HYPP and the Impressive line of American Quarter Horses: The facts

Jan Swan Wood

In 1969, an American Quarter Horse foal was born that changed the history of the halter horse in three breed registries. Born with outstanding conformation, disposition and pedigree, the colt named Impressive, eventually set the standard in halter horses for the Quarter Horse, Paint and Appaloosa breeds.

Sadly, because he was the biggest name in the halter world, he and his offspring were used extensively for breeding, and when a devastating genetic defect was isolated, it had already affected tens of thousands of horses. The “Impressive” line, once noted for their exceptional halter conformation, became known as the hyperkalemic periodic paralysis (HYPP) line of horses.

In 1992, researchers designated the Impressive line of horses as carriers. Not all horses of that line had HYPP, but all horses with HYPP were descended from Impressive himself. In 1994, a genetic test that utilized DNA from hair or blood, was perfected, this determined whether a horse had HYPP or not.

Hyperkalemic periodic paralysis is a genetic disorder that causes horses to have episodes of muscle spasms, weakness, “dog sitting” due to hind quarter weakness, collapse, recumbency, sweating, high serum potassium levels, third eyelid twitching and yawning. Levels of distress during an attack vary, with some horses showing only mild symptoms, while others have very severe symptoms. Due to respiratory paralysis or respiratory failure during an attack, some horses can suffocate and die.

The attacks happen when sodium channels – which are gateways in the cell membranes of the muscle that regulate muscle contraction, close, which interrupts the flow of sodium in and out of the muscles. As potassium levels in the blood rise, the channel that controls influx becomes stuck, and sodium floods into the cells. Uncontrolled muscle twitching and trembling are the results.

As the potassium levels continue to rise, the muscles become unable to contract, and the horse becomes paralyzed. This cycle continues until excessive potassium is worked through the kidneys and expelled in urine, or is reabsorbed into the cells. Episodes vary widely in duration and can last several minutes even to hours.

This constant flexing of the muscle cells is what leads to the pronounced muscle patterns on the afflicted horses, hence the popularity of the line in the stock horse breeds.

Attacks of HYPP can be triggered by many things. Environmental factors include chilling, fasting, transport, stressful events, such as weaning or surgery, and a change in diet. Beginning a new training regimen, or even the cessation of training, can also bring it on. Most episodes don’t occur during exercise, but at rest afterwards.

Diagnosis through DNA is classified at three different levels.

• H/H – are horses that have the gene mutation and are homozygous for the trait, hence the fact that this mutation is always passed on to the offspring.

• N/H – are horses have the mutation, but are heterozygous and will pass it on 50 percent of the time.

• N/N – are horses that do not have the mutation and will not pass it on.

Linebreeding of the Impressive bloodline is common, but does not increase the risk of the mutation due to saturation. The increased risk is due to there being more individuals of Impressive genetics breeding in the pedigree, thus increasing the odds of an occurrence for the mutation in one of those lines.

No dilution of bloodlines will remove the risk of HYPP. It carries through the genetic pool as a dominant trait, for example, a horse that is within seven generations from Impressive is just as likely to carry the gene as one that is within two generations of him.

The Quarter Horses with Impressive bloodlines used in breeding today are required to be tested for HYPP in order to be registered, and AQHA will no longer register a foal that tests H/H for the gene.

Breeding decisions for the horse carrying the gene are critical in order to lessen the chance of producing a foal that is positive for the gene. Four possibilities exist when breeding. When a N/N horse is bred to a N/H horses, 50 percent of the resulting foals will be a N/H carrier and 50 percent will be a N/N carrier. When a N/N horse is bred to an H/H horse, 100 percent of the foals will be a N/H carrier. When a N/H horse is bred to a N/H horse, 50 percent of the offspring will be a N/H carrier, 25 percent will be a H/H carrier, and 25 percent will be a N/N carrier. When a N/H horse is bred to a H/H horse, 50 percent of the foals will be a H/H carrier and 50 percent will be a N/H carrier.

The only sure way to not achieve a HYPP positive offspring is to breed only N/N horses. If only N/N horses are utilized for breeding, the mutation is not present and will never be.

Management of the HYPP positive horse is carried out through dietary changes and medication. Potassium is high in most horse’s diets. Limiting potassium in the diet, not to exceed 1 percent, is recommended. Different activity levels make potassium requirements vary. Sedentary horses require very little potassium, while very active horses require much more. An equine nutritionist can formulate a plan for a horse with HYPP, thus simplifying the management of the individual.

Horse diets are constituted primarily of forage, whether that be pasture, hay, or hay cubes. Horses with HYPP have those forage choices reduced dramatically. Legumes, such as alfalfa, are very high in potassium, so they should be used minimally or not at all. Clover is also a legume and needs to be limited.

Low potassium feed, whether on pasture or hand fed, should be basis for the diet of an HYPP horse. Testing of all forage is the only way to be certain of potassium levels.

Feedstuffs can contain various levels of potassium, so care should always be taken to determine what can be fed in conjunction with low potassium forages.

High potassium feedstuffs – which should be avoided, include: molasses (from sugar beets or sugarcane), soybean meal, alfalfa, reed canary grass, orchard grass, rich spring pasture, canola oil, and electrolyte supplements.

Medium potassium feedstuffs include: brome grass hay, fescue hay, clover hay, timothy hay, oat hay, coastal Bermuda grass hay, Kentucky bluegrass and stabilized rice bran.

Low potassium feedstuffs include: oats, corn, barley, wheat, wheat middlings, wheat bran, soybean hulls, beet pulp (without molasses), and corn and other vegetable oils.

Additional add-on supplements often contain potassium, so careful label reading is required when using them in the diet of the HYPP horse.

Medications which help manage HYPP include diuretics, such as acetazolamide, which stabilizes blood glucose and potassium by stimulating the excretion of insulin. It is often used in young horses fed a high protein diet to promote growth. Many halter horses are maintained on the medication for long periods of time while being fed alfalfa hay for conditioning. It is a restricted use drug in some breed registries, so care should be taken to be in accordance with those regulations.

Since HYPP horses are sensitive to anesthesia, it is necessary that veterinarians be aware before administering general anesthesia to those horses.

Other management practices that benefit the HYPP horse are frequent meals and steady access to water, even when hauling, where water should be offered often.

The blood of Impressive is in the pedigrees of thousands of horses today, including three different breed registries. Being knowledgeable about the disorder is key to making decisions about horses already owned or being considered for purchase or breeding.

The blood of Impressive is capable of adding the “pretty” to many horses, plus the athletic ability and speed, due to the pedigree behind Impressive himself. He was a linebred Three Bars (TB) and destined for the racetrack before being promoted as a halter horse, so the speed is there for performance events. (Note that Three Bars (TB) was NOT an HYPP horse.)

Impressive died at age 26 in 1995, after siring 2,250 foals in 24 crops. He was a superior individual and his blood can add quality to horses for many generations.

Due to genetic testing, it is possible to be absolutely sure whether a horse has HYPP or not, or whether they are capable of passing it along. Do the research on any horse with the bloodline, and know what the tests reveal. The American Quarter Horse Association can tell you if a horse has been tested and what the results are with a simple phone call. Education is the key to HYPP.