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What is Glycogen-Branching Enzyme Deficiency?

Glycogen-branching enzyme deficiency (GBED) is a disease that inhibits a horse’s body from properly storing sugar. Sugar (glucose) is normally stored within the body and is then converted to glycogen. In American Quarter Horses, as well as American Paint Horses, this deficiency is inherited and is a condition that is fatal. Affected horses generally die in the womb, are stillborn, or succumb to the deficiency soon after birth. Unfortunately, there is no treatment for this condition.

The protein that is needed to increase glycogen in the system is known as glycogen-branching enzyme. Glycogen is glucose, which is sugar, arranged in a complex tree-like arrangement. The glycogen-branching enzyme is the protein that forms the branches. When foals are missing GBE, the glycogen within the tissue has an abnormal, non-branching structure. This causes glucose, or sugar, to not be stored properly. Therefore, horses have no “fuel” for any energy within the heart and other organs.

Glycogen-branching enzyme deficiency in horses is the inability of horses’ bodies to effectively use and store glucose. It is a fatal condition, and many foals die in utero or shortly after birth.

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Symptoms of Glycogen-Branching Enzyme Deficiency in Horses

Foals that are born with GBED have a wide-range of symptoms associated with the lack of proper glucose storage. Symptoms include:

  • Low energy
  • Weakness
  • Difficulty standing up
  • Muscle contractions
  • Seizures
  • Decreased body temperature
  • Sudden death

Types

For determining the condition if your horse has the symptoms, and for having the data for future research, your horse may be genetically tested to see if he is affected, a carrier of the gene, or negative. Types of results will be:

  • Carrying two copies of the mutated gene, homozygous, affected with the disorder
  • A detection of the mutant copies and normal copies of the gene, 50 % chance the horse can pass a copy of the gene to its young, carrier of the disorder
  • Gene mutation is negative, will not pass on the gene to any offspring, negative carrier of disorder

Causes of Glycogen-Branching Enzyme Deficiency in Horses

One thing is for certain, GBED is a genetic, inheritable condition. Causes of GBED may include:

  • There is a responsible mutation for the gene
  • Carried by 10% of all Quarter Horse Breeds, Paint Horse Breeds, related breeds
  • Is specifically an autosomal recessive trait
  • GBED prevents the horse’s body from producing the proper enzyme for sugar storage

Diagnosis of Glycogen-Branching Enzyme Deficiency in Horses

If your horse is showing any of the above symptoms, make an appointment with your veterinarian. He will observe the foal, and determine the health status and begin testing for the condition. Your veterinarian will perform several laboratory tests, including the basic baseline testing of bloodwork, urinalysis, and biochemistry profile. 

A PCR test may be conducted, along with several biopsies of the muscles from your foal. There is a staining pattern that is characteristic of glycogen-branching enzyme deficiency. Studies and researchers have compared normal muscle to GBED muscles to see the significant difference. 

Your veterinarian may also recommend genetic testing, depending on your location and whether this is feasible. There is a specific genetic test for GBED, as this condition has been intensely studied over time. There are only certain laboratories in the US that are licensed to test for this disorder, and your veterinarian will communicate this with you and give you more information.

Treatment of Glycogen-Branching Enzyme Deficiency in Horses

There is not a treatment method for foals diagnosed with glycogen-branching enzyme deficiency. The foal will succumb to the illness very early in life. There has been much research conducted on this disorder. It is a genetic disorder and there is genetic testing for it. Much of the research has been conducted at various universities, such as the University of Minnesota. This disorder is fatal, and the foal will typically succumb within 8 weeks of age, if he is not aborted during the pregnancy.

Recovery of Glycogen-Branching Enzyme Deficiency in Horses

Unfortunately, this disorder is fatal. If the foal is not born stillborn or if it does not die in utero, once it is born it will face a very early death. Research is still being conducted on this disorder.

It is important to understand the genetics of this condition. If your horse is affected or is a carrier, it is imperative to not use the horse for breeding, as it will be passed to the foal. If you have any questions about the genetics and research of this condition, be sure to ask your veterinarian and he will give you the information you need so you can further understand this disorder.