HERDA, or hereditary equine regional dermal asthenia, is a genetic skin condition in horses that causes the skin to tear, stretch abnormally, and heal poorly. It primarily affects American Quarter Horses and is linked to a specific bloodline. The condition is present from birth but typically goes unnoticed until a horse begins training under saddle, usually around age 2, when friction from tack triggers the first visible lesions.
How HERDA Affects the Skin
At the core of HERDA is a problem with collagen, the protein that gives skin its strength and structure. Affected horses carry a mutation in a gene called PPIB, which produces a protein involved in collagen folding. The mutation doesn’t completely shut down the protein’s function, but it disrupts its ability to interact with other proteins needed during collagen production. The result is collagen that folds more slowly, gets secreted inefficiently, and lacks normal chemical modifications that contribute to strong, stable fibers.
Under a microscope, the skin of a HERDA horse looks distinctly different from healthy tissue. The collagen fibers are thin and short, clumping together in clusters rather than forming the dense, interwoven network that gives normal skin its toughness. There is also separation between the upper and deeper layers of the skin. This structural weakness is why even routine contact, like a saddle sitting on the back, can cause the skin to tear apart.
When Symptoms Appear
HERDA is present at birth, but foals usually look completely normal. The disease commonly reveals itself around 2 years of age, when horses are first saddled and ridden. The friction and pressure of tack and equipment create lesions along the back that would not occur in a healthy horse. Saddling and riding often result in the skin tearing, and these wounds are noticeably slow to heal. The skin along the back is the most common area affected, but lesions can appear anywhere that experiences repeated contact or pressure.
Because the onset coincides with training, HERDA is sometimes mistaken at first for saddle sores or an allergic reaction to equipment. The key difference is the severity and persistence: the skin doesn’t just get irritated, it separates and peels away, and the resulting wounds form fragile, unstable scar tissue that re-tears easily.
Which Horses Are at Risk
HERDA is most common in American Quarter Horses, particularly those bred for cutting. The condition traces back to the Poco Bueno bloodline, one of the most influential foundation sires in cutting horse breeding. Because Poco Bueno’s genetics are so widespread in certain segments of the Quarter Horse population, the carrier rate is significant: an estimated 2.5 to 3.8 percent of all Quarter Horses carry one copy of the HERDA gene. Breeds that share Quarter Horse bloodlines, such as American Paint Horses, can also carry the mutation.
Carriers are completely healthy. They show no symptoms and perform normally throughout their lives. The disease only appears when a foal inherits two copies of the mutated gene, one from each parent.
How HERDA Is Inherited
HERDA follows an autosomal recessive inheritance pattern, meaning both parents must carry the mutation for a foal to be affected. When two carriers are bred together, there is a 25 percent chance the foal will have HERDA, a 50 percent chance it will be a carrier like its parents, and a 25 percent chance it will be completely clear of the gene. Breeding a carrier to a non-carrier will never produce an affected foal, though about half the offspring will be carriers themselves.
This inheritance pattern is why HERDA persisted in the population for decades before being identified. Carriers look and perform normally, so the gene was passed along through generations of successful show and cutting horses without anyone knowing it was there.
Genetic Testing
A DNA test is available through the UC Davis Veterinary Genetics Laboratory and other labs. The test identifies whether a horse is clear (no copies of the mutation), a carrier (one copy), or affected (two copies). Testing can be done from a hair sample pulled with the roots intact, making it simple and non-invasive.
For breeders working with Quarter Horse lines connected to cutting or reining pedigrees, testing both the sire and dam before breeding is the most reliable way to prevent affected foals. If only one parent is a carrier, the pairing is safe from producing a HERDA foal, though testing the offspring is still useful for future breeding decisions.
Living With HERDA
There is no cure for HERDA. The underlying collagen defect is built into every cell in the horse’s body, and no treatment can correct it. Management focuses on preventing skin trauma. Affected horses cannot be ridden or worked under saddle, and they need careful handling to avoid injuries from normal activities like rolling, playing with herd mates, or rubbing against fences and stalls.
Some mildly affected horses can live as pasture companions with attentive management, but the prognosis is generally poor. Wounds accumulate over time, heal slowly, and produce weak scar tissue that breaks down again. Many owners and veterinarians ultimately face the decision of humane euthanasia when the horse’s skin damage becomes too extensive to manage and quality of life deteriorates. The severity varies between individual horses, but HERDA is a progressive, lifelong condition with no path to recovery.
Why Testing Matters for Breeders
Because carriers are healthy and often excel in performance disciplines, the HERDA gene can be unknowingly concentrated in competitive bloodlines. A carrier stallion used heavily in breeding programs can produce hundreds of carrier offspring before anyone realizes the gene is present. The availability of a simple DNA test has made it possible to make informed breeding decisions without removing valuable carrier horses from the gene pool entirely. Carriers can still be bred, just not to other carriers.
Breed registries and organizations have encouraged testing, and many buyers now request HERDA status before purchasing Quarter Horses with cutting or reining pedigrees. Testing both parents before every breeding is the single most effective way to eliminate affected foals from being born.