American Quarter Horses - Genetic Diseases

Genetic Diseases

There are several genetic diseases of concern to Quarter Horse breeders:

• Hyperkalemic periodic paralysis (HYPP), which is caused by an autosomal dominant gene linked to the stallion Impressive. It is characterized by uncontrollable muscle twitching and substantial muscle weakness or paralysis among affected horses. Because it is a dominant gene, only one parent has to have the gene for it to be transmitted to offspring. There is a DNA test for HYPP, which is required by the AQHA. Since 2007, the AQHA bars registration of horses who possess the homozygous form (H/H) of the gene, and though heterozygous (H/N) horses, are still eligible for registration, altering that status is currently being discussed. Additionally all Quarter Horses born 2007 or later that are confirmed to be descendants of Impressive must carry a note about the risks of HYPP on their registration papers. Due to HYPP, the halter classes are undergoing significant changes. Halter classes are dominated by the Impressive bloodline. Impressive, a very prolific halter horse, brought to the stock breeds the muscle mass that is popular in halter competition today. This muscle mass is linked to HYPP, and as the condition is reduced within the breed, the style of horse in halter classes is also likely to change. Already there have been rule changes, including the creation of a "Performance Halter class" in which a horse must possess a Register of Merit in performance or racing before it can compete.

• Malignant hyperthermia. A causative mutated allele, ryanodine receptor 1 gene (RyR1) at nucleotide C7360G, generating a R2454G amino acid substitution. has been identified in the American Quarter Horse and breeds with Quarter Horse ancestry, inherited as an autosomal dominant It can be caused by overwork, anesthesia, or stress.

• Hereditary Equine Regional Dermal Asthenia (HERDA), also known as hyperelastosis cutis (HC). This is caused by an autosomal recessive gene, and thus, unlike HYPP, HERDA can only be transmitted if both parents carry the gene. When a horse has this disease, there is a collagen defect that results in the layers of skin not being held firmly together. Thus, when the horse is ridden under saddle or suffers trauma to the skin, the outer layer often splits or separates from the deeper layer, or it can tear off completely. It rarely heals without disfiguring scars. Sunburn can also be a concern. In dramatic cases, the skin can split along the back and even roll down the sides, with the horse literally being skinned alive. Most horses with HERDA are euthanized for humane reasons between the age of two and four years. The very hotly debated and controversial theory, put forth by researchers at Cornell University and Mississippi State University is that the sire line of the great foundation stallion Poco Bueno is implicated as the origin of the disease. As of May 9, 2007, Researchers working independently at Cornell University and at the University of California, Davis announced that a DNA test for HERDA has been developed. Over 1,500 horses were tested during the development phase of the test, which is now available to the general public through both institutions.

• Glycogen Branching Enzyme Deficiency (GBED) is a genetic disease where the horse is lacking an enzyme necessary for storing glycogen, the horse's heart muscle and skeletal muscles cannot function, leading to rapid death. The disease occurs in foals who are homozygous for the lethal GBED allele, meaning both parents carry one copy of the gene. The stallion King P-234 has been linked to this disease. There is a DNA blood test for this gene.

• Equine polysaccharide storage myopathy, also called EPSM or PSSM, is a metabolic muscular condition in horses that causes tying up, and is also related to a glycogen storage disorder. While also seen in some draft horse breeds, PSSM has been traced to three specific but undisclosed bloodlines in Quarter Horses, with an autosomal recessive inheritance pattern. 48% of Quarter Horses with symptoms of neuromuscular disease have PSSM. To some extent it can be diet controlled with specialized low-starch diets, but genetic testing is advised before breeding, as the condition exists at a subclinical level in approximately 6% of the general Quarter Horse population.

• Lethal White Syndrome. Although "cropout" Quarter Horses with Paint markings were not allowed to be registered for many years, the gene for such markings is a recessive and continued to periodically appear in Quarter Horse foals. Thus, it is believed that some Quarter Horses may carry the gene for Lethal White Syndrome. There is a DNA test for this condition.