HERDA: DNA Tests Available for Disfiguring Skin Disease

Scientific research has scored another significant victory in the equine world. Two DNA tests are now available to detect carrier status for hereditary equine regional dermal asthenia (HERDA), also known as hyperelastosis cutis (HC).

Today horse owners desiring to know whether they own a carrier mare, stallion, or foal can have it tested at either the University of California Davis or Cornell University in New York, where tests have been developed independent of each other. The cost per individual horse at each institution is $50. Hair follicles provide DNA for the respective tests.

HERDA is a skin disease that is almost always fatal and has been traced back in large part to the Poco Bueno bloodline in Quarter Horses. It is primarily found in the performance and pleasure lines, especially cutting, and seldom shows up in racing or halter horses.

When a horse is afflicted with HERDA, there is a lack of adhesion within the dermis, the deep layer of skin, due, perhaps, to a collagen defect. One can think of it like glue holding the skin layers together. However, with HERDA or 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. The wound might heal, but often with disfiguring scars. New damaged areas arise continuously, sometimes even without obvious trauma.

The disease often manifests itself at about a year and one-half of age, but can show up earlier.

The gene implicated in HERDA is autosomal recessive. This means that both sire and dam must have the gene in order for it to be expressed in the offspring.

Two researchers who have been heavily involved in research to develop a genetic test to detect carrier status are Nena Winand, DVM, PhD, research scientist, Department of Molecular Medicine, Cornell University, and Danika Bannasch, DVM, PhD, associate professor, Department of Population Health and Reproduction, School of Veterinary Medicine, University of California-Davis.

They have a simple, straightforward message for the owners of horses that appear to be within the population where HERDA has surfaced most frequently--have them tested.

"Carrier frequency is significant in the pleasure and performance industry," Winand said, "and the trait is most frequently seen in the Poco Bueno line of horses, and that's the segment of the population that needs to be tested for breeding purposes. If they're normal riding horses and you're not going to breed them, or if they are geldings, you don't need to test them unless you have a personal interest in doing so. But, if a horse is to be used for breeding purposes, it's very important that you test it."

Bannasch, who collaborated with colleagues Robert C. Tryon, PhD, and Stephen D. White, DVM, Dipl. ACVD, on a paper published in the May issue of Genomics, had this to say: "On the scale of bad diseases, this is one of the worst. It is a disease that kills young adult horses just as they reach the age that all owners look forward to--riding age. Testing horses to avoid breeding carriers to each other is a way to never produce another affected horse."

The genetic probability, the researchers say, is that when carrier is bred to carrier, there is a 50% chance the offspring will be a carrier; a 25% chance that it will be affected and a 25% chance that it will be normal.

The good news is that breeders, especially in the cutting industry, are apparently embracing the test. Winand says the laboratory at Cornell has already conducted about 1,300 tests for the general public. A major breeder, Buffalo Ranch, recently paid what has been termed an industry-high price for the cutting stallion Hydrive Cat, and as part of the breeding evaluation, had the stallion tested at Cornell for HERDA. He tested negative.

Bannasch said the UC Davis laboratory tested 1,500 horses during the research phase and now is testing for the horse-owning public.

Cecilia Panedo, PhD, associate director for the genetics laboratory at UC Davis, said that testing of horses for the public has just begun there. She said the laboratory waited with testing for the public until May 9 when the Tryon-White-Bannasch paper on HERDA was published.

Funding a portion of the research at UC Davis has been the American Quarter Horse Association (AQHA). Tammy Canida, senior manager of registration, said that there are no rules or official recommendations from AQHA concerning HERDA at the moment, though the association did publicize the availability of testing this spring. She said the topic likely will be on the agenda at a future AQHA convention as members decide just what role the association should continue to play.

HERDA is a disease for which there is no known cure. However, the researchers say, its spread can be controlled if horse owners make use of the DNA testing procedure that is now available.

Currently, the Veterinary Genetics Laboratory at UC Davis is offering the test and can be reached at 530/752-2211. The Cornell University Animal Health Diagnostic Laboratory is, or will be shortly, making testing forms available on its Web site. They can be reached at 607/253-4136.

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.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More