Stephanie Valberg, DVM, PhD, professor of large animal medicine and director of the University of Minnesota’s Equine Center, presented information on GBED at the AAEP Convention 2006. The disorder involves glycogen storage in Quarter Horse or Paint horse foals, and it is associated with abortion or high neonatal mortality.

Glycogen-branching enzyme deficiency, a genetic mutation affecting a particular glycogen-storage enzyme, is traced back to Quarter Horse sire King or his sire, Zantanon. Up to 8% of Quarter Horses and Paint horses carry the GBED defect. Pedigree analysis is not particularly helpful in identifying if a horse has this gene because both of these horses are foundation stallions for the breed.

Researchers believe GBED has caused significant fetal and neonatal mortality in Quarter Horses and Paints for decades, although sophisticated histopathology of muscle tissue biopsy is required to identify it. Currently, the recommendation for diagnosis is genetic testing through the Veterinary Genetics Lab at UC Davis ( using mane or tail hairs with intact roots or samples of fetal liver tissue.

Many affected foals are aborted in late term or are stillborn. If the mare delivers a live GBED-affected neonatal foal, it is born weak with a low body temperature, is slow to stand to nurse, and might have slightly contracted tendons. With good clinical care, a GBED foal might respond for a short while and seem to improve, then deteriorate. Others might initially experience respiratory distress, collapse from hypoglycemia, seizures, then sudden death. GBED-affected foals have poor survivability, with most succumbing by eight weeks of age. One foal survived until 18 weeks old with aggressive nursing care.

There is no treatment for GBED foals. The best prevention is through early recognition that aims to limit an owner’s heartache and financial investment in attempts to save a dying foal. It is important to know that rebreeding a mare to the same stallion will give an owner a 25% chance of repeating the same fatal outcome.

Four percent of Quarter Horses have GBED and do not survive, and 8% are carriers. In Paint horses, 7% are carriers. No GBED defect has been found in Thoroughbreds.

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse’s AAEP 2006 Wrap-Up. Files are available as free PDF downloads.