Registry Services - Genetic Disorders
Other Genetic Disorders/Defects

GBED (glycogen branching enzyme deficiency)
causes late-term abortion, stillbirths, and death in young foals. Some foals are born alive but are often weak with low body temperature and require warming and assistance to nurse after birth. These foals may appear healthy for a time but eventually they may develop seizures from low blood sugar, become too weak to stand or in some cases they die suddenly when their heart, brain and skeletal muscles are unable to function. GBED-affected foals often have contracted tendons in all four legs, a high respiratory rate and weakness of the muscles used for breathing, and difficulty getting up from lying down. Glycogen branching enzyme (GBE) is a protein that is necessary for the body to build glycogen, the complex sugar that is a source of fuel for many tissues in the body. Normal glycogen consists of glucose (sugar) arranged to resemble a highly-branched tree and GBE is the protein that arranges the branches. When a foal is missing GBE the glycogen in its tissues lacks the normal branched structure and thus cannot effectively store sugar molecules; these tissues are unable to function, leading to rapid death. The tissues that rely heavily on glycogen as a fuel are skeletal muscle, heart muscle and the brain. This disease is fatal. It is inherited as an autosomal (not sex linked) recessive trait which means that an affected foal receives a copy of the defective gene from each parent. Horses can be carriers and not show signs of the disease but they can have affected offspring. GBED can be prevented if parents are tested and carriers not permitted to breed.




Many other equine diseases and disorders may have a genetic origin but research has so far not identified the specific mutations that cause them. Diseases that have been found in Quarter Horses as well as other breeds include the following:


PSSM (polysaccharide storage myopathy)
is a muscle disease causing muscle soreness, reluctance to engage the hindquarters, muscle atrophy, weakness, difficulty in backing up and picking up hind feet. Horses with PSSM have signs typically associated with tying-up – muscle stiffness, sweating and reluctance to move. The signs are most often seen in horses when they are put into initial training or after spelling. Episodes usually begin after very light exercise such as ten to twenty minutes of walking or trotting. Horses seem lazy, have a shifting lameness, tense up their abdomen and develop tremors in their flank area. When these horses stop moving they often stretch out as if to urinate. Some horses will paw and try to roll immediately after exercise. Most horses with PSSM have a history of numerous episodes of muscle stiffness at the commencement of training but mildly-affected horses may have only one or two episodes a year.

PSSM is caused by the abnormal accumulation of the normal form of sugar stored in the muscle (glycogen) as well as an abnormal form of sugar (polysaccharide) in muscle tissue. Horses with PSSM accumulate muscle glycogen due to an unregulated uptake of sugar (glucose) into their muscles and the synthesis of its storage form in the muscle. One aspect of the defect involves enhanced sensitivity of the muscles to insulin, resulting in more transport of sugar from the bloodstream to skeletal muscles. The diet can be adjusted to decrease the affects of this disease by avoiding carbohydrates that are high in starch such as corn, sweet feed, wheat, oats, barley and molasses and providing extra calories in the form of fat. An important part of the management of PSSM horses is daily exercise.


DSLD (degenerative suspensory ligament desmitis) is defined as any failure of the suspensory apparatus which results in a horse being unable to support itself. It is a chronic, untreatable connective tissue disease that causes pain and abnormalities, primarily in leg tissues and to a lesser degree in the other tissues in the body. There is speculation that the horse develops an abnormal response to normal stresses on its suspensory ligaments by failing to manufacture new collagen for repairing tiny tears and strains. Instead of new collagen fibres being laid down in line with the stresses on the ligaments, cartilage is laid down, resulting in loss of tissue strength or elasticity. Without the ability to stretch, even normal weight-bearing on the limbs can become too much and, over time, ligaments lengthen and break down. No matter how an injury or stress occurs, DSLD cases improperly heal the ligament injury with cartilage instead of collagen. DSLD has been identified in a number of breeds of horses, including Quarter Horses, and it can be relieved or cured by using therapeutic shoeing, medications, surgery, bone marrow injections and shock wave therapy.

OCD (osteochondritis dessicans)
is characterised by fragments of cartilage within the joint and it most commonly affects the hock, fetlock and stifle. OCD is the best-known form of developmental orthopaedic disease and is related to how a horse’s bones grow. Because a foal needs to grow to the size of its mother in a short time, its bones are specially engineered to grow quickly in length and strength through cartilage – growth plates – on each end. Bones grow by the growth plates calcifying or hardening into bone, adding length. However, in joints the cartilage in the growth plates forms bone covered with a thin lining of cartilage. For unknown reasons, sometimes growth plate cartilage at the joint surface remains as an island of cartilage instead of converting to bone and if that situation occurs OCD often results. This cartilage is weak and prone to damage, leading to cracks and cysts. OCD may cause lameness and swelling in an affected joint and treatment depends on the joint affected, the intended use of the horse and the owner’s budget. Conservative treatment involves rest, correction of nutritional problems and injections into the joint but the treatment of choice is arthroscopy – surgical removal of the cartilage defect.

At the direction of the Association any horse may be required to be tested for any genetic disorder/defect at any time:

a) On notification from the Association, a registered veterinarian will, using an official testing kit, be required to take the specified sample and forward it to the place of testing directed by the Association.

b) The horse’s registration papers, where applicable, must be forwarded to the Association where they will be marked with the official result.

Source: Article by MaryAnne Leighton