associated with the brutal affliction known as hyperelastosis cutis (HC) or hereditary equine regional dermal asthenia (HERDA). He was a giant among horses. Built like a rock, he possessed power and speed. He made his mark in the show ring as a champion, then retired to the breeding shed. His greatness was carried on through his get, grandget, and on down the line through succeeding generations of Quarter Horses.
VERNON DAILY RECORD
It is primarily down 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 basically carries with it a death sentence. Although affected horses can be made more comfortable and their lives prolonged, there is no cure.
While Poco Bueno’s sire line is implicated as the origin of the disease, the majority of horses with HC trace directly to him.
The disease first surfaced in 1971. Today as more and more breedings double up on the Poco Bueno line through the mating of close-up and distant cousins, more and more cases of HC are showing up.
HERDA is a term favored by researchers at the University of California, 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 research involving hyperelastosis cutis.
“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 to refer to this hereditary disease.
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 they are treated as pets and great care must be taken to prevent trauma that can rip the skin. Sunburn can also be a concern. In dramatic cases, says Rashmir, the skin can split along the back and even roll down the sides, with the horse literally being skinned alive. Generally speaking, she says, the average lifespan for an HC horse is two to four years.
For some time, evidence pointed to Poco Bueno as the culprit bloodline, but up until now, Winand says, inadequate case numbers limited information available to researchers.
It is not supposition at this point, Winand and Rashmir state. It is scientific fact. They say 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 Poco Bueno’s and Old Grand Dad’s sire King and perhaps even beyond. The statistical evidence, Rashmir says, is not just from Mississippi State and Cornell, but includes HC horses from around the country as veterinarians have shared information.
“All of the horses diagnosed with HC are related,” Rashmir says.
Pinpointing The Start
Just 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 to Poco Bueno. 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 this far back, or further.”
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, and it takes two to tango. 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. With HC, both parents must carry the recessive gene for the horse to be afflicted.
The HYPP gene has been identified, and horses can be tested for it through a DNA procedure. Researchers around the country are working to identify the HC gene, but estimates are that they might be a couple of years from success, barring some unforeseen scientific breakthrough.
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. “You must optimize the test to ensure specificity (thus avoiding 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 that is an HC gene carrier is never afflicted with the disease. It is clinically normal and shows no signs of 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, according to the two researchers. When a normal horse (one that is 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 (can develop the 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.
There was a time in the past when some in the cutting horse industry referred surreptitiously to HC as the Doc O’Lena affliction because a number of HC horses carried his bloodline. Doc O’Lena was indeed a “prolific carrier,” says Rashmir, but it went beyond that--back to his grandsire Poco Bueno and beyond.
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.
That, says Winand, is unfortunate. Under the genetic law of averages, there was only a 25% chance that both would be carriers.
The Poco Bueno bloodline begins with a mystery horse, according to American Quarter Horse Association (AQHA) pedigree records. The horse was named Traveler, believed to have been foaled in the late 1800s, and he literally came out of nowhere. No one has a clue as to what his pedigree might be.
The late Robert Denhardt, author, and one of the founding fathers of the American Quarter Horse Association, described Traveler thusly in his book, Quarter Horses: “Traveler’s history has been traced back to Eastland County, Texas, where he was working on the railway. He was just a sorrel work horse in a large remuda owned by the contractor. It has never been adequately explained just how it happened that a stallion was allowed with the horses, but there was no disagreement on this part of the story. Traveler was not a young horse when he left the railroad--his age has been estimated between eight and 10. He had to be broken to the saddle, even though trace-chain marks showed on his side and collar marks on his shoulders. He had been worked plenty, but not ridden. According to one old-timer, he pitched terrifically, but showed great intelligence and soon quieted down.”
Despite his age, Traveler raced successfully. However, it was as a sire where he shined. He sired an excellent racehorse in Little Joe. That stallion in turn sired Zantanon, and Zantanon sired King, the sire of Poco Bueno. Poco Bueno was born in 1944. King and Poco Bueno became legends in their time, and in the breeding shed they sired outstanding cutting and performance horses.
One of the greatest sons of Poco Bueno was Poco Tivio, foaled in 1947. The researchers say that 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. It was a magic cross, with many of the offspring becoming champions and big money winners in the cutting arena. Unfortunately, under the genetic law of averages, many of them also would be HC gene carriers.
It wasn’t long before cutting horse enthusiasts reasoned that if a little of the Poco Bueno bloodline through descendants like Poco Tivio and Doc O’Lena was good, a lot might be even better. Through the years there has been a good deal of inbreeding, which has allowed HC to flourish in the gene pool reservoir and 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. This thing (HC) is going to mushroom in the next several years, and there is no way to stop it. Because of the popularity of sires that are (or were) carriers and the use of assisted reproductive technologies (shipped cooled and frozen semen, for example), it is likely that the HC gene is present in thousands of horses.”
“We didn’t have that many cousins carrying the HC gene being bred to each other in the past,” says Rashmir. “Now we do.”
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 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 horses. “It is not just a problem with cutting horses,” she says. “It can involve all disciplines when Poco Bueno is involved.”
“It’s not the horses’ fault,” says Rashmir. “It’s the way we have bred them. Many of these horses have been, and are, wonderful athletes and, though they are carriers, have no outward signs of the disease. Breeders are going to have to take responsibility for their decisions and not breed known carrier to known carrier.”
Mississippi State University has taken a step to facilitate that process. Horse owners can send pedigrees involving potential matings to Rashmir and she will analyze them and seek to determine the degree of probability of the offspring inheriting either the gene or the disease itself. 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.
With Rashmir’s help, the first step in curtailing spread of the HC gene pool can be taken by individual breeders.
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 University of California, Davis, which is attempting to identify the gene responsible for HC. Hopefully, such research, he says, will provide a simple genetic test that will identify HC carriers. “All of the information stemming from research into this problem will be passed on to the appropriate (AQHA) committees for consideration and action,” he says.
There is good news for owners of Poco Bueno-bred horses--not all of them are HC gene carriers. Many are not. The goal is to determine which ones are and avoid breeding carrier to carrier.
HERDA TEST AVAILABLE
CORNELL UNIVERSITY DEVELOPS TEST TO IDENTIFY CARRIERS OF DISEASE.
A DNA test for the autosomal recessive connective tissue disorder HERDA has been developed by genetics researchers at Cornell University and is now available to the public.
According to CU geneticist Nena Winand, D.V.M., Ph.D., the test unambiguously identifies normal, carrier and affected horses. Affected foals can be definitively identified at birth.
“I want this to be used as a tool for breeders,” Winand said. “This test will enable us to manage this disease by testing for it and breeding intelligently.”
HERDA (hereditary equine regional dermal asthenia) is a disease caused by a homozygous recessive gene. When expressed, it causes a collagen defect in the skin of affected horses resulting in a lack of adhesion in the deep layers of skin. Trauma to the skin results in skin wounds that don’t heal. Most affected horses have a poor quality of life and are typically euthanized.
“As with tests for other genetic disorders, we hope being able to genetically test for HERDA will aid in the humane management of affected horses, and reduce the financial loss for breeders,” Winand said.
The test can be used with hair or blood samples. Testing inquiries can be made to:
Nena Winand, D.V.M., Ph.D.
Department of Molecular Medicine
College of Veterinary Medicine
Cornell University, Ithaca, NY 14853-6401
Phone: (607) 253-3608
E-mail at: firstname.lastname@example.org