Hereditary Disease Research Into HERDA

Research into the disease known as 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.



A typical sore from hyperelastosis cutis, or hereditary equine regional dermal asthenia.

The first part collected information and skin samples from more than 50 horses which were examined at veterinary schools including UC Davis and Colorado State University, and by private practitioners. "We looked at clinical signs and histopathology of the skin to see if there were distinguishing factors to help diagnose these cases," says White. That phase has been completed and a paper on the results has been accepted for publication by a scientific journal, he says.

"We looked at different staining techniques to see if we could find out if some of the collagen and elastin fibers were abnormal," he says. "But our pathologists, looking blindly at biopsies of 46 of these horses and 10 normal horses, were not able to see any difference between groups using the staining techniques."

"We mainly diagnose this disease by seeing clinical signs--lesions over the back and sometimes the sides of the neck, and separation in the deep dermis," White explains. "But not seeing these signs does not rule out the disease."

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

Part three is seeking to develop a test for this trait. "Rather than having to guess at the bloodlines that carry it, we will be able to definitively show if an animal is a carrier," says White. "If we have any doubt about a horse with lesions that look like HERDA (also called HC), we can test to see if that animal does have this disease. Usually it's easy to diagnose, once you've seen a few."

But since it's not a common disease across the whole horse population, some veterinarians have not seen it.

"We are still working on this phase; our geneticists tell us it will be two or three more years," says White. "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. The parents should have half the defect (one normal allele and one abnormal) whereas the offspring inherited the two abnormal ones."

Another part of the research, which White is doing in conjunction with Ann Rashmir, DVM, MS, Dipl. ACVS, associate professor of surgery and head of the Hyperelastosis Cutis Research Program at Mississippi State University, involves looking at some affected foals during their first year of life to see if they can find differences in skin biopsies (or on electron microscopy of the biopsies) compared with normal healthy foals.

About the Author

Heather Smith Thomas

Heather Smith Thomas ranches with her husband near Salmon, Idaho, raising cattle and a few horses. She has a B.A. in English and history from University of Puget Sound (1966). She has raised and trained horses for 50 years, and has been writing freelance articles and books nearly that long, publishing 20 books and more than 9,000 articles for horse and livestock publications. Some of her books include Understanding Equine Hoof Care, The Horse Conformation Handbook, Care and Management of Horses, Storey's Guide to Raising Horses and Storey's Guide to Training Horses. Besides having her own blog,, she writes a biweekly blog at that comes out on Tuesdays.

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