What's the Hype over HYPP?
Questions answered about hyperkalemic periodic paralysis

by Dr. Sharon Spier

"HYPP" has been the buzz word around certain show circuit circles since 1992. It has made some horsemen turn white, some others turn red and still others shrug their shoulders. What is the big deal about this disease?

The American Association of Equine Practitioners and Dr. Sharon Spier from the University of California at Davis have collaborated to respond to many unanswered questions about HYPP. Though still many questions will remain unanswered simply because of the disease's youth, education about HYPP can reduce its spread and severity.

What is the cause?
Hyperkalemic periodic paralysis (HYPP) is an inherited disease of the muscle which is caused by a genetic defect. In the muscle of affected horses, a point mutation exists in the sodium channel gene and is passed on to offspring.

Sodium channels are "pores" in the muscle cell membrane which control contraction of the muscle fibers. When the defective sodium channel gene is present, the channel becomes "leaky" and makes the muscle overly excitable and contract involuntarily. The channel becomes "leaky" when potassium levels fluctuate in the blood. This may occur with fasting followed by consumption of a high potassium feed such as alfalfa. Hyperkalemia, which is an excessive amount of potassium in the blood, causes the muscles in the horse to contract more readily than normal. This makes the horse susceptible to sporadic episodes of muscle tremors or paralysis.

This genetic defect has been identified in offspring of the American Quarter Horse sire, Impressive. To date, confirmed cases of HYPP have been restricted to descendants of this horse.

How is it spread?
HYPP is an autosomal dominant genetic trait, which means only one copy of the gene is required to produce the disease, and that the disease can equally occur in both sexes (mare, gelding or stallion).

To elucidate, suppose a mare has tested "HYPP H/N". This means she is heterozygous, or carries one copy of the HYPP gene. Breeding her to a normal sire, or HYPP N/N, will result in a 50 percent chance the offspring will carry the HYPP gene (HYPP H/N) and a 50 percent chance of the foal being normal. Breeding her to a heterozygous sire, or HYPP H/N, will cause only a 25 percent chance of the offspring being normal (HYPP N/N), a 50 percent chance of the offspring to carry the gene (HYPP H/N) and 25 percent chance of the offspring to be homozygous (HYPP H/H, or carry both copies of the HYPP gene).

A carrier of the defect (HYPP H/N) is affected with HYPP. These horses can show clinical signs of the disease and can pass the gene on to their offspring. Though horses which are homozygous show more severe clinical symptoms, heterozygotes also obtain the disease because it is a dominant trait.

Are there any signs?
Diagnosis of HYPP can be very difficult based on observance of the horse. There are many different symptoms which can mimic other diseases such as "tying-up" or colic. Classic signs include periodic attacks of muscle spasms, tremors, weakness, sweating and recumbency. During any attack, the horse may have rapid, labored or difficult breathing. Homozygotes will have noisy breathing due to paralysis of the muscles in the throat.

Furthermore, severity of clinical signs and frequency of episodes vary among horses even though they possess the same genetic defect. The duration of clinical signs also varies, with episodes of muscle tremors or paralysis lasting only minutes to three hours or more.

"The classification of severity of clinical signs and frequency of episodes is difficult," Dr. Spier states, "because horses may experience spasms that are not witnessed."

In fact, some HYPP-affected horses may show no signs at all. Studies suggest that spontaneous attacks of classic HYPP are rare in affected horses. They can appear clinically normal the majority of the time. This makes the disease even more difficult to detect.

So how can you tell if your horse has HYPP?
"In order to verify that a horse has HYPP," Dr. Spier states, "a blood test must be performed by your veterinarian."

How common is HYPP?
Another study by Dr. Spier determined the frequency of HYPP in the horse population. The study found that the gene is not being diluted in subsequent generations. Instead, it is being passed on with high frequency.

In still another study to determine the gene frequency, 1,000 horses were chosen at random by the University of California at Davis to test for HYPP. The data concluded that the HYPP gene is infrequent among American Quarter Horses, yet is substantially linked to pedigrees tracing back to a common sire.

"It is hopeful that HYPP will decrease among breeding stock during the next decade," Dr. Spier states, "as long as breeders continue to select stock free from the gene."

What about testing?
All offspring of Impressive should be tested for HYPP. Horses with suspicious symptoms of the disease should also be tested.

Testing can be performed by contacting a veterinarian or calling the Veterinary Genetics Laboratory at (916) 752-7416. A recorded message will explain how to submit a sample. Can management induce or reduce an HYPP attack?

Environmental factors can actually cause an attack of muscle weakness. Owners of HYPP-positive horses should be aware that external stimulus and events can increase the chance of paralysis onset. These factors include dietary changes, fasting, general anesthesia, concurrent illness and exercise restriction.

"Management factors remain an important cause of symptoms," Dr. Spier states. "Owners of affected horses should be aware of possible precipitating factors and be able to treat or manage the symptoms."

For control of HYPP episodes, Dr. Spier recommends the following management procedures; Maintain a regular feeding and exercise schedule. Avoid fasting or water deprivation. These horses do better if allowed access to a paddock or pasture rather than strict stall confinement. Daily or nightly turnout is helpful. Mix alfalfa with grass hay or oat hay and grain (oats are best) to decrease potassium content of the diet. Many owners report their horses do very well on grass hay alone or on pasture. Feed equal amounts of hay and grain two to three times daily. Avoid rapid changes in the diet.

Inform your veterinarian of the HYPP condition prior to any general anesthesia, as this may precipitate an episode of paralysis. If your horse is receiving medication, maintain him or her on therapy before and after surgery or anesthesia. Use common sense while hauling. Be sure to stop and water the horses frequently (every two hours).

"Many horses with this trait are asymptomatic and have very successful careers," Dr. Spier comments. "Most horses can be managed easily with good care and can still bring great pleasure to their owners. Put this condition into perspective with regard to many of the other conditions horses can have."

Should an HYPP-positive horse be ridden?
According to Dr. Spier's studies, the chance of a paralysis episode occurring while the horse is being ridden is unlikely. However, muscle spasms are unpredictable and can occur with only slight symptoms. Therefore, it is recommended that only experienced horsemen (those aware of symptoms and familiar with the horse's behavior) handle and ride affected horses. Contact a veterinarian immediately if any abnormal clinical signs are observed.

What if my horse goes into a mild HYPP episode?
Dr. Spier recommends the following procedures for mild episodes of an HYPP attack (muscle tremors, but the horse is not down)

• Contact your veterinarian immediately.
• Exercise horse (walking or longeing). Use caution as the horse could stumble and fall.
• Exercise stimulates adrenaline which helps replace potassium inside cells.
• Feed dry grain (oats, or corn-oats-barley, or you can use light Karo syrup for a glucose supplement). Feeding carbohydrates supplies glucose which stimulates the release of insulin which promotes potassium uptake in cells.

Dr. Sharon Spier is an associate professor in the Department of Medicine and Epidemiology at the University of California at Davis. She is the Chief of Service in the Equine Field Service. Her clinical interests are in general equine practice, internal medicine disorders including diseases of muscle and gastrointestinal disorders. Dr. Spier is the recipient of the AVMA Council on Research Award in 1994 for excellence in equine research. She has been involved with research on hyperkalemic periodic paralysis in horses since 1985.