More Than Skin Deep?
- Apr 1, 2006
It was two years ago--in April of 2004--that this magazine published the announcement that the Poco Bueno Quarter Horse sire line had been identified as carrying the recessive gene that causes hyperelastosis cutis (HC, also known as hereditary equine regional dermal asthenia or HERDA). This is an affliction involving the skin that very often carries with it a death sentence for the affected horse. During the past two years, a good deal more has been learned about HC, but at the same time, it remains an elusive entity as researchers seek to pinpoint its genetic location. This would be a major step in developing a DNA test that would identify carriers. (The gene is characterized as being autosomal recessive, which means that both parents must pass it on for their offspring to be affected.)
University of California-Davis researchers have reported progress in their genetic investigations of this disease, but have not yet identified the specific gene. They have identified the chromosome on which the HC gene is located and have narrowed the field as to which gene might be involved, but have not reported identifying the gene itself.
A new urine test can be administered to foals to determine if they are affected, but there is not yet a DNA test to determine which horses are carriers and can pass the condition to offspring. And, as knowledge expands, there is a growing concern that HC might involve more than just the skin of an affected horse.
What is HC?
When a horse has HC, there is a lack of cohesion within the dermis, the deep layer of skin, due to a collagen defect. Collagen is the protein constituent of bone, cartilage, tendon, and other connective tissue that in a manner of speaking, plays the role of glue in keeping the layers of skin together. However, when a horse has HC, the glue is inferior and the layers separate. When the horse is ridden or suffers trauma to the skin, the outer layer often splits or separates from the inner layer. It can also tear off completely. When the wound heals, there very often is a disfiguring scar. New damaged areas arise continuously, often without obvious trauma.
Some affected horses have lived to old age, but for the most part, they have been the recipients of tender, loving care that prevented skin trauma. Sun damage is also a concern, and affected horses must be kept out of direct sunlight as much as possible.
Ann Rashmir, DVM, MS, Dipl. ACVS, a researcher who heads up the HC research program at Mississippi State University, says that the affliction continues to strike with varying degrees of severity. With some, it is so bad that portions of the horse's skin literally fall off its body. With others, the horse is affected only mildly and can often be ridden in the performance ring if handled correctly, she says. However, one thing has not changed: There still is no cure for HC. For most horses, it is a death sentence, with the average life span being in the two- to four-year range.
Much has been learned about handling affected horses, Rashmir says, with overall quality care and proper nutrition playing valuable roles.
There normally are 24 affected horses in the herd at MSU that are being continually studied in an effort to learn more about HC and how to deal with affected horses. Unfortunately, says Rashmir, the often-virulent nature of the disease means that there is a fairly rapid turnover of animals in the herd. For example, two affected yearlings that had been donated didn't reach their 2-year-old years before progress of the disease and its side effects made it necessary that they be euthanatized.
When a DNA test is developed, it could reveal if a sire or dam carry the recessive HC gene. The genetic law of averages indicates that if both parents are carriers, 50% percent of the offspring will be carriers, 25% will have HC, and 25% will be normal. When a horse with HC is crossed with a carrier, the odds go up. Under that scenario, the genetic law of averages indicates that 50% of the offspring will be carriers, and 50% will have the disease. When a horse with HC is crossed with a normal horse, 100% of the offspring will be carriers.
Thus, one could reason, if the DNA test reveals that both stallion and mare have the HC gene, the wise thing is to avoid breeding them to each other. As a practical matter, however, that will not happen. Some of the HC gene carriers are talented stallions and mares in the cutting and performance horse arenas. Just being a carrier has no known negative effect on the horse's ability to perform.
One breeder, who chooses to remain anonymous, says that he has bred three of his mares to a known carrier stallion that has a reputation for siring winning cutting horses. The mare owner does not know whether his mares are carriers, but says he was willing to "roll the dice," gambling that they are not. Even if they were known carriers, he says, he perhaps would have bred them to the stallion anyway, gambling that the offspring from the two carriers would be in the 50% group that were carriers or the 25% group that were normal.
If one or more of the offspring from those matings fell within the 25% that were HC-affected, he would simply accept his loss, he says.
New Test for Foals
One of the more significant accomplishments to date at MSU involves development of the urine test mentioned earlier that can identify HC-affected horses when they are foals. There are changes in the horse's urine similar to what is seen in humans with Ehlers-Danlos type VI. (In this disease, tissue fragility and easy bruising are seen.)
The prime researchers involved in developing the test were Rashmir; Cyprianna Swiderski, DVM, PhD, assistant professor in the department of clinical sciences at Mississippi State; and Marzia Pasquali, PhD, associate professor of pathology and director of the Biochemical Genetics and Supplemental Newborn Screen at ARUP Laboratories at the University of Utah.
The test, says Rashmir, leaves some questions unanswered, even though it identifies affected foals. "It does tell you that this is a horse you will have to worry about because it has HC," she says, "but it doesn't tell you how seriously the horse is affected. You don't know whether it will turn out to be a mild case or one that is so severe that it results in the horse being unusable."
More Than Skin Deep?
There are other unanswered questions concerning HC. A number of stallions with HC are cryptorchids, Rashmir says. Is it merely the result of continued inbreeding, or did HC play a role?
A number of HC foals also suffer what sometimes develops into a life-threatening infection. This appears to be happening with too much frequency to be coincidence, she says.
Does this mean that HC horses are different from other horses in more ways than just defective collagen involving the skin? Nena Winand, PhD, a geneticist at Cornell University who has been researching HC, thinks that might be the case. "This is not just a skin disease," she says. "There is an impact on other tissues that needs to be studied."
Winand says, for example, that she has seen evidence that HC might have an orthopedic impact. Her investigation into that possibility is continuing.
There also is the question of whether there is a connection between vision problems and HC. MSU researchers are working on this and suspect that there might be, Rashmir says, but as yet do not have definitive answers.
Interestingly, the two researchers who authored the first paper on HC in 1978 also mentioned that one of the two affected horses studied had a reported vision problem. The two researchers were Dana J. Lerner, DVM, MS, an equine veterinarian in Monticello, Ill., and Malcolm McCracken, DVM, PhD, a professor in the College of Veterinary Medicine at the University of Tennessee.
The two horses featured in that first paper were brought to the clinic at the University of Illinois in 1976 and ultimately were diagnosed as being afflicted with HC, according to the researchers' published report in the Journal of Equine Medicine and Surgery. In the report, they described what had been found in a 2-year-old Quarter Horse colt and a 5-year-old Quarter Horse mare. The researchers suspected that genetics might be involved because both horses had the same grandsire. The grandsire was not identified.
What the two researchers reported after examining the two affected horses would be repeated over and over in the future as more and more cases came to light. Here, in part, is what they had to say about the colt: "A black colt was presented in late February with an 18-month history of abnormal skin. The horse had recently been put into a training program when several abnormal areas in the saddle path appeared to be unusually sensitive. Previous treatment for this had included oral griseofulvin, which was ineffective. Skin lesions were present on the left foreleg below the elbow and on the left lateral thorax at the level of the 18th rib. These areas appeared dry and were hairless. They varied in diameter between 4 and 6 centimeters. Additional skin lesions were noted over the back, between the 13th and 18th ribs (two in number) and one on the right rump over the gluteal musculature. Each of the latter areas was very hyperelastic. With tension, the center of each lesion was readily elevated 3 to 5 centimeters off the body. No resentment was evidenced by the animal. Immediately after the tension was released the skin returned to the original site. Extreme thinness was readily appreciated..."
With that horse, the researchers reported, there was no evidence of abnormalities within the muscoskeletal or ocular systems. However, with the second horse examined-the 5-year-old mare--the owner had reported a vision problem.
The concern among horse owners and researchers alike is that the HC problem is continuing to grow because of continued inbreeding that concentrates the gene pool. Horses carrying Poco Bueno blood, in many cases, are prized for their athletic ability and, in the minds of some breeders, if a little Poco Bueno blood is good, a lot is better.
There has been some speculation that many of the cutting horses carrying Poco Bueno blood have skin that is more sensitive and flexible and, as a result, are more apt to be affected than their counterparts who are not cutting horses.
As of a couple of months ago, according to Rashmir, the number of horses positively identified as HC carriers stood at 465, an increase of about 300 over the known number of carriers two years ago.
There can be little doubt that the 465 horses identified as HC carriers represent the tip of the iceberg. So, one might wonder, how will this all play out? Will a definitive DNA test in the future result in fewer carriers being mated to carriers, or will breeders knowingly continue to "roll the dice?" Where does responsibility lie in identifying carriers? Many of the horses offered for sale at the prestigious auctions during the NCHA Cutting Horse Futurity have the potential to be carriers. Are sale companies responsible for alerting the public as to a horse's HC status?
No, says Rashmir to the last question. "A sales company's job is to sell horses, not guarantee their health status. It comes down to the owners. We can hope that someone will guarantee a horse to be a not a carrier or not affected and, as a result, will get more money for it. That would encourage others to test their horses (when a DNA test for carriers becomes available) or test the foals with the urine test to guarantee that they are not affected."
At least one case involving a breeding that resulted in a pair of HC foals via embryo transfer has been in the court system, and the result of that suit might also have a bearing on how the industry handles disclosure of HC carrier status.
About the Author
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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