Screening panel for four genetic diseases requiring management in stock breeds (Quarter Horse, Paint and related breeds). It brings together glycogen branching enzyme deficiency (GBED), a lethal glycogenosis of the foal; hereditary equine regional dermal asthenia (HERDA), a genodermatosis caused by extreme skin fragility; hyperkalaemic periodic paralysis (HYPP), a sodium channel myopathy linked to the sire line Impressive; and polysaccharide storage myopathy type 1 (PSSM1), a muscle glycogenosis manageable with diet and exercise. It allows carriers to be identified and matings to be planned.

Applicable breeds: Quarter Horse and Paint (all four diseases); Appaloosa through Quarter Horse ancestry, with more limited data. In the Quarter Horse, the allele frequency of GBED is around 5% in a random population (Tryon 2009) and close to 11% in reining lines (Brown 2026); that of HERDA is 0.021 in controls and 0.142 in the cutting subgroup (Tryon 2009); HYPP is linked to descendants of Impressive, with much higher frequencies in halter; PSSM1 has been documented in Quarter Horse, Paint, Appaloosa and draught and cold-blooded breeds.

- GBED: abortions in the last trimester and stillbirths; weak foals that cannot stand, severe hypoglycaemia, tremors, seizures and flexural deformities; death or euthanasia in the first weeks\n- HERDA: hyperextensible and fragile skin from the start of training (18-24 months); tears, seromas and haematomas on the back and withers with minimal trauma; poor healing and secondary infections\n- HYPP: episodes of tremors and sudden weakness, intermittent prolapse of the third eyelid, sweating and decubitus with lucidity; death from respiratory or cardiac failure in severe episodes\n- PSSM1: stiffness and muscle pain (tying-up) after rest and resumption of exercise, dark urine, elevated CK and AST, and poor performance without an overt crisis

The four tests were developed independently. GBED was described in the early 2000s in Quarter Horse foals (Valberg 2001) and its mutation in GBE1 was identified shortly afterwards (Ward 2003-2004). HERDA was recognised in cutting lines of the American Quarter Horse and its mutation in PPIB was identified in 2007 (Tryon). HYPP was recognised in the mid-1980s in halter horses and traces back to the sire Impressive; the SCN4A mutation was characterised by linkage in the early 1990s. PSSM1 was clinically characterised in the 1990s (Valberg 1992) and its molecular basis (GYS1 R309H) was identified in 2008 (McCue). Systematic screening in North America has markedly reduced the incidence of clinical cases.

- Test all breeding stock of Quarter Horse, Paint and Appaloosa before the breeding season; the panel covers the four diseases in a single sample\n- GBED and HERDA (recessive): never mate carrier with carrier (25% affected homozygotes); carrier × clear with testing of the offspring\n- HYPP and PSSM1 (dominant): every carrier transmits the allele to 50% of the offspring; the decision to breed a positive animal must be justified and communicated, and never mate two positives (25% more severely affected homozygotes)\n- The birth of an affected foal confirms that both parents are carriers: do not repeat that mating\n- Always inform the veterinarian of the status before anaesthetising, castrating or transporting the animal, especially in HYPP\n- Record the results with the breeders' association if a control programme exists

Differential diagnosis of the neonatal foal: neonatal maladjustment syndrome ('dummy foal'), sepsis, prematurity and hypoglycaemia due to poor lactation (GBED), and of skin conditions such as epidermolysis imperfecta, photosensitisation and deep pyodermas (HERDA). In the acute HYPP episode, heat stroke, PSSM-related rhabdomyolysis, atypical pasture myopathy, tetanus and colic must be ruled out; between episodes serum potassium is usually normal. In PSSM1, biopsy shows diastase-resistant PAS-positive deposits, but a normal GYS1 does not rule out PSSM2. Confirmation of GBED and HERDA requires clinical, histological and DNA test correlation.

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 (2020-2024). J Am Vet Med Assoc. PMID: 42633773
6. White SD et al. 2004. Hereditary equine regional dermal asthenia ("hyperelastosis cutis") in 50 horses. Vet Dermatol. PMID: 15305927
7. Tryon RC et al. 2005. Inheritance of hereditary equine regional dermal asthenia in Quarter Horses. Am J Vet Res. PMID: 15822588
8. Tryon RC et al. 2007. Homozygosity mapping approach identifies a missense mutation in equine cyclophilin B (PPIB) associated with HERDA in the American Quarter Horse. Genomics. PMID: 17498917
9. Rudolph JA et al. 1992. Periodic paralysis in quarter horses: a sodium channel mutation disseminated by selective breeding. Nat Genet. PMID: 1338908
10. Rudolph JA et al. 1992. Linkage of hyperkalaemic periodic paralysis in quarter horses to the horse adult skeletal muscle sodium channel gene. Anim Genet. PMID: 1323940
11. Naylor JM. 1997. Hyperkalemic periodic paralysis. Vet Clin North Am Equine Pract. PMID: 9106348
12. Valberg SJ et al. 1992. Polysaccharide storage myopathy associated with recurrent exertional rhabdomyolysis in horses. Neuromuscul Disord. PMID: 1284408
13. Valberg SJ et al. 1996. Familial basis of exertional rhabdomyolysis in quarter horse-related breeds. Am J Vet Res. PMID: 8669756
14. McCue ME et al. 2008. Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis. Genomics. PMID: 18358695
15. McCue ME et al. 2008. Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy. J Vet Intern Med. PMID: 18691366
16. Herszberg B et al. 2009. A GYS1 gene mutation is highly associated with polysaccharide storage myopathy in Cob Normand draught horses. Anim Genet. PMID: 18822097
OMIA:000420-9796; OMIA:000327-9796; OMIA:000785-9796; OMIA:001158-9796.