Hyperelastosis cutis (HC), more correctly referred to as hereditary equine regional dermis asthenia or HERDA, causes the skin on a horse's back to separate, lift off and even tear away when the horse carries a saddle or suffers a skin trauma.
In affected horses, there is a lack of adhesion within the deep layers of the dermis, which attaches the skin to the horse. Researchers believe HERDA is due to a collagen defect resulting in fragile attachment of the skin. Even light contact with the skin on the back, neck, hips and sometimes lower legs of an affected horse can cause it to separate and peel off in large sheets, resulting in a gaping wound that exposes the muscle underneath.
It has been reported that even normal activities such as rolling in the pasture can cause the skin on an affected horse's back to rip apart.
Currently there is no successful treatment or cure; therefore, affected horses are unsuitable for any purpose other than as research animals for this poorly understood disease.
Researchers at Mississippi State University have reported that affected horses may show behavioral problems as a result of the pain associated with the skin disorder. These behavior issues can be crucial, subtle indicators that the horse is beginning to develop clinical signs of the disease.
Many affected horses do not want their backs to be groomed. They may bite, repeatedly move away, pin their ears, swish their tails or show other signs of discomfort during grooming or saddling.
Some horses may show signs of discomfort for some time before they begin to have skin separation or peeling, especially if they are only mildly affected Affected horses typically do not show signs of HERDA until they enter training and carry a saddle because the horse's skin appears normal until something pulls or pushes on the affected area.
Younger horses are typically diagnosed with HERDA after suffering an injury that tears, scrapes or punctures the affected skin. The healing of these wounds is dramatically impaired and produces awful scars.
It is not known how long HERDA has been around, but approximately 200 cases have been documented since the early 1970's. At least forty-two of those cases have been reported within the last two years.
The disease is thought to be caused by an autosomal recessive gene, meaning that both the sire and the dam must possess the gene in order for the offspring to be affected. This also means that horses that have the gene can be carriers without actually being affected.
As long as a carrier is bred to a HERDA gene free individual, the offspring cannot develop the disease. The problem arises when two carriers of the gene mate. This mating will have a 25% chance of producing an affected foal, a 50% chance of producing a carrier, and only a 25% chance of producing a genetically normal foal.
However, it is important to understand that breeding a carrier stallion to a normal mare would result in 50% of the offspring being HERDA gene carriers as well. The other 50% would be genetically normal.