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Glycogen Storage Disease IV (GSD IV Horse)

Glycogen Storage Disease IV (GSD IV) causes energy deficiency in muscles and organs due to a disorder in glycogen metabolism. The disease occurs in the Quarter Horse and related breeds.

Synonym: Glycogen Branching Enzyme Deficiency (GBED)


  • Weakness
  • Low body temperature
  • Accelerated breathing
  • Tremors
  • Spontaneous abortions

General Information

  • In glycogen branching enzyme deficiency (GBED), glucose cannot be stored in the form of glycogen.
  • This leads to a lack of energy for muscles and organs.
  • Foals may die shortly after birth (within the first 8 weeks) due to organ failure (liver or heart).

Breeds affected

Quarter Horse and related breeds like Appaloosa or Paint Horse

Test Information

This test detects a single basepair change in exon 1 of the GBE1 gene.

Genotype and Lab Report

Inheritance: autosomal recessive

→ Only horses with two copies of the variant (gbed/gbed) are affected. Horses with only one copy (N/gbed) are clinically healthy carriers.




N/N = normal

The horse has no copies of the genetic variant causitive for GBED/GSD IV and therefore cannot pass it on to its offspring.


N/gbed= a carrier

The horse is clinically healthy.  It has one copy of the genetic variant causitive for GBED/GSD IV, which will be passed on to its offspring with a probability of 50%.


gbed/gbed= affected

The horse is affected and will not live long enough to reproduce.


  • Carriers may be bred to normal animals (N/gbed x N/N) without any risk of producing affected offspring. The offspring should also be tested before breeding to determine if they are carriers or normal.
  • Breeding two carriers (N/gbed x N/gbed) is not recommended due to the possibility of 25% of the offspring being affected.


Ward, TL., Valberg, SJ., Adelson, DL., Abbey, CA., Binns, MM., Mickelson, JR.: Glycogen branching enzyme (GBE1) mutation causing equine glycogen storage disease IV. Mamm Genome 15:570-7, 2004. Pubmed reference: 15366377.


Further information is available at Online Mendelian Inheritance in Animals.

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