Greater awareness and ongoing research mean fewer foals are being born with this genetic skin condition

Attention stock horse owners: Have you heard of HERDA? For the past 20 years, learning about hereditary equine regional dermal asthenia (HERDA) has been a major goal of stock horse associations and HERDA researchers alike. Up to 3.8% of Quarter Horses—and specifically up to 28.3% of those in the cow horse population—are carriers for this debilitating genetic condition that causes horses’ skin to wrinkle, stretch, and slough. While HERDA horses can be maintained with specialized management, most affected animals are euthanized due to the impracticalities of that care, the inability to be ridden, and painful scarring.

With simple genetic testing, though, owners can find out if their animals are affected by or carriers of HERDA so they can make sound management and breeding decisions. As awareness increases, the industry should see fewer foals born with HERDA—and aim to eliminate clinical cases altogether.

How HERDA Happens

In 1978 scientists first described an unusual super-elastic skin condition and suspected it might have a hereditary link. They were right: In 2007 scientists discovered HERDA arises from a simple mutation on a gene called PPIB, located on Chromosome 1. If a foal inherits that same mutation from both parents, he or she will develop clinical signs of the disease, says Rebecca Bellone, PhD, Veterinary Genetics Laboratory director and a professor at the University of California, Davis.

Scientists traced the mutation back to its origin, a 1944 foundation Quarter Horse colt named Poco Bueno, who sired 405 registered foals. The stallion possessed mutated genetic codes for defective collagen processing. Collagens build connective tissues in skin as well as in cartilage, bones, tendons, ligaments, neurological membranes, and some other tissues. The most obvious effect of these defects is abnormally stretchy and wrinkly skin, with layers often detaching from each other so the skin peels away and sloughs. These gaping skin wounds—along with any other contact wounds—heal slowly and poorly, leaving disfiguring scars.

Owners might first notice frequent lacerations, cuts, or scrapes on young horses eventually diagnosed with HERDA, but the strongest clue appears when these animals are first saddled. At this point they often start to develop wounds along the back and neck. Ultraviolet rays from the sun seem to make the condition even worse, creating new lesions and exacerbating existing ones.

However, because horses have connective tissue throughout their bodies, they might also develop problems elsewhere, says Abby McElroy, DVM, MS, PhD candidate and researcher at the University of Massachusetts Chan Medical School, in Worcester. In particular, researchers know HERDA to be associated with an increased risk of corneal ulcers as well as abnormal heart valves, she says.

“If you have an affected horse, it’s obviously very serious for both the horse and the owner,” says Tammy Canida, registrar at the American Quarter Horse Association (AQHA), whose headquarters are in Amarillo, Texas. “The humane thing to do is put the horses down in most cases.”

It Just Takes Two Alleles

Early research in 2009 revealed 3-4% of Quarter Horses, Paint Horses, and other stock horses probably have the HERDA mutation. However, most of these horses show no signs of disease because they’re merely heterozygous carriers, meaning they have only one copy of the HERDA mutation