HERDA
Hereditary Equine Regional Dermal Asthenia
recessive traitWhat is HERDA?
HERDA (Hereditary Equine Regional Dermal Asthenia) is a connective tissue disease that weakens the skin's attachment layers. Affected horses have fragile skin that tears, slips, or develops open wounds from normal handling, tack pressure, or minor trauma. The condition is painful and progressive. Because it is recessive, a horse must inherit two defective copies to show clinical signs — carriers with one copy appear completely normal.
How is it inherited?
HERDA is autosomal recessive. Both parents must carry at least one copy of the Hr allele for an affected foal to be born. Carrier-to-carrier breeding produces a 25% chance of an affected foal, 50% chance of a carrier, and 25% chance of a clear foal.
What do the results mean?
| Result | Status | What it means |
|---|---|---|
| N/N | Normal | No HERDA alleles. Horse is clear and cannot produce affected offspring unless bred to a carrier. |
| N/Hr | Carrier | One normal and one HERDA allele. Horse is completely healthy with no skin symptoms but will pass the allele to ~50% of offspring. |
| Hr/Hr | Affected | Two HERDA alleles. Horse has the condition and will show skin fragility, open wounds, and chronic pain. Affected horses typically cannot be ridden and are often humanely euthanized. |
No HERDA alleles. Horse is clear and cannot produce affected offspring unless bred to a carrier.
One normal and one HERDA allele. Horse is completely healthy with no skin symptoms but will pass the allele to ~50% of offspring.
Two HERDA alleles. Horse has the condition and will show skin fragility, open wounds, and chronic pain. Affected horses typically cannot be ridden and are often humanely euthanized.
What should buyers know?
N/N and N/Hr horses are safe to own and ride. If you plan to breed, avoiding carrier-to-carrier pairings is essential. Hr/Hr horses require intensive care and cannot be used as riding horses.
Which breeds are affected?
Found almost exclusively in American Quarter Horses and closely related breeds (Paints, Appaloosas). Highest prevalence in cutting horse bloodlines, particularly those tracing to Poco Bueno.
This page is for general education only. Always consult a veterinarian or equine geneticist before making purchasing or breeding decisions based on genetic test results.
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