| Contents |
|---|
| Hereditary Equine Regional Dermal Asthenia (HERDA) |
| Summary |
| Understanding the Results |
| Disease Name and Genes |
| Inheritance |
| Resources |
Hereditary Equine Regional Dermal Asthenia (HERDA)
Summary
Hereditary equine regional dermal asthenia (HERDA), or hyperelastosis cutis (HC) is an inherited condition causing skin lesions and ulcerations, poor healing and scarring of wounds, and skin sloughing along the back of affected horses beginning around 1.5-2 years of age and progressively worsening over time. This disease has been found in Quarter Horses and related breeds. Most affected horses are euthanized as they cannot be ridden and are not suitable for breeding. There is no effective treatment. The condition resembles Ehlers-Danlos syndrome in humans, which has multiple genetic causes. The skin defects resemble those seen in the Warmblood Fragile Foal Syndrome (WFFS), which has an earlier onset of symptoms. There are also reports of Ehlers-Danlos-like syndromes of unknown origin in a number of breeds.
The carrier frequency is about 3.5% in Quarter Horses, with an incidence of over 25% in Cutting horses. It is especially prevalent among horses descended from the Quarter Horse sire, Poco Bueno. The American Quarter Horse Association requires genetic testing for HERDA and four other genetic conditions (GBED, PSSM1, MH, and HYPP).
Date of Last Update: 08/02/2016
Results
Understanding the Results
Results of the genetic test for HERDA are presented as shown below.
| Hereditary equine regional dermal asthenia (HERDA) | ||
|---|---|---|
| N/N | Clear | This horse tested negative for HERDA. |
| N/Hrd | Carrier | Both the normal and mutant alleles are present. This horse is positive for the PPIB-G39R mutation but will not develop symptoms of HERDA. |
| Hrd/Hrd | Affected | This horse carries two copies of the PPIB-G39R mutation and will develop HERDA. |
Disease Name and Genes
Hereditary equine regional dermal asthenia (HERDA) is caused by a G to A substitution at base 115 of the gene for peptidyl-prolyl isomerase B (PPIB), causing a glycine to arginine substitution at amino acid position 39 in the PPIB protein (PPIB-G39R).
Inheritance
HERDA is caused by a recessive variant of the PPIB gene (PPIB-G39R). The recessive allele is commonly abbreviated as Hrd, with the dominant wild-type allele abbreviated as N.
Carriers of the recessive allele (N/Hrd) have no symptoms of the disease. If two carriers are bred, each foal has a 25% chance of having two copies of the normal allele (N/N), a 50% chance of being a carrier (N/Hrd), and a 25% chance of being affected (Hrd/Hrd).
If a carrier of the recessive allele (N/Hrd) is bred to a normal horse (N/N), each foal has a 50% chance of having two copies of the normal allele (N/N) and a 50% chance of being a carrier (N/Hrd).
