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What is HERDA (HC)?

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. Characterized by hyperextensible skin, scarring, and severe lesions along the back of affected (H/H) horses, they 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 (H/H) 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. The diagnostic DNA test for HERDA that has been developed allows identification of horses that are affected (H/H) or that carry (N/H) the specific mutation. UCD is capable of performing this simple test via a hair pull w/follicle submission. Other skin conditions can mimic the symptoms of HERDA. The DNA test will assist veterinarians to make the correct diagnosis. Any horse with the below mentioned bloodlines should be HERDA tested prior to breeding. (UCD)

The genes that cause HC are recessive, and it takes two for HC to occur. This means that both sire and dam must possess the recessive gene in order for an offspring to possibly be afflicted (H/H) with HERDA.

In this way, the disease differs from hyperkalemic periodic paralysis (HYPP), which descended through the bloodline of the Quarter Horse stallion Impressive. HYPP is caused by a dominant gene that can be carried by only one of the parents and still cause the affliction. With HC, both parents must carry the recessive gene for the horse to be afflicted.