Hereditary equine regional dermal asthenia (HERDA) is an inherited autosomal recessive connective tissue disorder that affects Quarter Horses and related breeds. It is caused by a homozygous recessive mutation that weakens collagen fibers in the skin, leading to abnormal skin along the back that tears or rips easily and heals into disfiguring scars. The skin is loose and hyper-elastic in affected horses, and their mechanical properties are inferior to native horses. HERDA can also affect the horses heart valves. Symptoms typically don’t appear until the horse is subjected to pressure or injury on their back, neck, or hips, usually around two years of age. However, foals can show signs when injured, while other horses mature and only show signs in the joints. The expression of HERDA is variable, and the phenotypic range of expression is still being determined.
After being tested, the results either show "normal", "carrier" or "affected" and can be determined by DNA testing of either a blood or hair sample. The following are the results of the test:
- N/N — Normal: does not have the HERDA gene
- N/HRD- Carrier: carries one copy of the gene
- HRD/HRD- Affected: has two copies of the gene
HERDA is not curable, and affected horses are usually euthanized due to the severity of the condition. However, some horses with minimal signs have been discovered to be homozygous for HERDA, and they can be managed with proper care and treatment.
