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Hyperkalemic Periodic Paralysis

By: Dr. Melissa Mazan

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Over 20 years ago, a horse called Impressive was winning all the halter classes. Impressive was well-named – he was so strikingly muscled that he never lost a halter class. It is easy to understand why such a grand specimen would be in huge demand as a breeding stallion. And Impressive showed that he had a very important trait as a breeding stallion – he had the ability to pass on his majestically sculptured musculature to his get. It wasn't for quite some years that people started noticing that Impressive's offspring had inherited more than just his looks.

For the gift of Impressive's massive muscles came with a far less welcome inheritance, which came in the form of intermittent episodes of weakness, collapse, or even death. When veterinarians found that these attacks were accompanied by a high level of potassium in the blood, the inherited disease was named hyperkalemic periodic paralysis (HYPP).

Today, HYPP continues to plague the quarter horse industry, although our ability to identify affected individuals, coupled with responsible action on the part of breeders, has helped us to gain control over this genetic, or inherited disease. We have learned much about this disease, yet it persists in the quarter horse and horses with quarter horse blood, such as Appaloosas, paints and palominos.

Although we have named the HYPP for the characteristically high potassium levels that occur during an attack, HYPP is actually due to a mutation in a gene that governs sodium channels in muscle cells.

Genes carry coded messages to the protein-making apparatus in the body. These proteins eventually become things that are as diverse as muscles, hooves, and microscopic channels in muscle cells. Mutations tend to occur spontaneously in certain individuals – indeed, we all likely will have mutations in our genetic code at some point – but, generally, no outward effect is seen. Occasionally, as with the Impressive mutation, the consequences are enormous.

As far as we know, HYPP is seen only in horses that have inherited the gene for the disease of Impressive's bloodline. That is why we generally see the disease in quarter horses and horses that have quarter horse blood, such as Apaloosas, paints and palominos. As fate would have it, not only did Impressive have a genetic mutation for his muscle sodium channel, but he also had an outwardly very desirable trait – his musculature – that resulted in him being a leading sire. Thus, by the early 1990s, over 50,000 quarter horses had Impressive inheritance. So, Impressive's physique was, in a way, a Trojan horse that allowed a huge and consequential flaw to be transmitted to a large number of horses.

HYPP in the horse is a dominant trait. We all receive two copies of most genes – one from our mother and one from our father. With a dominant trait, the mutation only has to be present on one copy of the gene in order for the trait to be seen (heterozygous). With recessive genes, on the other hand, both genes must have the mutation for the disease to be seen (homozygous).

So, if an HYPP positive horse is bred to an HYPP negative horse, at least 50 percent of their offspring will have HYPP, and up to 100 percent of their offspring will have HYPP, depending if the positive horse has inherited the trait from both parents or only from one. It also seems that homozygous horses are more severely affected than heterozygous horses – it may be that with two copies of the abnormal gene, more of the muscle cells are affected.

What to Watch For

  • Muscle trembling
  • Prolapse of the third eyelid. You may note the third eyelid flickering across the eye or covering more of the eye than normal
  • Generalized weakness
  • Weakness in the hind end. The horse may look as though it is 'dog-sitting'.
  • Complete collapse
  • Abnormal whinny because the muscles of the voicebox are affected as well as other muscles

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