Equine Hyperkalemic Periodic Paralysis (HYPP)

1. What is HYPP?
2. What are effects of HYPP?
3. What is the origin of the genetic mutation causing HYPP?
4. What are the chances of a positive horse producing a positive foal?
5. How is HYPP diagnosed?
6. Why should veterinarians, breeders and showers of halter type horses be concerned about HYPP?
7. Where and how can I have my horse tested?

What is HYPP?

Hyperkalemic periodic paralysis (HYPP) is a muscular disease that affects both horses and humans. It is caused by a hereditary genetic mutation that disrupts a protein called a sodium ion channel, a tiny gateway in the membrane of muscle cells. The mutation affects the channel's normal opening and closing, such that uncontrolled sodium influxes occur. These influxes in turn change the voltage potential of muscle cells, causing uncontrolled muscle twitching or profound muscle weakness. High levels of potassium in the blood usually are present when the disruptions in the ion channel occur.

What are effects of HYPP?

Horses with HYPP can experience unpredictable attacks of paralysis which, in severe cases, can lead to collapse and sudden death. The cause of death usually is cardiac arrest and/or respiratory failure. The disease is characterized by intermittent episodes of muscle tremors manifested by generalized or localized shaking, trembling and weakness. Occasionally, episodes are accompanied by respiratory noises resulting from paralysis of the muscles of the upper airway.

What is the origin of the genetic mutation causing HYPP?

The original genetic mutation causing HYPP was a natural mutation that occurred as part of the evolutionary process. The majority of such mutations, which are constantly occurring, are not compatible with survival. However, the genetic mutation causing HYPP produced a functional, yet altered, sodium ion channel. This gene mutation is not a product of inbreeding. The gene mutation causing HYPP inadvertently became widespread when breeders sought to produce horses with heavy musculature, but this does not mean that all horses with well developed musculature possess the disease. The mutant gene causing HYPP presently has been identified in the descendants of the horse "Impressive". The American Quarter Horse Association has more than 102,000 descendants of "Impressive", the American Paint Horse Association and the Appaloosa Horse Club also have registered a substantial number of impressive-bred horses, but have no precise figure. In a recent study (Naylor et al, 1992. J. Am. Vet. Med. Assoc., 3:340-343), it was found that over 50,000 registered Quarter Horses are positive for HYPP. Thus, the disease may be widespread, and many of the most successful show horses may be HYPP positive.

What are the chances of a positive horse producing a positive foal?

If a horse carrying one copy of the mutant gene - a heterozygote - is mated with an unaffected horse, there's a 50 percent chance that the foal will have HYPP. If the horse carries two copies of the mutant gene - a homozygote - all offspring will be HYPP-positive, regardless of the other parent. If two heterozygotes are mated, there's a 75 percent chance that the foal will be affected. Two unaffected horses cannot produce a positive foal. Homozygous horses are affected more severely than heterozygous horses. Under ideal management practices, a mutant gene does not appear to have adverse effects, but stress and/or increased potassium in the serum can trigger clinical signs of muscle dysfunction.

How is HYPP diagnosed?

The best way to determine whether a horse is HYPP positive is a DNA test. The identification of this gene mutation is the basis for the DNA test used to diagnose HYPP. There is no cure for HYPP, but a consistent diet and regular exercise may minimize the risk of attack.

Why should veterinarians, breeders and showers of halter type horses be concerned about HYPP?

Veterinarians are likely to encounter the problems in pre-purchase examinations of Quarter horses, Paint and Appaloosa horses. Affected horses usually appear clinically normal and yet the presence of HYPP has major adverse implications for breeding, insurance, and, possibly, for strenuous performance. This HYPP DNA test offers veterinarians and breeders a tool which can be applied as a safer and more reliable diagnostic test for this disease, and thus allow to direct their breeding programs. To eliminate this trait from certain lines, stallion and mare, owners should have their horses tested prior to breeding and select normal horses for their breeding stock. If only unaffected horses are used for breeding, no HYPP-affected offspring wil1 be produced. Without prompt action, inbreeding will produce more homozygous affected individuals and the indications are that this may greatly increase the expression of serious clinical disease. Veterinarians can play an important role in testing suspect horses and in education the public about this condition.

Where and how can I have my horse tested?

VITA-TECH Canada Inc. proudly offers the HYPP DNA test at a highly competitive rate. Simply submit a completed requisition form and a whole blood sample (1-5 ml) in an lavender top EDTA Vacutainer tube labeled with the horse's name, the name of the owner or the responsible agent. Requisition forms are available upon request at no extra cost. Th sample may be shipped directly to us using any two-day or next-day courier or mail service in a padded and leak-proof package. U.S. clients are asked to attach an importation permit to the package which we also provide at no extra charge.

We are proud to offer this quality service and look forward to your patronage. Our friendly, helpful staff is available to answer any questions you may have from early morning to late evening, six days a week. For inquires regarding HYPP and other DNA tests or any of our other services, please do not hesitate to contact us.