Equine HYPP

Hyperkalemic Periodic Paralysis


  • Also called "Impressive Syndrome".
  • Genetic defect originally traced back the the American Quarter Horse "Impressive".
  • Impressive was one of the top Halter horses in the world.
  • Impressive had many offspring, estimating over 55,000 which would carry the HYPP trait that wasn't known until later.
  • It is an inherited disease of the muscles that is caused by a genetic defect.
  • Most commonly found in heavily muscled horses such as halter horses.
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Symptoms of Disease

  • Muscle tremors
  • Prolapse of the third eye lid
  • Sweating
  • Weakness
  • Yawning
  • Depression
  • Inability to relax muscles
  • Noisy breathing
  • High heart rate and respiratory rate
  • Staggering
  • Dog sitting or collapse
  • Can lead to death from heart attack or respiratory failure
Padukah's HYPP episode.avi

HYPP Episode


  • Affected homozygous (H/H)
  • Affected heterozygous i.e. carriers (N/H)
  • Normal (N/N)
  • Gene/locus: SCN4A


  • Heavily muscled
  • Double muscled

Genetic Transmission

  • Autosomal Dominant
  • The trait is inherited from generation to generation with equal frequency.
  • It does not get diluted or skip generations.
  • Caused by leaky sodium channels in the muscle that makes the muscles excitable and contract involuntarily like an uncontrollable twitch.
  • The channels become leaky when potassium channels fluctuate in the blood.

Why is HYPP an issue?

HYPP is a problem because if you have a H/H horse then you will have to very closely monitor it's diet and try to feed low potassium feeds. Not everyone wants to deal with high maintenance horse that may have tremors that could lead to possible fatality. You also will NOT want to breed an H/H horse. If you have an N/H horse who is only a carrier they may have problems or they may not, so it's like a gamble. However, if there will be any issues they generally show up within the first few years of life, but it is not impossible to happen at an older age. Horses that are N/H should also not be bred so that the gene is not passed on.

Methods to eradicate HYPP

  • First of all, foals born beginning in 1998 had to be tested and required notification on their registration certificate. However, there is no rule to keep HYPP horses from breeding or being in the show ring.
  • In 2004 AQHA made a statement that starting in 2007 foals that were H/H would no longer be allowed to be registered, however N/H horses could still be registered.
  • If your foal is a descendant of Impressive then an HYPP test had to be performed. However, once a negative line is established then testing is not required.
  • All other HYPP testing is voluntary.
  • It is still a controversy as to rather or not N/H horses should be allowed to be registered as some say it will eliminate the disease completely. However, some argue that N/H horses are manageable and should still be allowed to breed.


In my opinion, I do think AQHA should put a stop to the registration of N/H horses. That way we can potentially eliminate HYPP all together. It may mean the sad loss of some good bloodlines, but there will always be bigger, better, faster, more elite bloodlines. So why not eliminate the problem sooner than later! So my question to everyone out there is, what is you opinion? Should AQHA ban N/H horses from being registered? If not, what other methods would you suggest?


UC Davis. 2016. Hyperkalemic Periodic Paralysis (HYPP). https://www.vgl.ucdavis.edu/services/hypp.php Accessed 27 April 2016.

America's HQSE Daily. 2014. HYPP Survival Guide. http://americashorsedaily.com/free-report-hypp-survival-guide/#.VyByDXoYGSo Accessed 27 April 2016.

Foundation Horses. http://www.foundationhorses.com/impressive_syndrom.aspx Accessed 27 April 2016.

American Quarter Horse Association. 1996. HYPP. https://aqha.com/media/3110/aqha_hypp_brochure_sm.pdf Accessed 28 April 2016.

EQUUS. 2004. Halting HYPP in Quarter Horses. http://equusmagazine.com/article/haltinghypp_042506 Accessed 28 April 2016.

American Quarter Horse Association. My AQHA Idea. http://aqha.ideascale.com/a/dtd/Eliminate-HYPP-in-the-breed/77538-28145#idea-tab-details Accessed 29 April 2016.

you can vote!

Below is a link to "My AQHA Idea" where people can vote rather or not to stop N/H horses from being registered.