Hyperkalemic Periodic Paralysis (HYPP) and Breeding

Q. I have a 6-year-old Quarter Horse mare N/H (heterozygous) for hyperkalemic periodic paralysis (HYPP), and a lot of vets have told me that I can breed her to a Thoroughbred, and the baby should be fine. She does have some problems with the warm weather. I would love to get her bred, but I do not want to have her so stressed out that it would kill her. Would she be OK if I get her bred this year? What is your opinion on this whole HYPP breeding?


A. Because the disease is inherited as a dominant trait, one-half of your mare's offspring will be affected, regardless of who you breed her to. In the early 1990s, we asked this question of whether the horse needed the Quarter Horse phenotype or "build" to express the disease. At the University of California, Davis, we bred an affected Quarter Horse stallion (N/H, as is your mare) to Thoroughbred, Standardbred, and Arabian mares. What we found from 21 offspring is that the disease occurred in roughly one-half of the offspring, regardless of breed or phenotype, proving the autosomal dominant mode of inheritance. This is how the disease can spread easily to other breeds of horses (as it already has in Appaloosa, Paint, Pony of the Americas, and other color breeds).

As there is a genetic test for this disease, the frequency of disease can be reduced if breeders choose not to breed affected horses. Enjoy your mare, but for the sake of the horse, do not breed your mare and perpetuate an avoidable problem.

About the Author

Sharon Spier, DVM, PhD, Dipl. ACVIM

Sharon Spier, DVM, PhD, Dipl. ACVIM, is a professor of the Department of Medicine and Epidemiology at the University of California, Davis. Her areas of interest include Corynebacterium pseudotuberculosis infection (also known as pigeon fever or dryland distemper), hyperkalemic periodic paralysis, and genetic diseases of horses.

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