American Quarter Horses are a four centuries old breed that originated in Virginia as a cross between Spanish and English horses. Horses of this particular breed are vulnerable to a serious genetic disease known as HERDA (Hereditary Equine Regional Dermal Asthenia) that affects their skin and eyes. It is a rare condition that causes skin lesions in the afflicted horses. The condition is recessive; that is, to have HERDA the foal needs one abnormal gene from the mare and another one from the stallion.
This genetic disorder is believed to have originated from the sire line of the American Quarter Horse Poco Bueno, a beautiful brown horse without any white spots. Because of the genetic defect, the outer layer of the carrier horse's skin fails to keep adhesion within the inner layers of skin. Consequently, the outermost skin breaks and splits from inner layers, greatly affecting the skin under the saddle area and gradually making the horse miserable and unrideable.
The disorder causes the afflicted horse’s skin to have defective fibers which fail to keep the layers of skin joined properly. The weakened fibers cannot sustain the pressure of external stress or strain. As a result, damaged skin is more prone to infection and, in extreme cases, it ‘de-gloves’ the horse – that is, skinning it alive. Unfortunately, the skin cannot be repaired because of the weakened state of the fibers. HERDA most often spreads along the back of horses, particularly around the saddle area and the neck, but can affect other parts of the body including the legs. Affected horses’ eyes also contain substantial amount of collagen; so, there is a possibility of developing ocular abnormalities in these horses which could potentially damage their vision.
In most cases, the signs of the disease are not visible until the victim horse is around 2 years of age and are in their initial training period. With occasional injury in the paddock, HERDA can be spotted earlier. However, there is no cure for this disease as it is a genetic disorder. Most of the horses carrying it genetically are euthanized as they become incapable of carrying riders due to the injuries the horse suffers whilst saddled.
Some of the horses diagnosed with HERDA survive and live into their old ages because of extraordinary care from their owners. Careful nursing is required for them as the diseased horses are extremely vulnerable to fatal injury and illness. A minor injury or sunburn may lead to complex physical condition. Sadly, most of the horses with HERDA are likely to have an average life of only two to four years.
Fortunately, the research into the genetics of HERDA is gaining prominence and DNA screening has made it possible to decisively reach a conclusion without having to wonder. Poco Bueno-bred horse owners may breath a sigh of relief as all of these horses are not carriers of the defective gene. Many of them are healthy and normal. Through conclusive identification of the carrier horses, further spread of the genetic disease can be avoided while breeding of the American Quarter Horse continues.
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