Hereditary Equine Regional Dermal Asthenia (HERDA)

What is HERDA?

Hereditary equine regional dermal asthenia (HERDA), also known as hyperelastosis cutis, is an autosomal recessive, inherited degenerative skin disease characterized by lesions that develop along the back, primarily under the saddle area. It has been identified in American Quarter Horses and other breeds with Quarter Horse lineage, such as American Paint Horses and Appaloosas, with the highest incidence in cutting horse lines. The disease causes the skin along the horse’s back and neck to stretch and tear easily, making the horse unsuitable for riding. It is present at birth, but commonly first noticed when training begins as the friction of tack and other equipment may induce the formation of lesions. A DNA test is available to confirm a HERDA diagnosis and to enable breeders to make educated breeding decisions to avoid producing affected offspring.

What are the clinical signs of HERDA?

The identified genetic mutation results in defects in collagen that cause the outer layer of skin to separate from the underlying layer. Affected horses are characterized by hyperextensible (i.e. stretchy) skin, severe lesions along their backs, and related scarring. These signs worsen in frequency and severity with age. Horses with HERDA can develop seromas (buildup of clear fluid under the surface of the skin), hematomas (blisters filled with serum or blood), and ulcerations over the back and sometimes the sides of the neck.  Occasionally similar lesions have been noted on the lower legs. The severity of clinical signs can vary from horse to horse even though the mutation is the same.

A study on a small number of horses with HERDA reported an increased incidence of corneal ulcers, as well as decreased corneal thickness, increased corneal curvature, increased corneal diameter, and mild corneal opacity in the eyes of affected horses, but it is currently unknown whether these structural changes affected the horses’ vision. Another recent study in horses with HERDA suggested that they may have weakened heart valves, but clinical correlations have not yet been reported.

How is HERDA diagnosed?

Although the disease is present at birth, horses are not normally diagnosed with HERDA until they are around 2 years of age and being broke to ride. Saddling and riding often result in tearing of skin along the back and the wounds are slow to heal. Diagnosis is based on clinical signs and genetic testing. Histologic findings are sometimes subtle but clumped or poorly organized collagen fibers below the level of the hair follicles may be seen. A DNA test is available to identify carriers and confirm diagnoses based on clinical presentations (other skin conditions can mimic some of the symptoms of HERDA).

How is HERDA treated?

There is no treatment or “cure” for HERDA.

What is the prognosis for HERDA?

Affected horses are unfortunately usually euthanized. Some may become pasture pets but often require specialized management (including restricting exposure to sunlight). Lesions may heal adequately, but often leave scars or hair that grows in white. The prognosis for carriers is excellent as there are no known health problems associated with carrier status for this disease.

How can HERDA be prevented?

Breeders can prevent HERDA in their horses by performing DNA testing on potential sires and dams and avoiding carrier matings. Hair samples (with the roots attached) can be submitted to the UC Davis Veterinary Genetics Laboratory. Offspring of two HERDA carriers have a 25% chance of being affected. Breeding a normal horse to a carrier will not result in an affected foal but will produce a HERDA carrier foal 50% of the time. The American Quarter Horse Association requires HERDA genetic testing for all breeding stallions and it is one of the tests included in the AQHA 5 panel test.

*The causative mutation for HERDA was identified at the UC Davis School of Veterinary Medicine in the laboratory of Dr. Danika Bannasch with support from the American Quarter Horse Foundation and the UC Davis Center for Equine Health.

For more information:

UC Davis Veterinary Genetics Laboratory HERDA DNA testing

HERDA: 20 Years Later, The Horse, July 2024

Tryon, R.C., White, S.D., Bannasch, D.L. Homozygosity mapping approach identifies a missense mutation in equine cyclophilin  B (PPIB) associated with HERDA in the American Quarter Horse. Genomics 90(1): 93-102.

UC Davis Center for Equine Health Horse Report, January 2006 

UC Davis Center for Equine Health Horse Report, January 2005

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu.