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The HYPP controversy: The outcome will eventually affect all breeds

by Kenneth J. Kopp, D.V.M., Article donated by the mane points horse resource center.

During the last several years, HYPP has been a topic of controversy. It is an inherited disorder called hyperkalemic periodic paralysis. Hyperkalemic means "elevated potassium," and a similar syndrome has been identified in humans.

HYPP has been primarily limited to Quarters Horses, but Paints and Appaloosas are affected if genetically linked to the leading Quarter Horse sire, Impressive.

Symptoms are that the horse has muscle cramping, quivering muscles, difficulty breathing, and paralysis. Death can occur from heart or respiratory failure due to toxic effects of high blood levels of potassium.

While HYPP is limited to these breeds, how the equine industry deals with the issues raised by HYPP will eventually impact all breeds.

New technology has allowed us to identify a genetic marker for this disease, and suspect horses can be tested by your veterinarian. It is estimated that more than 2 percent of all Quarter Horses carry this gene.

This is the first of the genetically linked diseases we can test for in horses, but others will no doubt come along.

What would happen, for example, if we found a gene in thoroughbreds linked to DOD (developmental orthopedic disease)? How would the Jockey Club respond? How will any breed respond if we find a gene linked to colic?

Molecular biology is opening up many difficult questions. The American Quarter Horse Association is addressing this issue head on and should be admired for its open approach.

Still, tough decisions need to be made. In the interest of improving this fine breed, should all carriers of the gene be restricted from breeding registration? Or, should we live with the syndrome?

As a purist, I would vote for restriction. As a pragmatist, I understand the difficult ramifications of this enforcement.

I have had a long history with HYPP because Impressive stood in my equine practice area. We diagnosed the Impressive syndrome in the early 1980's but could not genetically test for it, nor did we know to treat the syndrome.

Despite the HYPP concerns, Impressive and his offspring made substantial positive contributions to the Quarter Horse breed.

And, in fairness, all breeds can have undesirable genetics. With line breeding, we can select for desirable traits and unknowingly concentrate defective genes. The reason this disease has received so much attention is that, since 1992, we've been able to test for it; the largest breed registry is involved; and a leading sire is the origin.

HYPP can show up any time. Many horses carry the gene, but are normal. To date, there is no method to predict the incidence of clinical episodes.

What's actually happening inside the horse with HYPP? For some reason still not understood, potassium leaks out of the muscle cell and sodium moves into the cell. This creates an electrical charge that over-stimulates the muscles as well as an elevated blood-potassium level.

In fact, one theory has been proposed that this stimulation explains why these horses have such well-defined muscles-their muscles are always in a slight state of contracture which results in enlargement and "impressive" muscle definition.

One proposed therapy is to reduce dietary potassium. By far, the greatest amount of potassium ingested by all horses is in forage. Alfalfa hay is high in potassium, and horses suffering this problem should avoid it. Hays that are lower in potassium are bermuda-grass, oat hay and some fescues.

Still, with HYPP horses, I would advise testing the hay for potassium concentration.

Since horses need fiber, a good alternative is beet pulp, which is one of the lowest potassium fibers available.

The best way to effectively lower a horse's dietary potassium is to use a beef pulp balance ration, such as Southern States Target. Although this feed indulges molasses, which is high in potassium, if Target is fed as the primary ration, the total dietary potassium concentration is dramatically reduced. It's not recommended that you feed beef pulp alone because it is low in critical minerals and vitamins.

If your horse is severely affected, it can be maintained on no hay by feeding Target to maintain body condition. (Target will have to be fed at 20-25 pounds per day per 1,000 pounds of body weight) if all hay is eliminated. (To prevent boredom and add supplemental fiber, try offering small amounts of grass or oat hay.)

Medication may also help. A common diuretic azetazolimide, can be prescribed by your veterinarian. This increases potassium secretion by the kidneys. However, some researchers believe the drug has its effect at the muscle cell membrane. For this reason, it may be that HYPP involves more than just potassium and sodium shifts.

If your horse is diagnosed with HYPP, don't panic. HYPP is not a death sentence. Horses with HYPP can lead healthy, productive lives.

However, we need much more research. The AQHA should be applauded for its generous funding of equine research, including HYPP.

HYPP Guidelines

  1. If your horse is genetically linked to Impressive, have it tested by your veterinarian.
  2. Remember that many horses can live a long, productive life with this gene.
  3. Have your forage tested for potassium concentration. (Your local Southern States store can help provide forage analysis.)
  4. If you and your veterinarian decide to limit potassium intake, feeding hay, especially alfalfa, should be minimized if not eliminated.

Read the next horse diseases article on Hypothyroidism.
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