Hereditary equine regional dermal asthenia (HERDA) in Quarter Horses: A review of clinical signs, genetics and research
Authors: Rashmir‐Raven A. M., Spier S. J.
Journal: Equine Veterinary Education
Summary
# Editorial Summary: Hereditary Equine Regional Dermal Asthenia in Quarter Horses Hereditary equine regional dermal asthenia (HERDA) is an autosomal recessive genetic condition affecting Quarter Horses and their crosses, characterised by progressive skin fragility and poor wound healing that typically manifests within the first two years of life. Rashmir-Raven and Spier's 2015 review synthesised the clinical presentation, genetic basis and current research surrounding this condition, which has become increasingly prevalent in the Quarter Horse breeding population due to high carrier frequencies. Affected homozygous horses present with loose, hyperextensible skin prone to spontaneous tearing or injury from minor trauma, frequently complicated by haematoma and seroma formation; the resulting disfiguring scarring and functional impairment typically necessitate early euthanasia, as no curative treatments exist beyond palliative care. Whilst the American Quarter Horse Association mandates DNA testing for all breeding stallions as part of a five-panel genetic screen, there are currently no restrictions on registering heterozygous or homozygous carriers, meaning the mutation continues to circulate in breeding populations. For equine professionals across all disciplines, this review underscores the critical importance of genetic testing not only in stallions but also in mares and horses from affected bloodlines, and highlights the need for informed breeding decisions to reduce carrier frequency and prevent further disease manifestation.
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Practical Takeaways
- •All Quarter Horse breeding stallions should undergo DNA testing for HERDA as part of the five-panel test; mares and horses from affected bloodlines should also be tested regardless of breed
- •Homozygous affected horses will develop clinical signs within the first 2 years of life with spontaneous or trauma-induced skin lesions—early identification allows for humane management decisions
- •Heterozygous carriers appear clinically normal but can transmit the mutation; breeding decisions should account for carrier status to reduce disease frequency in the population
Key Findings
- •HERDA is an autosomal recessive genetic disorder affecting Quarter Horses and Quarter Horse lineage horses, caused by a single genetic mutation
- •Affected homozygous horses appear normal at birth but develop loose, hyperextensible skin and excessive scarring from minor trauma within the first 2 years of life
- •The disease has high frequency of heterozygote carriers in the Quarter Horse population, making genetic testing a significant industry concern
- •No curative treatments exist; most severely affected horses require euthanasia due to progressive disfiguring lesions and complications including haematomas and seromas