Panel of four equine hereditary diseases with molecular testing, applicable to breeding animals of stock breeds and to any horse with Quarter Horse ancestry. It brings together glycogen branching enzyme deficiency (GBED), hereditary equine regional dermal asthenia (HERDA), hyperkalaemic periodic paralysis (HYPP) and equine malignant hyperthermia (EMH). GBED and HERDA are recessive and lethal or severely disabling in homozygosity; HYPP and EMH are dominant with incomplete penetrance and may manifest as potentially fatal acute muscle episodes. Each test reports the clear, carrier or affected genotype and allows matings to be planned. The panel does not replace clinical examination or biopsy when the clinical picture does not fit.

Applicable to stock breeds and to horses with Quarter Horse ancestry. GBED, HERDA, HYPP and EMH are documented in those populations; there are no reliable population frequency figures for the species as a whole (limited data).

The signs depend on the condition:\n- GBED: abortions and stillbirths, weak foals with hypoglycaemia, tremors, seizures, contractures and neonatal death.\n- HERDA: hyperextensible and fragile skin, with tears and seromas on the back and withers when training begins (18-24 months).\n- HYPP: episodes of tremors, sudden weakness, prolapse of the nictitating membrane, sweating and recumbency with lucidity; triggered by potassium-rich diets, fasting, stress, transport or anaesthesia; they can be fatal.\n- EMH: rapid hyperthermia, tachycardia, rigidity, acidosis and rhabdomyolysis during anaesthesia or in the face of stress/intense exercise; risk of sudden death.

GBED was characterised in the early 2000s 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 mid-1980s in halter lines linked to the sire Impressive, was linked to the SCN4A gene in the early 1990s and its test was incorporated into breeding programmes. EMH was described in the horse in the early 2000s and in 2004 the RYR1 C7360G variant associated with the disease was identified; later studies confirmed the association in the Quarter Horse and its coexistence with PSSM1.

Genotype breeding animals before mating, especially in stock lines or with ancestry from the sire Impressive.\n- GBED and HERDA (recessive): do not mate carrier with carrier (25 % affected); carrier with clear produces 50 % carriers and no affected animals, and allows valuable lines to be preserved by selecting clear offspring.\n- HYPP and EMH (dominant): every carrier transmits the allele to 50 % of the offspring; breed preferably with N/N and avoid H/N x H/N because of the 25 % severely affected homozygotes. Deliberate breeding of HYPP H/H foals is incompatible with welfare.\n- Do not breed with affected animals; HERDA and GBED have no treatment.\n- Communicate the status to the veterinarian before anaesthetising, castrating or transporting (HYPP and EMH) and to the buyer.

Equine rhabdomyolysis has multiple causes: PSSM1, PSSM2, atypical myopathy due to hypoglycin A, HYPP, infectious or traumatic causes and nutritional deficiencies. In the face of an anaesthetic reaction, EMH must be considered, but also heat stroke, sepsis and anaphylaxis. In HYPP or EMH animals, plan anaesthetic protocols with non-triggering agents and dantrolene available. A negative test does not rule out myopathy of another origin. HERDA skin can be confused with photosensitisation or deep pyodermas; biopsy shows disorganisation of the dermal collagen.

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