Hereditary Equine Regional Dermal Asthenia (HERDA)

Hereditary equine regional dermal asthenia (HERDA) is a genetic skin disease predominantly found in the American Quarter Horse. Within the breed, the disease is prevalent in particular lines of cutting horses. HERDA is characterized by hyperextensible skin, scarring, and severe lesions along the back of affected horses. Affected foals rarely show symptoms at birth. The condition typically occurs by the age of two, most notably when the horse is first being broke to saddle. There is no cure, and the majority of diagnosed horses are euthanized because they are unable to be ridden and are inappropriate for future breeding.

HERDA has an autosomal recessive mode of inheritance and affects stallions and mares in equal proportions. Research carried out in Dr. Danika Bannasch's laboratory at the University of California, Davis, identified the mutation causing HERDA. HERDA is caused by a single base change in the gene PPIB (c.115G>A). This missense mutation codes for a change in the gene product, specifically the normal  glycine at position 39 is changed to an arginine (denoted as p.G39R) altering the Peptidylprolyl Isomerase B (PRIB) protein. PPIB is one of the proteins involved in proper collagen formation. Collagen is is an important structural protein of all connective tissues including skin. Functional studies have shown that the HERDA mutation delays proper collagen folding and secretion and is presumed to alter collagen organization thus leading to the clinical manifestations.

This diagnostic DNA test for HERDA allows identification of horses that are affected or that carry the specific mutation. Other skin conditions can mimic the symptoms of HERDA; this DNA test will assist veterinarians to make the correct diagnosis. For horse breeders, identification of carriers is critical for the selection of mating pairs. Breedings of carrier horses have a 25% chance of producing an affected foal. Breedings between normal and carrier horses will not produce a HERDA foal, although 50% of the foals from this type of cross are expected to be carriers.