HYPP: To Be Or Not To Be?

HYPP, or as properly addressed Hyperkalemic Periodic Paralysis, has become a topic in the stock horse industry that has virtually divided it’s membership. In 2004, The American Quarter Horse Association, in a well documented and debated decision, passed a bill to discontinue the registration of homozygous (H/H) foals born in 2007 and beyond. Additionally passed would be the non registration of heterogygous (N/H) foals, 20 years hence, 2027.

Both sides of the issue have fought and defended their positions with great fervor. Yet upon researching this article, I discovered the majority of people I interviewed had only general knowledge of HYPP, and a much lesser degree in regard to specific characteristics and idiosyncrasies significant to the genetic disorder. So, let’s attempt to distinguish between the facts and myths of HYPP.

To begin, HYPP is a genetic flaw. It has been identified to have originated from the stallion Impressive, as a mutated gene. He did not inherit this from either parent. Impressive had 3 full siblings and seven half siblings out of his dam. There is no evidence of the disorder in any of their history. Due to the fact that Impressive sired N/N foals, he could only have been N/H himself. HYPP also occurs in humans and swine, along with other species. A mutation of this nature can occur at any time, in any given individual. If this had generated itself in a stallion of lesser quality and demand, it may never have been brought to public attention, thereby wiping it out due to lack of interest in propagating the bloodline. Impressive was so sought after as a sire, that inbreeding and line breeding became the rule rather than the exception. Each generation of champions from these crosses, continued the breeding trend, shrinking and intensifying the genetic pool.

HYPP is a muscular disease that is by all means hereditary, but not a mutation of inbreeding. The genetic defect affects a protein called a sodium ion channel. The defect disrupts the channel’s ability to open and close, causing uncontrolled muscle twitching or intense weakness. Roaring respiratory noises often accompany a severe attack due to the tightening of muscles in the Larynx and Pharynx. Not all attacks are severe, and often pass with little or no notice. Some horses are non symptomatic their entire lives, with no special management at all. When one does occur, a blood test identifies high levels of potassium present. Therefore a low potassium diet is strongly recommended for positive horses. Alfalfa, originally thought to be a problem, is now accepted and has a lower potassium content than that of grass hay, in the volume needed to support a horse. Many major feed companies are doing research and offering special low potassium products to accommodate the HYPP horse. Traditionally oats have been the feed of choice. Another no no is red salt blocks, as it is extremely high in potassium levels, while white salt is highly recommended. Management has been the key to providing these horses with a normal and happy life. Diet, exercise, lots of available water and a daily stress free routine, with medication if necessary, can lead to a totally non symptomatic life.

Historically, N/H horses have been sought after for their excessive muscling by trainers and owners who dream of trophies and large shiny buckles. To achieve this, many sought to breed horses with at least one H in his DNA. H/H to H/H will always produce H/H. N/N to N/N will never produce a carrier. But any combination that includes at least one H, will have the ability of producing an N/H foal. N/H to H/H can produce H/H.

Interestingly enough, as much as has been achieved by N/H horses in the pen, H/H stallions have never dominated competition or the breeding shed. Breeders repeatedly spoke and wrote to me of their commitment to breed for a great horse, not a positive horse. Few professional breeders reported having any problems due to their proper management and regiment of affected horses.

New developments on the horizon include research into a potassium rich semen extender that would effectively incapacitate only sperm cells carrying the mutated gene. This lofty goal may not be realized in the near future as research grants may need to be privately funded. Dr. Sam Prien, Ph.D Associate Professor from Texas Tech University is spearheading the project along with Clayton Dehn of YL Ranch in Texas.

The produce of Impressive can now be seen in all disciplines of horses. Although his major impact has been on the Halter Horse industry, Impressive’s stamp of beauty, elegance size, muscling and athletic ability, have influenced everything from Pleasure Horses to Ropers.
It has been often warned that Impressive bred horses should never be ridden, as there is concern of having a seizure with a rider on. In fact, exercise is the major contributor to positive horses leading a normal useful life. At the first sign of an attack, one should aggressively walk or lunge the horse. Administering Karo syrup or the drug Acetozolamide, is also highly recommended, both as a daily supplement and during an attack. Acetazolamide increases potassium excretion from the kidney and also affects glucose metabolism, while exercise stimulates adrenalin, which helps replace potassium inside the cells.

According to Dr. Sara Caldwell. DVM, Ocala Florida, a graduate of Florida State University, College of Veterinary Medicine, “The non riding myth is propagated by the veterinary teaching schools, We are actually taught and directed in school to discourage clients from riding HYPP horses in order to protect ourselves, veterinarians, from possible legal action, should an accident occur. Not at all for reasons of veterinary health or management, it is strictly a legal issue.”

This discrepancy, to ride or not to ride, brought a flurry of e mails. Nearly a hundred exhibitors and pleasure riders contacted me in a one week period with loving stories of their fabulous mounts. From World Champions to Back Yard Heroes, these Americans are riding and showing positive horses. There appears to be no more incidence of injury or evidence of danger than any other horse. No one reported any attack while under saddle. Some over a period of as much as 16 years. HYPP attacks are often confused with riding horses tying up or exhibiting Equine Polysaccharide Storage Myopathy, which is prevalent in several bloodlines. Many contributors spoke of their concerns of not having a market to sell their positive horses, and what would be their fate. All communications were sentimental, upbeat, yet tinged with sadness. Their horses encompassed all age groups and disciplines, making it an excellent sampling.

It is with great interest that we will be able to follow the fate of HYPP horses in the future. It is with great hope and expectation that science will render a solution to those horses carrying the defect. It is with great pride these horses have helped developed the looks and athletic ability of today’s stock horse, whether AQHA, APHA or ApHC. It is with great promise that our registries, trainers and judges will give us unanimous direction. It is with great anticipation that we look to the marketplace to be an influence in developing healthy preferences. And finally, it is with great respect that we acknowledge an individual’s right to make educated and personal choices in regard to their own horses.

I wish to thank everyone who responded to my inquiries with such wonderful information and insight, and the following for their wisdom and contributions to this article.
Candace Jussen, owner of World Champion Mr GQ, N/H
Zoerado Mills, Owner of World Champion Noble Tradition, N/N
Penny Steward, Siesta Quarter Horses
Sara Caldwell, DVM, legal issues
Sharon Spier, DVM UC Davis published research

About the author: Karen Berk owns and operates Equine Reproduction Services (ERS) Stallion Station in Ocala Florida. Along with Bryan Huggins she stands 9 stallions and travels nationwide as a speaker and clinician. Her published articles have appeared in over 30 publications in 4 languages, worldwide. She can be contacted through www.frozen-semen.com.

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