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Hyperkalemic Periodic Paralysis

Fact Sheet | Glossary | More ResourcesInterview with Dr. Spier | Test Kit Order Form

HYPPFor some time now, you have probably heard, or had first-hand knowledge, of the condition known as hyperkalemic periodic paralysis (HYPP). This condition is characterized by intermittent episodes of muscle tremors (shaking or trembling, weaknesses and/or collapse).

At the 1996 AQHA Convention in Seattle, Washington, the AQHA Board of Directors approved some rules recommended by the AQHA Stud Book and Registration Committee and approved by the Board of Directors. Among the changes was a rule requiring disclosure of HYPP status on the registration certificates of foals born on or after January 1, 1998, which descend from any bloodline determined to carry the HYPP gene.

Beginning with the 1997 AQHA Official Handbook, HYPP is in rule 205 among conditions commonly considered undesirable traits or genetic defects, such as parrot mouth and cryptorchidism. These conditions do not prevent a horse from being used as breeding stock or from participating in AQHA-approved events, subject to rules of the individual event.

Beginning with 1998 foals, the rule requires the following notification to be placed on the registration certificates of foals descending from any bloodline determined to carry the HYPP gene:

"This horse has an ancestor known to carry HYPP, designated under AQHA rules as a genetic defect, AQHA recommends testing to confirm presence or absence of this gene."

Facts about HYPP have been gained through research projects funded in part by AQHA, through the University of California, Davis and the University of Pennsylvania. The first report, from Drs. Sharon Spier and Gary Carlson of U.C. Davis, was delivered to AQHA in the summer 1992, and published in-full in the September 1992 issue of The Quarter Horse Journal. As additional information has been made available, AQHA has promptly published it. I invite you to refer to [the Publications] page for a list of AQHA publications and others which contain information about HYPP.

AQHA has a duty to its members and American Quarter Horse owners to keep them abreast of current information on HYPP, and indeed, all health matters, so that they may make informed decisions concerning their equine programs.

AQHA's HYPP Fact Sheet

  • Hyperkalemic periodic paralysis (HYPP) is an inherited disease that leads to uncontrolled muscle twitching or profound muscle weakness, and in severe cases, may lead to collapse and/or death.
  • HYPP is listed as a genetic defect in AQHA's rules, along with Parrot Mouth and Cryptorchid conditions.
  • To date, HYPP only has been traced to descendants of IMPRESSIVE, #0767246.
  • Having negative (N/N) results on file may prevent a horse's offspring from being tested.
  • Foals born in 1998 and later and tracing to IMPRESSIVE will have a statement placed on their Certificates of Registration that recommends testing for the condition unless test results indicating the foal is negative (N/N) are on file with AQHA.
  • AQHA will test any foals who are required to be parentage verified and who trace to IMPRESSIVE for HYPP prior to them being registered. This testing will be performed with the same DNA sample submitted to the laboratory for parentage verification.
  • AQHA will accept HYPP test results only if performed through a licensed laboratory.
  • Beginning with the 2007 foals, all Impressive progeny are required to be parentage verified and HYPP tested subject to the conditions listed in rule 205. Any that test H/H will not be eligible for registration.
  • Possible results of HYPP testing are N/N, N/H and H/H. HYPP is inherited as a dominant trait, which means a heterozygous (N/H) stallion or mare bred to a normal (N/N) horse will result in approximately half of the offspring being affected and half being normal. The rare homozygote (H/H) usually is severely affected with the disease and will pass the gene to its offspring 100 percent of the time.
  • HYPP can be treated through diet and medication in most cases.
  • AQHA Testing Kits can be ordered for $40 when requested with a new foal registration, or $50 for a registered horse.

HYPP Glossary

Sodium channel - A membrane "pore" or channel in the muscle membrane which opens and closes allowing for exchange of the electrolyte sodium from outside to inside of the muscle cell. Proper function of the sodium channel is vital for electrical activity and contraction of the muscle fibers. There are two parts to the sodium channel, the larger alpha-subunit and the beta sub-unit. The HYPP mutation causes a change in the protein structure of the alpha-subunit. 

Allele - One copy of a pair of genes located at the same location in paired chromosomes. One allele is inherited from the sire, and one from the dam, for each gene.

Heterozygote - An individual possessing different alleles for a given gene or trait. For example, the classification N/H is given to horses which contain one normal sodium channel allele and one altered allele.

Homozygote - An individual possessing identical alleles for a given gene or trait. A horse may be homozygous normal as in the classification N/N, or homozygous affected H/H .

Myotonia - Increased muscle irritability which results in sustained muscle contraction or delayed muscle relaxation. A muscle cramp or spasm where the muscle fails to relax normally.

mRNA - Messenger RNA is the intermediate template providing the code to assemble amino acids.

More Resources For HYPP-Related Information  

Many persons have requested a list of publications on HYPP. AQHA's official publications, including The American Quarter Horse Journal, are excellent resources on issues affecting the American Quarter Horse breed and the industry. Click here for subscription information. The following are publications/articles on equine hyperkalemic periodic paralysis:

• Cox, J.H., DeBowes, R.M. "Episodic weakness caused by hyperkalemic periodic paralysis in horses." Comp Cont Educ Pratt Vet (Equine) 1990; 12:83-89.

• Naylor J.M., Robinson J.A., Bertone J. "Familial incidence of hyperkalemic periodic paralysis in Quarter Horses." J Am Vet Med Assoc. 1992; 3:340-343.

• Pickar J.G., Spier S.J., Snyder J.R., et al. "Altered ionic permeability in skeletal muscle from horses with hyperkalemic periodic paralysis." Am J Physiol (Cell Physiol) 1991; 26O:C926-C933.

• Rudolf J.A., Spier S.J., Byrns G. Hoffman E.P. "Linkage of hyperkalemic periodic paralysis in Quarter Horses to the horse adult skeletal muscle sodium channel gene." Animal Genetics 1992, 23~241-250.

• Rudolf, J .A., Spier, S.J., Byrns, G. Et al. "Periodic paralysis in Quarter Horses: a sodium channel mutation disseminated by selective breeding." Nature Genetics 1992; 2:114-147.

• Spier S.J., Carlson, G.P. "Hyperkalemic periodic paralysis in certain registered Quarter Horses." The Quarter Horse Journal, September I992, p.p. 68069, 120. • Spier, S.J. Carlson, G.P., Holliday, T.A, et al. "Hyperkalemic periodic paralysis in horses." J Am Vet Med Association 1990; 197:1009-1017.

• Steiss J.E., Naylor J.M. "Episodic muscle tremors in a Quarter Horse: Resemblance to hyperkalemic periodic paralysis." Cn Vet J 1986; 27~332-335.

Questions and Answers: An Interview with Dr. Sharon Spier

Did Impressive's ancestors pass along HYPP to him? 

We are unable to answer this question because there are no living ancestors to test and we have no genetic material from these horses to test. 

Why are most horses tested heterorygous rather than homozygous? 

This genetic disease is inherited as a dominant trait, which means that only one copy of the mutated gene must be inherited to possess the disease. Fifty percent of all offspring of heterozygotes will possess the disease. The mating of two heterozygotes will produce 25 percent homozygous-affected foals, 50 percent affected and 25 percent homozygous normal. 

Isn't there a proportionately greater number of heterozygotes tested than homozygotes-proportionately meaning greater than what the statistics would indicate would exist within the population? 

There is a higher than expected number of heterozygotes in this pedigree. Theoretically, if breeding stock were selected at random with respect to HYPP, the gene frequency would decrease with each generation. The high frequency of HYPP positive that we found in our research suggests that more affected horses were maintained as breeding stock than normal horses. The most likely reason is that the trait has been selected for by breeders seeking phenotypic characteristics which may be linked to this gene. 

Is there any relation between the number of times Impressive traces to one particular stallion [Three Bars (TB)] and this - or any - genetic mutation? 

No, there does not appear to be any association. 

How would you respond to someone who said a horse had to be HYPP-positive to win at halter? 

We were curious as to what effect the gene would have on muscle mass, which is just one of the qualities that are selected for in halter horses. We studied muscle cell (muscle fiber) diameter and muscle fiber type distribution (slow twitch and fast twitch fibers) in HYPP positive and negative horses. We found no relationship between large muscle diameter and the gene mutation. There was no difference in muscle fiber types between HYPP-positive and HYPP-negative horses. While there may be some other effect the mutation has on the appearance of muscle that we could not measure (for example, an increase in muscle tone), the appearance of heavy muscling is regulated by a separate group of genes. Impressive had numerous qualities, including excellent conformation, which gave him tremendous success in the halter ring. The DNA test allows breeders to preserve the other qualities of this bloodline yet select away from the genetic disease. Decreasing the incidence of HYPP is important for the long-term health of the Quarter Horse breed. 

If a horse is HYPP-positive, but is asymptomatic, does it lower the chances of its offspring being HYPP-positive? 

No, the gene mutation is identical for those horses that are asymptomatic and those horses that require medication to control symptoms. The expression of clinical symptoms is quite variable, which is identical to what occurs in humans with HYPP and occurs with many other genetic diseases. Unfortunately, offspring of asymptomatic HYPP-positive horses have just as high a chance of inheriting the gene (50:50 chance) and just as high a chance of showing clinical symptoms as other offspring of HYPP-positive horses. 

What other equine health problems could conceivably have a genetic link? 

There are numerous other equine health problems with a probable genetic basis. Many conditions are under intensive study in hopes to identify the casual genes. HYPP is the first equine disease which can be identified by a DNA test, but it will certainly not be the last. Some examples of other inherited defects of horses include parrot mouth, lethal white syndrome, combined immunod eficiency, cerebellar abiotrophy, epitheliogenesis imperfecta, hyperelastosis cutis, degenerative suspensory ligament desmitis, recurrent uveitis, polysaccharide storage myopathy, osteochondrosis and limb deformities. 

Do you personally feel additional research on HYPP is needed?

HYPP is just one of the many problems that can plague our horses. Research on other diseases of muscles is ongoing and needs additional support. Colic and laminitis remain the leading causes of death in horses and are major areas of research. While there are still many unanswered questions about HYPP, we do know a great deal. We know how to treat the symptoms and can control the disease in horses very well. We now have a useful tool to prevent the disease in future generations. Personally, while I do not wish to downplay other with this disease, I feel that some of these questions are more of an academic interest only.