HERDA: Skin Characteristics

Horses with the hereditary skin disease known as hereditary equine regional dermal asthenia (HERDA) or hyperelastosis cutis (HC) have been recognized since the 1970s, but only recently have researchers defined just how much weaker their skin is compared to that of healthy horses. At the 2009 American Association of Equine Practitioners Convention, held Dec. 5-9 in Las Vegas, Nev., one study discussed the results of biomechanical and molecular investigations into the skin characteristics of horses with HERDA.

"Hereditary equine regional dermal asthenia is an autosomal recessive skin disease associated with certain Quarter Horse bloodlines," began Ann M. Rashmir-Raven, DVM, MS, associate professor of Large Animal Clinical Sciences at Michigan State University. "Affected horses have loose, hyper-extensible, fragile skin and are frequently euthanized because of poor wound healing and disfiguring scars. Lesions can occur anywhere on the body, but are most common dorsally (over the top of the body, such as the back)."

Rashmir-Raven explained that affected horses’ skin has loosely integrated, thin, shortened collagen fibers in the deep dermis layer, which results in disintegration of parts of the layer in response to very little or no trauma. "On scanning electron microscopy we see clear spaces with fibrosis and/or granulation tissue in the middle of the layer that never heals," she noted.

For the biomechanical portion of the study, skin samples were taken from several locations on horses immediately following euthanasia. Ten unaffected and 10 affected horses were studied; their skin samples were stretched to the point of failure or until they were under a load of 1,000 N, whichever came first. Rashmir-Raven reported that HERDA-affected horses' skin was 2-3 times weaker than normal skin throughout the horse, and that HERDA-affected skin failed differently as well—it tended to shred and fail, whereas normal skin failed all at once in one spot.

The HERDA-affected skin also had a much lower modulus of elasticity, meaning it would not deform (stretch) very far and still return to its original shape/size. It also had significantly higher amounts of soluble collagen. "Skin from carriers (horses that inherit only one copy of the HERDA gene; two copies results in the disease) exhibits a wide range of biomechanical values, both above and below normal values," she noted. "But preliminary results indicate that they have higher levels of soluble collagen.

"Horses affected with HERDA showed uniformly weaker skin over their dorsum, ventrum, and extremities than age-matched controls," Rashmir-Raven summarized.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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