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Hyperkalemic periodic paralysis (HYPP) in horses

Quick facts

  • HYPP is a genetic disease noted by mild to severe episodes of muscle spasms and trembling.

  • HYPP links back to the Quarter Horse sire Impressive.

  • Breeders should refrain from using any HYPP horses for breeding.

  • Lowering diet potassium levels and providing regular exercise can help manage HYPP in affected horses.

What is HYPP?

HYPP is a genetic disease noted by episodes of muscle twitching and shaking. Horses only need one copy of the mutated gene to be affected. HYPP occurs in the following breeds.

  • Quarter Horses

  • American Paint Horses

  • Appaloosas

  • Quarter Horse crossbreds

The disease links back to the Quarter Horse sire Impressive. About 4.4 percent of the Quarter Horse breed may be affected.

Signs of illness 

HYPP signs include intermittent

  • Muscle twitching

  • Shaking

  • Trembling

  • Weakness

  • Collapse

The signs usually begin by two to three years of age. Severely affected horses may have a hard time swallowing or make loud breathing noises. Sometimes sudden death can occur following a severe episode.

HYPP horses may appear normal between episodes.

What causes episodes in horses with HYPP?

HYPP results from a mutation in the muscle’s sodium channel. The channel becomes leaky, which causes the muscle to be more sensitive. As a result the muscles contract involuntarily with blood potassium changes. This can occur with stress or fasting followed by eating high potassium feed like alfalfa.

Breeding considerations 

Horses with two copies of the mutated gene are more severely affected. The severity of HYPP in horses with one copy can vary. A horse with one copy that shows little sign of disease still has the same chance of passing the gene to its offspring as a horse showing more severe signs.

You should refrain from breeding HYPP horses for the long-term health of the Quarter Horse and other related breeds. Breeding an affected horse to a normal horse will result in a 50 percent chance of producing a foal with HYPP.

Since 2007, the American Quarter Horse Association no longer registers Impressive progeny who have two copies of the mutated gene.

Testing for HYPP

The Veterinary Genetics Laboratory at the University of California Davis tests for HYPP mutations using main or tail hair. Quarter Horse foals born after 1998 that are offspring of an affected parent have a statement recommending DNA testing for HYPP on the Certificate of Registration.

Caring for HYPP horses


Authors: Nichol Schultz, DVM, former graduate student, College of Veterinary Medicine and Molly McCue, DVM

Reviewed in 2021

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