Hyperkalemic Periodic Paralysis

September 2007

At the 2006 American Association of Equine Practitioners (AAEP) Annual Convention, Dr. Sharon Spier gave a talk called "Hyperkalemic Periodic Paralysis: 14 Years Later." The title is significant because it was at the 1992 AAEP convention that she dropped a bombshell on the horse industry: Impressive, the legendary quarter horse sire, was the only bloodline that carried the mutant gene which causes hyperkalemic periodic paralysis, or, HYPP.

Since then, the American Quarter Horse Association (AQHA) has taken dramatic steps to reduce the frequency of the disease in their breed:

1996 - AQHA officially recognizes HYPP as a genetic defect or undesirable trait.

1998 - Mandatory testing for HYPP with results designated on the Registration Certificate begins for foals descending from Impressive born after January 1.

2004 - Foals born in 2007 or later that test homozygous for HYPP (H/H) will not be eligible for registration.

Approximately 4% of the quarter horse breed may be affected and this number does not appear to be going down despite AQHA’s efforts and an inexpensive, readily available DNA test to identify carriers of the mutant gene. What is hyperkalemic periodic paralysis and why is everyone so concerned about it?

The mutant gene mentioned above causes an error in an electrolyte "gate" in muscle cells. This gate is supposed to closely control the amount of charged sodium (Na) and potassium (K) ions that come in and out of the cell during a muscle contraction. However, because the gate was made wrong, it sometimes gets stuck in the open position, allowing uncontrolled flow of Na and K ions. When this happens, horses show characteristic signs of the disease.

A horse having a mild episode may sweat and tremble in the flanks, neck and shoulders. This muscle twitching or shaking may progress to the rest of the body. In severe episodes, muscle cramping may occur, or the horse may become weak and stagger, sway or even dog-sit. The entire event lasts anywhere from 15 to 60 minutes and horses can actually die during episodes, due to cardiac or respiratory arrest. Due to the risk of personal injury, only experienced adults should ride and handle horses with HYPP.

Owners who notice early signs of an episode are encouraged to try and head it off by giving the horse something with sugar in it (such as sweet feed or corn syrup), or by lightly exercising the horse (such as lunging). A veterinarian should be called for this or any other muscle disorder, to provide emergency care if necessary.

In most cases, HYPP is a manageable disease. The first step is having a DNA test of mane or tail hair performed on a quarter horse or quarter horse cross that either has Impressive in the pedigree or has shown any of the signs described above. The next step is working with a veterinarian to design a nutrition, exercise and turnout, and medication plan that works for the particular owner, horse and situation.

The typical recommendations include:
  • Avoiding high potassium feeds such as alfalfa hay, brome hay, soybean meal, sugar and beet molasses, and electrolyte and kelp-based supplements (plain salt is okay)
  • Using low potassium feeds like later cuts of timothy or Bermuda grass hay; grains such as oats, corn, wheat, and barley; and beet pulp
  • Feeding multiple small meals throughout the day so that no one meal exceeds 33 grams of potassium
  • Allowing the horse turnout time, especially onto grass pasture
  • Being aware of trigger factors such as fasting or abrupt changes in diet, anesthesia or heavy sedation, trailer rides, stress and others
  • Medicating with oral acetazolamide to remove excess potassium from the body
For more information, or to order a testing kit, visit www.aqha.com.

Hyperkalemic Periodic Paralysis (HYPP)
By: Lydia F. Gray, DVM, MA
SmartPak Staff Veterinarian and Medical Director
September 2007