Hereditary equine regional dermal asthenia (HERDA; "hyperelastosis cutis") is a disease that occurs early in life in horses. Most affected horses are Quarter Horses, but registered Paint Horses and Appaloosas with Quarter Horse lineage have been afflicted with this disease.
Many of the Quarter Horses are from high-quality cutting lines. The disease (or something very similar) has also been reported in a cross-bred Arabian mare, a Thoroughbred gelding, a Hanoverian foal, and a Haflinger horse.
The working hypothesis is that these horses have a defect in their collagen fibers or in the way those fibers are structurally organized in the mid to deep
dermis. Typically, these areas are over the back and sides of the neck. The skin in these areas may seem to be easily torn or stretched, and it
often develops seromas and hematomas ("blisters" filled with either serum or blood)). Healing per se is usually adequate but often leaves rather
unsightly scars.
Diagnosis is often based on the clinical signs alone; histologic findings are sometimes subtle, but "clumped" or poorly organized collagen fibers below the level of the hair follicles may be seen. A zone of mid to deep-dermal separation has been reported in two horses, and it has been present in some of the biopsy samples that the author has seen.
"Poorly oriented" collagen fibers are sometimes seen on electron microscopy. There is no blood test to confirm the disease.
This condition is almost certainly present at birth; however, it is often not noticed until about two years of age when horses start being trained with tack, and the
friction/trauma of this induces the typical lesions.
As with many genetic diseases, there is no effective treatment or cure, but some of these horses have been maintained as "pasture pets."
This disease follows an autosomal recessive mode of inheritance; therefore, for the foal to be affected, both the sire and the dam must carry the gene, and
if they were bred again, there would be about a 25% chance that the next foal would also be affected.
Obviously, clinically affected horses with the disease should be removed from
breeding programmes.
There is now a diagnostic test for HERDA developed by researchers at the University of California, Davis.
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