Glycogen branching enzyme deficiency (GBED) is a lethal glycogenosis of the Quarter Horse and Paint, with limited data in the Appaloosa. The absence of the enzyme prevents the formation of normal branched glycogen and causes accumulation of abnormal polysaccharide in muscle, liver and heart. Affected foals are born weak, suffer hypoglycaemia and neuromuscular deterioration, or die as abortions or stillbirths. There is no treatment and control consists of avoiding matings between carriers.

Affected breeds: Quarter Horse and Paint. The previous record included the Appaloosa without sufficient support (limited data). The allele frequency described in a random Quarter Horse population is around 5% (Tryon 2009) and close to 11% in reining lines (Brown 2026); there are no robust figures for Paint or outside North America.

- Abortions in the last third of gestation and stillbirths\n- Weak foals of low vitality, unable to stand\n- Severe recurrent hypoglycaemia\n- Tremors and seizures\n- Flexural limb deformities (contractures)\n- Poor growth and failure to develop\n- Death or euthanasia in the first weeks of life

The disease was described in the early 2000s in Quarter Horse foals, when Valberg and colleagues characterised it as a glycogenosis due to branching enzyme deficiency. Shortly afterwards, the same team identified the responsible mutation in the GBE1 gene, which enabled the development of a carrier test. Systematic screening in North America has markedly reduced the incidence of clinical cases. Together with HERDA, PSSM1 and HYPP, it is one of the genetic diseases requiring mandatory management in stock breeds.

- Test all Quarter Horse, Paint and Appaloosa breeding animals before the breeding season\n- Never mate carrier with carrier: 25% risk of affected foals\n- The birth of an affected foal confirms that both parents are carriers: do not repeat that mating\n- Carriers can be mated to clear animals, keeping only clear offspring for breeding if you aim to eliminate the allele\n- Record the results with the breed association if a control programme exists

Differentiate from other neonatal foal conditions: neonatal maladjustment syndrome ('dummy foal'), sepsis, prematurity and hypoglycaemia due to inadequate lactation. Confirmation is achieved by measuring branching enzyme activity in tissues and observing abnormal polysaccharide deposits (PAS-positive, diastase-resistant) in biopsies. DNA testing on blood or hair is decisive for diagnosis and breeding advice.

1. Valberg SJ et al. 2001. Glycogen branching enzyme deficiency in quarter horse foals. J Vet Intern Med. PMID: 11817063
2. Ward TL et al. 2004. Glycogen branching enzyme (GBE1) mutation causing equine glycogen storage disease IV. Mamm Genome. PMID: 15366377
3. Ward TL et al. 2003. Genetic mapping of GBE1 and its association with glycogen storage disease IV in American Quarter horses. Cytogenet Genome Res. PMID: 14970703
4. Tryon RC et al. 2009. Evaluation of allele frequencies of inherited disease genes in subgroups of American Quarter Horses. J Am Vet Med Assoc. PMID: 19119976
5. Brown BN et al. 2026. Allele frequencies of 7 inherited disorders in performance and random cohorts of American Quarter Horses. J Am Vet Med Assoc. PMID: 42633773
6. OMIA:000420-9796.