Hyperkalemic Periodic Paralysis, commonly known as HYPP, has been widely spread throughout the equine industry over the last several years. This heritable disease is caused by a genetic defect that has been found through research to trace back to the great American Quarter Horse stallion, Impressive.
HYPP affects the sodium channels of afflicted horses and overloads the system with high potassium levels that cause episodes which may include: mild muscle twitching that is undetectable to the human eye; noticeable muscle twitching; "crawling" skin, ranging from slight to very noticeable and usually from the back flank area forward; hind quarter paralysis; excessive yawning; and paralysis of the muscles surrounding the heart and/or lungs, causing death due to heart attack or suffocation.
There are three testing statuses for HYPP:
H/H This status means that a horse carries a double copy of the defective gene and will pass at least one copy of the gene and the disease to 100% of it's offspring.
N/H This status means that the horse carries one normal gene and one HYPP gene and statistically an N/H horse will pass the gene and the disease to 50% of it's offspring when bred to a N/N or non-Impressive bred horse. N/H to N/H cross will statistically result in 25% N/N progeny, 50% N/H progeny, and 25% H/H progeny.
N/N This status means that the horse carries two normal genes. It does not have the disease, nor can it be passed on.
A dominant gene disease requires only ONE PARENT to have and pass on the gene AND the disease. It can be avoided by not breeding diseased animals. HYPP is a dominant gene disease. All N/H and H/H horses are carriers and all have the disease and any of them could have an attack at any moment.
A recessive-gene disease requires both parents to pass a copy of the gene for offspring to inherit the disease. These parents are carriers, but do NOT have the disease themselves. The disease can be easily avoided by testing and not breeding carriers to each other. HERDA and OLWS are recessive gene diseases.
Not all horses affected with HYPP will show outward signs and many will live full productive lives. BUT, there is no way of knowing at the time of conception or at foaling which horse will lead a productive life, which horse will have mild, intermittent attacks that are easily controlled, which horse will have severe, ongoing attacks and which horses will die. And they may die from the very first attack. How many horses have to suffer and die to make breeders and the registries realize that something needs to be done about N/H as well as H/H?
HYPP was identified in 1985, and a test was made available for public use in 1992. At that time breeders were encouraged by vets and vet schools to test their breeding stock and choose horses who did not carry HYPP to be used as breeding stock. New research shows that not only has the test not been used to cull the disease from the equine industry, but the number of homozygous (H/H) and heterozygous (N/H) carriers have increased! Samples submitted by Quarter Horse owners for HYPP testing in 1992-1996 showed that 1.1% were H/H (homozygous), while 35% were N/H (heterozygous). Testing in 2005 indicated that 2.2% of horses tested were H/H genotype, and 37% were N/H. Keep in mind that these numbers are indications only of the ones actually tested. You can read more detail of testing at http://www.thehorse.com/ViewArticle.aspx?ID=9909&nID=5
While the article from The Horse is a very enlightening one, it noted mandatory testing for HYPP with the AQHA beginning in 1988. I believe that date to be 1996, when HYPP was added to the AQHA handbook. The 1996 rule applied only to foals born of Artificial Insemination (AI) or Embryo or Multiple Embryo Transfer (ET or MET). AQHA just started requiring mandatory HYPP testing for all descendants of Impressive effective January 1, 2007. January 1, 2007 was also the date set by AQHA to no longer allow registry of foals testing H/H. Both rules regarding mandatory testing and denial of registration for H/H tested foals were passed at the 2004 AQHA convention. N/H was not ruled against at that time.
While the AQHA mandatory testing rule is good news for the horse industry, it is not good enough. We still need rulings on HYPP from the all of the registries that have Impressive descendants, such as APHA and ApHC. Additionally, there is now anecdotal evidence (from horses for sale sites and breeders web sites) that HYPP may be crossing over into the Arabian Horse Association, the Pinto Horse Association, and even the POA association. All stock horse registries are at risk, including the international Buckskin Horse Association, Palomino Horse Breeders and the American Buckskin Horse Association. There are also many part and half blood registries who need to follow suit and pass rulings on both positive statuses and mandatory testing thereof. Grade horses are also at risk, especially since evidence is now coming to light that AQHA breeders are continuing to breed carrier to carrier and getting H/H horses. These H/H horses cannot be registered with AQHA, and unless they can be registered with another registry, they are dumped into the grade horse market. BOTTOM LINE: There are no SAFE horses because the American Quarter Horse is the most popular breed of horse in our country. And the one most people will cross breed to, if given a chance.
No American Paint Horse Association (APHA) rule proposals for voluntary testing for HYPP have been passed at this time. APHA does not have voluntary or mandatory testing and will not unless a new rule is submitted and passed.
The ApHC (Appaloosa Horse Club) has passed the following rule for HYPP:
5. Hyperkalemic Periodic Paralysis (HYPP) - meaning a muscular disease caused by hereditary genetic defect that leads to uncontrolled muscle twitching or profound muscle weakness, and in severe cases, may lead to collapse and/or death. According to research, this condition exists in certain descendants of the stallion Impressive, AQHA registration number 0767246.
a. The ApHC recommends testing of any horse known to have an ancestor carrying the HYPP gene, designated under ApHC rules as a genetic defect, to confirm the presence or absence of this gene. The results of such a test shall not be noted on the horses Certificate of Registration, however, the results can be forwarded to the ApHC for documentation under the horses file.
b. ApHC-eligible foals resulting from AQHA-registered stallions and mares born on or after January 1, 2007 and having HYPP status of NH or HH will be required to be HYPP tested at the same time they are parentage verified and to have their HYPP status designated on their ApHC Certificates of Registration. Further descendants of NH or HH ApHC-registered horses will also be required to be HYPP tested/Parentage Verified and have that status designated on their ApHC Certificates of Registration. As far as we know at this time, there are no other registries with rules regarding HYPP.
In pulling progeny of well known Quarter horse sires for our lists, records indicate that 50% or less of foals sired by each positive stallion, through 2006, are tested with results on record. Of those tested through UC Davis laboratory, their newest results are showing approximately 160,000 who are tested to be heterozygous or homozygous for HYPP. Those numbers would show a dramatic increase if all registries required mandatory testing for registry and would make status publicly available as the AQHA does.