Glycogen-branching enzyme disorder (GBED) has likely been a cause of neonatal mortality in Quarter Horses for decades, according to Stephanie Valberg, DVM, PhD, who gave an update on her research on the disorder at the recent conference of the American Quarter Horse Association, held March 11-14 in St. Louis, Mo. Additionally, she reported that all the known affected foals and carriers of GBED are descendants of the Quarter Horse sire Zantanon and his son King, and about 8% of King descendants are carriers of GBED.

Valberg and Jim Mickelson, PhD, associate professor veterinary pathobiology at the University of Minnesota College of Veterinary Medicine, identified GBED and described it to the public in 2004. The disease causes a wide variety of clinical signs, so it might be confused with other conditions. It can cause abortion or stillbirth.

Alternatively, foals can be weak at birth and need assistance to suck and stand. If helped through the first few days, foals might survive for up to two months. All foals diagnosed with GBED have died or had to be euthanatized because of muscular weakness by 18 weeks of age.

Valberg indicated in her presentation that GBED-affected foals have a gene mutation that causes the animals’ cells to stop reading the GBE gene. This results in an inability to produce the GB enzyme necessary to store sugar as glycogen in the body. GBED is a recessive genetic disorder, thus parents show no clinical signs of disease and are only carriers of the mutant gene for the GBED trait. For a foal to be affected with GBED, it needs to receive a mutant gene from both parents. If a carrier stallion is bred to a carrier mare, there is a 25% chance of the foal being affected, and a 50% chance the foal will be a carrier.

The disease is not sex-linked, so colts and fillies are equally affected. To date, GBED has only been recorded in Quarter Horses and American Paint horses.

Valberg and Mickelson’s research determined the GBED link to Zantanon and his son King. It is important to realize that although 92% of descendants of King do not carry the GBED trait, a DNA test is necessary to identify the 8% of horses that are carriers and can produce an affected foal. 

The University of Minnesota has a patent pending on a GBED genetic test that is able to determine if a horse is a carrier of the mutation responsible for the disease. The University of California, Davis, is licensed to perform testing and is currently accepting hair samples for this purpose. Forms and information on testing are available at Valberg recommends horses be tested if they have previously lost foals to abortion or neonatal death for unknown reasons. The only way to know if horses that trace back to King are carriers of GBED is to send hair samples in for testing.