Posted by Dr. Cory Reng on June 16 2004 08:16
Hyperkalemic Periodic Paralysis (HYPP)
Many of you have heard about the hot topic causing loads of controversy in the American Quarter Horse Association. Many quarter horse owners remain blissfully ignorant of a fight that could potentially rock the AQHA to its core. Many people are very vocal of their opinion and taking up the fight, and many are standing quietly on the sidelines withholding comment until a decision is made. I think regardless of the final outcome, the debate about HYPP will affect all quarter horse owners before it’s done.
Hyperkalemic Periodic Paralysis (HYPP) is an inherited disease that affects horses. This genetic mutation originated with a single sire "Impressive". Because "Impressive" was such an outstanding sire, many, many quarter horses are related to him. The heavy musculature associated with "Impressive" and HYPP makes these bloodlines especially popular in halter horses.
It is caused by an autozomal dominate genetic defect that disrupts the sodium ion channels in muscle cells. The defect disrupts the channel’s opening and closing, allowing sodium to flow in and out of cells uncontrolled. These influxes change the voltage of muscle cells, causing a wide spectrum of symptoms. On the minor side, constant micro-twitches can cause advanced muscle mass a highly desirable trait in halter horses. A higher level will cause muscle twitches, muscle weakness, or even collapse and death.
With the advent of genetic testing, it is possible to test horses for the HYPP factor. Because the trait is on a pair of alleles, you get two notations upon testing. You can get a horse that tests negative on both alleles (N/N), you could get a horse that tests (N/H) which will result in HYPP in half of the offspring, and a horse could test (H/H). These (H/H) horses will pass the trait on to 100% of their offspring. While the genetic test does a good job determining whether or not your horse will pass HYPP on to offspring, it does not predict how severe symptoms may be. For example a (N/H) horse may die of an HYPP attack, and a (H/H) horse may never have severe symptoms, especially if managed correctly.
Managing HYPP horses has become something of an art form. Especially since many of the top horseman showing Halter horses have some of the most severely effected horses. Because symptoms are related to potassium metabolism, feeding a diet low in potassium is paramount. This means avoid feeding alfalfa to HYPP horses. Feeding a diet of grass hay, and grain usually minimizes symptoms. If a horse is starting a minor attack, feeding grain will often cause an insulin release that will direct potassium back into the cells. Severe attacks will need to be treated with IV fluids by your veterinarian. These attacks are an emergency.
The controversy is based around whether or not HYPP horses will be able to continue to qualify for registry in AQHA. There is a great deal of pressure to consider HYPP a genetic defect that would disallow registration. The other side is that many of the top winning halter horses are either homozygous or heterozygous HYPP. Does AQHA want to disqualify for registration the bloodlines that have produced most of the World Champion quarter horses for the last ten years?
Those who argue that HYPP is a genetic defect that should be eliminated will cite other times that a show ring fad have almost ruined a breed. The example most often cited is "Snorter Dwarfs" in Hereford cattle. This genetic defect became common when very short blocky cattle were winning in the show ring. Heavy selection on short blocky bloodlines made the genetic defect of tiny cattle with deformed nasal passages common. Because genetic testing was not possible at that time, all cattle in bloodlines that had produced "Snorter Dwarfs" were eliminated from the breed.
The other side of the argument is that many of AQHA’s top producing and most desirable bloodlines carry the HYPP gene. Many argue that by eliminating these bloodlines, they may put the industry back 25 or more years.
Whichever side of the argument you are on, or even if you choose to stay out of it completely, testing will become more and more important. Though there is a blood test available most breeders are now using the tail hair DNA test. This test only requires a number of tail hairs from your horse. The test can be run through the vet clinic. Horses that are related to Impressive will need to be tested for registry. There is a discount available for AQHA members.