Researchers Examine HERDA Horses' Eyes

HERDA-affected horses have important ocular changes that could potentially negatively impact vision, but more research is needed, the team said.

Photo: Anne M. Eberhardt/The Horse

What do a horse's skin and eyes have in common? If the horse is a Quarter Horse, both organs could be affected by a serious genetic condition called HERDA, or hereditary equine regional dermal asthenia. Brazilian researchers recently took a closer look at the impact of HERDA, which up to 3.5% of Quarter Horses might carry the genes for, on affected horses' eyes.

“HERDA is a recessive condition, which means that for the foal to have HERDA it must have two copies of the abnormal gene—one from the mare and one from the stallion,” said Alexandre Borges, DVM, MSc, PhD, from the Department of Clinical Science, UNESP, Univ Estadual Paulista, Rubião Junior, São Paulo, Brazil.

The genetic anomaly disrupts the production of collagen, which helps maintain connective tissues' structure and strength. Classic clinical signs include hyperextensible skin, susceptibility to bruising and scarring, and frequent ulcers and/or cuts, especially around the area where a saddle might sit.

“The eyes of horses also contain substantial amounts of collagen, which means that ocular abnormalities that could potentially affect a horse’s vision might develop in affected horses,” Borges said.

Little information regarding ocular abnormalities associated with HERDA exists in the literature, prompting Borges; Peres Badial, DVM, MSc, PhD; and colleagues to conduct comprehensive ophthalmic examinations on 10 Quarter Horses—five with HERDA and five without.

The team identified some important differences between the eyes of horses with and without HERDA, including:

  • HERDA horses had thinner corneas, particularly in the central and dorsal (top) parts;
  • HERDA eyes had a significant increase in both curvature and diameter of the cornea;
  • Some affected HERDA eyes had increased opacity (cloudiness); and
  • There was no difference in tear production, intraocular pressure, or overall ocular dimension between the two groups of horses.

“These findings suggest that HERDA-affected horses have important ocular changes that could potentially negatively impact vision, but more research is needed,” Borges and Badial concluded.

The study, “Ocular dimensions, corneal thickness, and corneal curvature in quarter horses with hereditary equine regional dermal asthenia,” was published in Veterinary Ophthalmology

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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