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Breeds: American Indian, Appaloosa, Appaloosa Cross, Appaloosa/Friesian, Appaloosa/Quarter Horse, Appaloosa/Thoroughbred, Appendix, AraAppaloosa, Arabian/Paint, Arabian/Quarter Horse, Australian Stock Horse, Azteca, Baroque Pinto, Canadian, Cleveland Bay, Clydesdale/Paint Cross, Colonial Spanish Horse, Crossbred, Curly Horse, Friesian Cross, Grade Horse, Grade Pony, Miniature Appaloosa, Mixed Breed, Mixed Breed (Horse), Morgan Horse, Origine Constatée, Paint Cross, Paint Horse, Paint Horse/Thoroughbred, Paint/Warmblood, Ponies of America, Pura Raza Espanola, Quarter Cross, Quarter Horse, Quarter Pony, Quarter/Paint Horse, Quarter/Stock Horse, Quarter/Thoroughbred, Quarter/Warmblood, Spanish Barb, Spanish Mustang, Stock Horse, Stonewall Sporthorse, Sugarbush Harlequin Draft, Thoroughbred, Unspecified, Welsh Mountain/Quarter Horse
Description
Hereditary Equine Regional Dermal Asthenia (HERDA), also known as hyperelastosis cutis (HC), is an inherited connective tissue disorder found predominantly in the American Quarter Horse and in breeds with Quarter Horse ancestry. Pedigree analyses have traced affected horses back to the stallion Poco Bueno.
HERDA is caused by a mutation in the PPIB gene, which encodes cyclophilin B, a protein required for the normal processing of collagen. Collagen is the main structural protein that gives skin its strength. As a result, affected horses have fragile, abnormally loose skin that separates within its deeper layers under minimal force. This leads to open wounds, fluid-filled swellings, and sloughing skin, most often along the back and under the saddle. Signs typically appear between 18 months and 2 years of age, frequently when saddle training begins. Healing often leaves permanent scars, and many affected horses cannot be ridden.
HERDA is inherited as an autosomal recessive trait: a horse must inherit two copies of the variant, one from each parent, to be affected. A horse with one copy is a carrier and shows no signs of the disorder, but can pass the variant to its offspring. When two carriers are bred, each foal has a 25% chance of being affected, a 50% chance of being a carrier, and a 25% chance of being clear. A carrier bred to a clear horse cannot produce an affected foal, although about half of the foals will be carriers. Testing breeding stock allows owners to avoid carrier-to-carrier matings.
There is no cure for HERDA. Animal Genetics offers a DNA test that detects the causative PPIB variant, allowing owners and breeders to identify affected, carrier, and clear horses before making breeding decisions.
Possible results
| Genotype | Description |
|---|---|
| Hrd/Hrd | Affected: Horse has two copies of the HERDA variant and is expected to develop signs of the disorder. It will pass one copy of the variant to every offspring. |
| N/Hrd | Carrier: Horse has one copy of the HERDA variant and is not expected to show signs of the disorder. Each offspring has a 50% chance of inheriting the variant. |
| N/N | Clear: Horse does not carry the HERDA variant and cannot pass it on to offspring. |