The Online Horse Magazine for Equestrians of all Ages



Daily exercise or regular turnout helps alleviate HYPP symptoms by regulating potassium levels.

The high water content of grass prevents HYPP horses from consuming large amounts of potassium quickly, thus reducing the likelihood of an attack. 

The information contained within this article does not constitute medical advice.
Please consult your horse's veterinarian for further information on Hyperkalemic Periodic Paralysis.

Hyperkalemic Periodic Paralysis



Unfortunately, there is no cure for HYPP. The genetic defect is carried on the dominant allele, thus horses can either be heterozygous (carrying one mutated gene and one normal gene) or homozygous (carrying a pair of mutated genes) for the disease. Homozygous horses are more symptomatic than heterozygous ones. Likewise, the number and severity of HYPP attacks in heterozygous horses can vary greatly among individuals. The reason for this phenomenon is not clearly understood but management practices are believed to have an influence.

To prevent or reduce the severity of attacks, the general recommendation is to closely regulate potassium levels in the diet. Feeds naturally high in potassium include alfalfa hay, brome hay, canola oil, soybean oil or meal, molasses and molasses-containing concentrates. These feeds should be avoided and replaced with feeds that are lower in potassium such as grass hay, particularly later cuts of timothy or bermuda grass, and grains such as oat, corn, wheat or barley. Beet pulp that doesn't have added molasses is an excellent fiber source. Pasture is ideal for HYPP horses as the high water content of grass prevents horses from consuming large amounts of potassium quickly. Likewise, feeding HYPP horses three to four meals a day will also reduce spikes in blood potassium levels.

Naturally, abrupt switches in feed should be avoided. It is believed that HYPP attacks are more likely to occur when changing from a diet low in potassium to a higher one. Regular exercise or daily turn-out is also recommended to help regulate potassium levels. Owners of HYPP horses should also notify their veterinarian of their horse's condition prior to the administration of sedatives or general anesthesia.

HYPP horses whose symptoms can't be controlled by management techniques alone may benefit from drug therapy. Both Acetazolamide and hydrochlorothiazide are diuretics which reduce blood potassium levels by eliminating it through the renal system. However the use of both of these drugs is restricted by some breed and show organizations.


A further indication that this was a genetic disease was presented in 1985. JH Cox addressed the American Association of Equine Practitioners at their annual convention and pointed out the similarities between this syndrome in horses and HYPP in humans. But it was not until 1992 that DNA research conducted at the University of California, Davis and the University of Pennsylvania confirmed that the mutation causing HYPP in horses traced back to Impressive's bloodlines. This confirmation occurred long after Impressive's foals began hitting the ground in the early 1970's.

In these DNA study, 1000 blood samples, collected and stored from 1989 to 1991, were tested. The frequency of HYPP positive horses in the sample set was 4.4%, indicating that the HYPP mutation affects only a small percentage of the Quarter Horse population. However, all the samples that tested positive for the HYPP mutation came from Impressive's descendants. Of the 1000 samples tested,100 of these horses traced back to Impressive. The frequency of HYPP positive samples in this subset was 43%, indicating that a large number of Impressive's descendants carry the mutated gene even if these horses don't show outward signs of the disease.

Since 1992, DNA testing for the HYPP gene has been available to breeders. Further research has indicated that the mutated gene is not intrinsically linked to the highly desirable traits that were also inherited from Impressive. These include both his well-defined musculature and his nearly-ideal conformation.

However, there has been resistance to removing HYPP positive horses from the Quarter Horse gene pool. Breeders and owners familiar with the disease have found it to be of little consequence as the periodic symptomatic episodes can be moderated through management techniques and medication. In 2003, more than ten years after DNA testing for HYPP became available, over 55,000 registered Quarter Horses can trace their lineage to Impressive.

The AQHA has taken steps to address this issue. As of 2007, foals testing homozygous positive for HYPP will no longer be eligible for registration with the AQHA. Homozygous positive animals are not only more symptomatic for the disease, but since they carry the mutation on both alleles of the gene they inevitably pass on a mutated copy to 100% of their offspring.

Although this ruling is designed to keep these horses from further perpetuating the disease in the AQHA registry, it has resulted in HYPP homozygous horses being sold as unregistered or grade Quarter horses. These horses can end up in the hands of youth and amateur adult riders, who have little, if any, familiarity with the disease.

In addition, the Quarter Horse has been influential in the development of breeds such as the American Paint, Appaloosa, Pony of the Americas, Palomino and the Buckskin. Due to its popularity, the Quarter Horse has also been used in crossbreeding programs. Since the HYPP mutation is a dominant allele, only one copy of the gene is necessary to produce HYPP symptoms in horses.

This gives the potential for the HYPP gene to have crossed the Quarter Horse breed boundary and become established in related breed registries as well as in grade and crossbred horses. This makes the eradication of the HYPP gene not only the responsibility of Quarter Horse breeders, but an issue for all horse owners to understand.                                                     

On March 15, 1968 a seemingly innocent chestnut colt was born in Oklahoma. He was a good looking fella with an outstanding pedigree that traced back through his maternal and paternal bloodlines to Three Bars, a Thoroughbred racehorse whose influence produced a multitude of champion Quarter Horses. Like many Appendix Quarter horses, the chestnut colt was eligible for registry in the American Quarter Horse Association (AQHA).

The colt's registered name became "Impressive" and like his name suggests, he went on to become a successful halter horse in the show ring and a sought-after sire in the breeding shed. Impressive was renowned for his unsurpassed musculature, a trait he reliably passed on to his offspring.

Today, Impressive is also known for his association to a genetic mutation that causes a potentially fatal disease in horses. Hyperkalemic periodic paralysis (HYPP) is an inherited disease that alters sodium ion channels in muscle cells and periodically increases blood potassium levels.

In order for muscles to contract, an electrical charge is initiated by sodium ions moving into the cell. In a normal individual, these sodium channels close to restrict the movement of sodium into the cell once the muscle has contracted. A mutation to the gene responsible for building these sodium channels can result in channels that don't close properly or remain open when they should not. This allows sodium to continue entering the cell. This not only prevents the muscle from relaxing, but osmotic pressure also increases the movement of potassium out of the cell. The end result is a muscle that remains tense until it fatigues and an abnormally high level of potassium in the blood.

Since the genetic mutation that causes HYPP affects how muscles work, the symptoms of the disease center around muscle function. Elevated levels of blood potassium, which is a condition called hyperkalemia, is not just the end result of leaky sodium channels in HYPP horses. Hyperkalemia makes muscles overly excited and caused them to contract more readily, thus setting up a cycle that produces more muscle contractions and allows more potassium to enter the bloodstream. An HYPP attack can also be precipitated by fluctuations in blood potassium due to environmental factors, dietary changes, illness and exercise restrictions.

During a mild attack, an HYPP horse may exhibit signs of muscle tremors and appear to shake or tremble. These tremors may be readily apparent on the horse's neck, shoulders and flanks. The third eyelid, which is a thin membrane that normally flickers across the eye to remove irritants, may prolapse and become visible. The horse will likely show signs of weakness and may sway or swagger, but remain standing.

In more severe attacks, the horse may sit dog-style due to weakness in the hindquarters. Widespread weakness can cause complete collapse of the animal. A severe attack can last from 15 to 60 minutes or longer and can be accompanied by elevated heart and respiratory rates. If the muscles of the larynx (voice box) are affected, an abnormal whinny may be produced. Death from suffocation can occur if the muscles of the diaphragm also become paralyzed.

Horses undergoing HYPP attacks remain conscious and alert. They do not appear to be in any pain. After the attack, the horse is able to stand and move normally. These factors help distinguish HYPP from other medical conditions such as tying-up syndrome, seizures and colic. Tying-up syndrome, which is also known as exertional rhabdomyolysis, azoturia, and Monday morning sickness, leave horses stiff and in pain. Horses experiencing seizures can tremble, shake, and collapse, but their eyes will glaze over and they are not aware of their surroundings. Colicky horses may have difficulty standing, but will show clinical signs of pain.

Copyright © 2012 - 2016 PonyZine, LLC​
All Rights Reserved

One Small Gene Mutation in a Quarter Horse, One Giant Issue for Horse Owners

Although it is believed that Impressive never exhibited any symptoms of the disease, the specific genetic mutation that causes HYPP has been identified in horses that descended from his bloodlines. It was not until the 1980's that breeders and veterinarians began to see the connection between this muscle-tremor syndrome and inheritance. This is due to a variation in the outward appearance of the disease in heterozygous HYPP horses. Not all horses carrying the mutated gene exhibited symptoms of the disease.