Panel of six inherited equine diseases with molecular testing, intended for Paint Horse and stock breeds with overo coat. It brings together equine malignant hyperthermia (EMH), glycogen branching enzyme deficiency (GBED), hereditary equine regional dermal asthenia (HERDA), hyperkalemic periodic paralysis (HYPP), lethal white foal syndrome (OLWS) and type 1 polysaccharide storage myopathy (PSSM1). GBED, HERDA and OLWS are recessive; OLWS is lethal in homozygosity and is linked to the frame overo coat. EMH, HYPP and PSSM1 are dominant with incomplete penetrance. Each test reports clear, carrier or affected genotype and allows matings to be planned, in particular avoiding two OLWS carriers. The panel does not replace clinical examination.

Applicable breed: Paint Horse. Panel of 6 inherited equine diseases.

Signs depend on the condition:\n- EMH: rapid hyperthermia, tachycardia, rigidity, acidosis and rhabdomyolysis during anaesthesia or in the face of stress/intense exercise.\n- GBED: abortions and stillbirths, weak foals with hypoglycaemia, tremors, seizures, contractures and neonatal death.\n- HERDA: hyperextensible, fragile skin with tears and seromas on the back and withers when training begins.\n- HYPP: episodes of tremors, sudden weakness, prolapse of the third eyelid, sweating and recumbency, triggered by dietary potassium, fasting, stress, transport or anaesthesia.\n- OLWS: white or nearly white foal with absence of meconium, obstructive colic and abdominal distension in the first 24-48 h; fatal prognosis.\n- PSSM1: muscle rigidity and pain (tying-up), dark urine and elevated CK/AST, especially after rest and resumption of exercise.

The six tests were developed independently. EMH was described in the horse in the early 2000s and the RYR1 C7360G variant was identified in 2004. GBED was characterised in Quarter Horse foals and its mutation in GBE1 was identified in 2004. HERDA was recognised in cutting lines and its mutation in PPIB was described in 2007. HYPP was recognised in the 1980s in halter lines linked to the stallion Impressive. PSSM1 was defined in the 1990s and its mutation in GYS1 (R309H) was identified in 2008. OLWS was described in white foals with obstructive colic and the mutation in EDNRB was identified in 1998.

Genotype breeding animals, especially any animal with an overo coat or overo ancestry, before mating.\n- Recessive (GBED, HERDA, OLWS): do not mate carrier with carrier. In OLWS mating two carriers means 25 % lethal white foals in each birth. Carrier with clear produces 50 % carriers and no affected animals.\n- Dominant (EMH, HYPP, PSSM1): every carrier transmits the allele to 50 % of the offspring; breed preferably with N/N and avoid carrier x carrier because of the 25 % more severe homozygotes.\n- Do not assume status from the coat: tobiano or sabino patterns do not depend on the OLWS allele; only the test clarifies it.\n- Communicate the status to the veterinarian before anaesthetising, castrating or transporting and to the buyer.

In a white foal with early obstructive colic, the differential diagnosis includes atresia of the colon or ileum, meconium retention, peritonitis and bladder rupture; in a foal from overo parents it is OLWS until proven otherwise, and the prognosis is nil. Frame overo expression may be minimal in carriers (cryptic frame), so the coat does not replace the test. Rhabdomyolysis has multiple causes (PSSM1, PSSM2, atypical myopathy due to hypoglycin A, HYPP, infectious or traumatic) and a negative test does not rule it out. HERDA skin fragility may be confused with photosensitisation or deep pyodermas.

1. Aleman M, et al. Association of a mutation in the ryanodine receptor 1 gene with equine malignant hyperthermia. Muscle Nerve. 2004. PMID: 15318347.
2. Aleman M, et al. Malignant hyperthermia associated with ryanodine receptor 1 (C7360G) mutation in Quarter Horses. J Vet Intern Med. 2009. PMID: 19220734.
3. Valberg SJ, et al. Glycogen branching enzyme deficiency in quarter horse foals. J Vet Intern Med. 2001. PMID: 11817063.
4. Ward TL, et al. Glycogen branching enzyme (GBE1) mutation causing equine glycogen storage disease IV. Mamm Genome. 2004. PMID: 15366377.
5. White SD, et al. Hereditary equine regional dermal asthenia ("hyperelastosis cutis") in 50 horses. Vet Dermatol. 2004. PMID: 15305927.
6. Tryon RC, et al. Homozygosity mapping approach identifies a missense mutation in equine cyclophilin B (PPIB) associated with HERDA. Genomics. 2007. PMID: 17498917.
7. Rudolph JA, et al. Periodic paralysis in quarter horses: a sodium channel mutation disseminated by selective breeding. Nat Genet. 1992. PMID: 1338908.
8. Naylor JM. Hyperkalemic periodic paralysis. Vet Clin North Am Equine Pract. 1997. PMID: 9106348.
9. Santschi EM, et al. Endothelin receptor B polymorphism associated with lethal white foal syndrome in horses. Mamm Genome. 1998. PMID: 9530628.
10. Metallinos DL, et al. A missense mutation in the endothelin-B receptor gene is associated with Lethal White Foal Syndrome. Mamm Genome. 1998. PMID: 9585428.
11. Santschi EM, et al. Incidence of the endothelin receptor B mutation that causes lethal white foal syndrome in white-patterned horses. Am J Vet Res. 2001. PMID: 11197568.
12. McCue ME, Valberg SJ, Miller MB, et al. Glycogen synthase (GYS1) mutation causes a novel skeletal muscle glycogenosis. Genomics. 2008. PMID: 18358695.
13. McCue ME, Valberg SJ, Lucio M, Mickelson JR. Glycogen synthase 1 (GYS1) mutation in diverse breeds with polysaccharide storage myopathy. J Vet Intern Med. 2008. PMID: 18691366.