While genetic diseases exist in a number of horse breeds, many researchers have focused expressly on issues occurring in the Quarter Horse breed. Thanks to their efforts, genetic tests for several disorders specific to this breed are now commercially available for breeders and horse owners.

"The size of the Quarter Horse industry, the commitment of the American Quarter Horse Association, and the development of the equine genome have all contributed to the identification of genetic diseases affecting Quarter Horses," said Stephanie Valberg, DVM, PhD, professor of large animal medicine and director of the University of Minnesota's Equine Center, who presented on the topic at the 11th Congress of the World Equine Veterinary Association in Guarujá, Sao Paulo, Brazil.

The most well-known genetic diseases affecting Quarter Horse and related bloodlines include the following:

Hyperkalemic Periodic Paralysis (HYPP) This genetic mutation affects 1.5% of the Quarter Horse breed andalmost 60% of halter horses that descend from Impressive, a prominent sire. Signs begin around 2-3 years of age and include intermittent muscle tremors and weakness. Severely affected horses can present with difficulty swallowing and respiratory distress. HYPP is caused by a dominant mutation that results in a single amino acid change in a sodium channel in skeletal muscle. Testing is available at the University of California, Davis.

Glycogen Branching Enzyme Deficiency (GBED) This is a mutation approximately 8% of Quarter Horses carry, particularly Western pleasure horses descended from Zantanon and King. Aa recessive mutation, causes GBED , and affected fetuses generally are aborted or stillborn. If they're born alive, foals are often weak, have contracted tendons, and typically die within 18 weeks. Genetic testing is available at the University of California, Davis, and VetGen.

Polysaccharide Storage Myopathy (PSSM) Type 1 PSSM, a form of exertional rhabdomyolysis (tying-up), affects about 10% of Quarter Horses. This dominant mutation results in enhanced function of a gene called glycogen synthase 1 (GYS1). Signs include firm, painful muscles, stiffness, weakness, gait abnormalities, and muscle wasting. PSSM is also present in at least 20 other breeds, including draft horses. Testing is available at the Neuromuscular Laboratory at the University of Minnesota.

Malignant Hyperthermia (MH) Affecting <1% of Quarter Horses, this dominant mutation results in a single amino acid mutation in the ryanodine receptor 1 (RYR1) gene. High fevers, metabolic failure, and death might occur while affected horses are under general anesthesia. Horses can also tie-up, which can be severe if the RYR1 mutation occurs in conjunction with the GYS1 mutation that's characteristic of PSSM. Testing is available at the Neuromuscular Laboratory at the University of Minnesota.

Hereditary Equine Regional Dermal Asthenia (HERDA or HC) This disorder is caused by a recessive mutation in a gene involved in processing collagen to anchor the skin to underlying tissues. About 4% of Quarter Horses are carriers (and 28% of cutting horses). Those with two copies of the mutated gene have skin that tears easily and heals with scars and white hairs after trauma. Testing is available at the University of California, Davis.

"Tests for genetic disease affecting other breeds are also available," noted Valberg. "These include tests for severe combined immunodeficiency (SCID) in Arabians and junctional epidermolysis bullosa in North American Belgians and Saddlebreds. These tests will assist owners when making breeding program decisions and during prepurchase examinations."

Valberg and colleagues also relayed that they anticipate a rapid expansion in the number of genetic tests available in the near future, thanks to the development of equine genetic tools that scan for genetic mutations.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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