What’s The Hype Over HYPP?
By Dr. Sharon Spier and The American Association of Equine Practitioners
“HYPP” has been the buzzword around certain show circuit circles since 1992. It has made some horsemen turn white, some others turn red and still others shrug their shoulders. What is the big deal about this disease?
The American Association of Equine Practitioners and Dr. Sharon Spier from the
What is the cause?
Hyperkalemic periodic paralysis (HYPP) is an inherited disease of the muscle, which is caused by a genetic defect. In the muscle of affected horses, a point mutation exists in the sodium channel gene and is passed on to offspring.
Sodium channels are “pores” in the muscle cell membrane which control contraction of the muscle fibers. When the defective sodium channel gene is present, the channel becomes “leaky” and makes the muscle overly excitable and contract involuntarily. The channel become “leaky” when potassium levels fluctuate in the blood. This may occur with fasting followed by consumption of a high potassium feed such as alfalfa. Hyperkalemia, which is an excessive amount of potassium in the blood, causes the muscles in the horse to contract more readily than normal. This makes the horse susceptible to sporadic episodes of muscle tremors or paralysis.
This genetic defect has been identified in offspring of the American Quarter Horse sire, Impressive. To date, confirmed cases of HYPP have been restricted to descendants of this horse.
How is it spread?
HYPP is an autosomal dominant genetic trait, which means only one copy of the gene is required to produce the disease, and that the disease can equally occur in both sexes (mare, gelding and stallion).
To clarify, suppose a mare has tested “HYPP H/N.” This means she is heterozygous, or carries one copy of the HYPP gene. Breeding her to a normal sire, or HYPP N/N, will result in a 50 percent chance the offspring will carry the HYPP gene (HYPP H/N) and a 50 percent chance of the foal being normal. Breeding her to a heterozygous sire, or HYPP H/N, will cause only a 25 percent chance of the offspring being normal (HYPP N/N), a 50 percent chance of the offspring to carry the gene (HYPP H/N) and 25 percent chance of the offspring to be homozygous (HYPP H/H, or carry both copies of the HYPP gene).
A carrier of the defect (HYPP H/N) is affected with HYPP. These horses can show clinical signs of the disease and can pass the gene on to their offspring. Though horses, which are homozygous, show more clinical symptoms, heterozygotes also obtain the disease because it is a dominant trait.
What does this mean for you if you are considering breeding your horse?
Is there any GOOD NEWS about horses carrying “Impressive” blood?
YES THERE IS!
ANY horse, that has been DNA tested and found to be N/N... or in other words, NOT carrying any copies of the HYPP gene, ABSOLUTELY CANNOT pass HYPP on to your foal, EVEN IF that horse is carrying “Impressive” blood.
IT IS AS SIMPLE AS...YOU CANNOT GIVE WHAT YOU DO NOT HAVE!
YOU do NOT need to FEAR or AVOID breeding to, or owning horses, who are related to Impressive—IF they have been tested for HYPP by the AQHA, or the UNIVERSITY OF DAVIS, and found to be negative for the gene, which is listed as “Results N/N."
If they have been tested, a seal or a written statement should be affixed to, or printed on, their Registration Papers stating their status, be it N/N, N/H, or H/H, AND the AQHA will have these results ON RECORD. Do not be FOOLED by claims that cannot be BACKED UP! Verify!
We at White Hawk Quarter Horses have ALL our breeding stock that carry Impressive blood TESTED and we will NOT breed a horse we own if it is found to be carrying HYPP. (Which is why we do not own anything, but horses who have tested N/N.) We will also be VERY RELUCTANT to breed a mare brought to us who carrys HYPP. There would have to be a VERY GOOD reason for it. Money is not the object when the welfare of the breed is at stake.
Impressive was a GREAT horse and the embodiment of what Quarter Horse conformation should be. His genes have far more good to offer than bad. If only people would test, and STOP breeding horses that are not N/N, his legacy could go on untarnished.
However, this is not the case, and so the fears of the public in general have only grown, along with FALSE rumors and misinformation.
As a result, very sadly, many fear any horse with Impressive in their pedigree. There is no need for this if a person is correctly informed, and they only buy or breed to tested stock, or buy foals that have been born to two tested horses who were both found to be N/N.
For those of us who do test, and will only breed N/N horses, there is nothing to fear and only good to be found in horses carrying Impressive’s blood.
Impressive breeding is NOT a bad thing!
Please see this great sire’s picture (which does not do him justice) and record in our Reference Sires and Dams page. Let’s get back to remembering, the reasons why so many American Quarter Horses DO carry the blood of this GREAT stallion, and stop making his name a bad word in the horse industry, when it rightly used to mean the most EXCELLENT of breeding.