"Glycogen branching enzyme deficiency (GBED) occurs in newborn foals. This fatal disease is seen in Quarter Horses and related breeds. The foals lack the enzyme necessary to store glycogen (sugars) in its branched form and therefore cannot store sugar molecules. This disease is fatal as the heart muscle, brain and skeletal muscles are unable to function.

Drs. Stephanie Valberg and James Mickelson from the University of Minnesota, St. Paul first identified this deficiency. The Veterinary Genetics Laboratory, University of California, Davis has obtained a license from the University of Minnesota and has developed a diagnostic test that is now available to horse owners and associations.

Glycogen branching enzyme deficiency (GBED) is a disorder first recognized by clinicians at the University of Minnesota that causes muscle weakness in Quarter Horse and related breeds. The clinical presentation of this disease is variable. Late term abortion or stillbirth is described for GBED. Recent research suggests that at least 3% of abortions in Quarter horses are due to GBED. Some foals are born alive but are often weak and require warming and assistance to nurse after birth. These foals may appear healthy for a time but eventually the may develop seizures, become too weak to stand, or in some cases, they die suddenly. Owners may note that GBED foals are less active than other foals. In spite of aggressive treatment, all known cases of GBED have been euthanized or died by 18 weeks of age.

Until recently, GBED was not recognized in horses because the wide variety of clinical signs resembles many other foal diseases.

The signs can be:
Abortion or still birth of a foal.
Weakness and low body temperature at birth. Treatment with a bottle, tubing the foal with milk, and assistance to stand and suckle regularly helps the foal become stronger.
Sudden death on pasture of foals from the heart stopping or from seizures (due to low blood sugar).
High respiratory rate and weakness of the muscles used to breathe in foals.
Contracted tendons found in all four legs of a foal.
Overall weakness and the inability of the foal to get up from lying on its side.

Research done at the University of Minnesota suggests that this mutation may be present in about 10% of all Quarter Horses and related bloodlines. This means breeding carriers would result in a 25% chance of getting a foal affected with GBED. All GBED foals verified to date have been Quarter Horses and American Paint horses.

The University of Minnesota College of Veterinary Medicine has licensed the University of California, Davis, Veterinary Genetics Laboratory to perform GBED testing.

The American Quarter Horse Association funded research at the University of Minnesota to investigate the possibility that this disease existed in Quarter Horse foals. Researchers were suspicious that this disease existed after examination of muscle biopsies from affected foals. Normal muscle glycogen stains a rich pink color using a special stain called PAS. When we examined biopsies from GBED foals, we saw that there was no background pink staining. Instead, researchers saw big clumps of purple staining indicating abnormal glycogen.

The appearance of these samples was similar to the human version of GBED. Further research involved study of the glycogen structure in these samples, and found it was not properly branched.

Researchers at the University of Minnesota measured activity of the GBE enzyme and other enzymes involved in glycogen metabolism in frozen muscle, heart, and liver samples from affected foals and foals that died from other causes. There was no activity of the GBE enzyme in the foals with the abnormal appearing muscle biopsies. Our researchers also discovered that the GBE protein was absent in the tissues, confirming the suspicion the foals died because of a new disease in horses called GBED.

GBED is not sex-linked, which means that both males and females are affected equally

For more information on GBED please visit the following websites.

University of Minnesota

University of California - Davis

Glycogen branching enzyme deficiency - Wikipedia