Glycogen is the chemical name for the complex sugar that our cells make from simple sugar to store for future use. As the glycogen is formed, the glucose strands are branched. As the glycogen is utilized, these branches are broken off as a first step to making energy. Without the branches, only the ends of the long chain can be used and utilization is greatly slowed down.

Dr. Stephanie Valberg and a group of researchers at the University of Minnesota recently found a fatal genetic condition in a small group of related Quarter Horse foals. The condition resulted in abnormal unbranched glycogen. This genetic disease has been reported in humans and cats but not in horses before. Early analysis suggests this is an autosomal recessive disease, which means that for disease to occur the foal must have two copies of this gene on his chromosomes, one obtained from the mare and one from the stallion. However, some siblings and parents that presumably have just one gene and no obvious clinical signs, show some abnormal laboratory values.

The so far identified affected foals have at least 2,600 half-siblings. Consequently, GBE deficiency may be a common cause of neonatal mortality in Quarter Horses that is obscured by the variety of clinical signs that resemble other equine neonatal diseases. This article discusses the clinical and laboratory signs associated with this disease.

Researching the QH horse lineages Dr Valberg has determined the earliest originating horse was Zantanon and was spread into the population by his son the great QH sire King. It is estimated that 8% of King's descendents have the gene for GBED.