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Common Genetic Diseases in Quarter Horses

Updated: Sep 8, 2019

Originally published by Kentucky Equine Research Staff


Quarter Horses are strong, muscular equines that were originally developed for ranch chores such as working with cattle and traveling over rough country. Horses of this breed are very fast over short distances and were given the name Quarter Horses for their ability to run a quarter-mile faster than horses of most other breeds. In addition to racing, they are used in many disciplines and are valued for their versatility and even temperament.

Along with their valued characteristics, some Quarter Horses have inherited genetic diseases that limit their ability to perform and stay healthy. Caused by defective genes that are passed from one or both parents to their foals, these diseases cause problems that may be mild and easily managed, or so severe that the affected horse is often euthanized.

Hyperkalemic periodic paralysis (HYPP) is a dominant condition, meaning that foals need to inherit only one gene to show signs of the disease. It was first seen in heavily-muscled halter horses and at one time affected more than half the horses bred for this purpose. It is caused by a defect in the generation and conduction of the electrical impulses that control muscle contraction. Horses that are mildly affected may show only a few muscle tremors, but severely affected animals may develop muscle cramping, paralysis, collapse, or sudden death. Careful nutritional management is successful in preventing signs of HYPP in many horses.

Polysaccharide storage myopathy (PSSM) is a dominant disease caused by a defect in the glycogen synthase gene. It affects more than 10% of Quarter Horses and is also found in other breeds. Affected horses may show muscle stiffness and cramping (tying-up) that may be mild or severe. In some cases, horses with PSSM are unable to exercise normally because of weakness, pain, and reluctance to move. In many horses, PSSM can be managed by dietary modification to minimize intake of carbohydrates.

Malignant hyperthermia (MH) is another dominant disease that may be present in a horse and never be discovered. MH can be triggered by extreme stress, use of the anesthetic halothane, and treatment with succinylcholine, a muscle relaxant. Under these special circumstances, an affected horse may show an extreme rise in body temperature accompanied by abnormal heart rhythms, high blood pressure, breakdown of muscle tissue, and sometimes death.

Glycogen branching enzyme deficiency (GBED) is a recessive trait, meaning that both parents must pass on the defective gene for offspring to be affected. GBED is carried by 8 to 10% of Quarter Horses and is also found in some Paints and Appaloosas that have Quarter Horse ancestry. This disorder causes a malfunction in the use and storage of glycogen in the brain, liver, and muscles. Mares may abort affected fetuses, or they may deliver a weak or dead foal. If the foal is born alive, it usually dies or is euthanized soon after birth. No treatment exists, so GBED is always fatal.

Hereditary equine regional dermal asthenia (HERDA) is another recessive trait that is less common, affecting fewer than 4% of the breed. Horses that have HERDA are born with a defect in the structure and function of collagen, a component of tissues such as skin, muscles, and cartilage. Foals appear normal at birth and the defect is often noticed only as training is begun. Wearing a saddle often causes the horse’s skin to tear away from the underlying muscles, leaving large raw areas that fail to heal. These horses can survive but are not able to be used or bred and are often euthanized.

These genetic conditions have unpleasant physical effects on horses as well as financial and emotional effects on owners and breeders. The only way to eliminate these diseases from the breed is to refrain from breeding horses that could pass the unwanted traits to their foals. Because a horse could have one or more defective genes and yet show no outward signs of a problem, genetic testing is the only way to know whether a certain horse is a carrier. An owner who is considering using a mare or stallion for breeding should schedule this testing to find out whether the prospective sire or dam is a safe candidate.

The American Quarter Horse Association (AQHA) offers a panel test for these five diseases. For more information on genetic testing, contact the AQHA.

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