Hyperkalemic periodic paralysis (HYPP) in horses
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.
American Paint Horses
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
The signs usually begin by 2 to 3 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.
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
Limit the amount of potassium your horse consumes. The following feeds are high in potassium.
Soybean meal or oil
Cool-season grass hays and other grains such as oats tend to be lower in potassium. Allowing horses to graze pasture is ok. The high water content in pasture makes it unlikely for horses to consume enough potassium to cause problems.
Avoid any sudden changes in feed.
Regular exercise or turnout can help prevent HYPP episodes. If your horse has a mild episode, walking them can reduce the potassium levels in their cells. Always consider safety first. During an episode, horses can be unstable and more likely to trip or fall.
Reviewed in 2018