Lauren Neumann
Hyungchul Han
AVS 3305
30 October 2018
Equine Hyperkalemic Periodic Paralysis
Equine hyperkalemic periodic paralysis is a fairly recently discovered genetic disorder, having been discovered in the mid 1980’s. Equine hyperkalemic periodic paralysis, also known as HYPP, is genetic disorder generally affects quarter horses in the halter ring. It can also affect muscular breeds of horses which have been influenced by quarter horse bloodlines including paints, appaloosas, mustangs and warmbloods. It is of major concern to people involved in the horse industry as clinical attacks of HYPP can be life threatening for horses and their handlers. The maintenance of HYPP positive equines can be costly and stressful for owners as maintaining horses with this disease is essential for the health and well-being of the animal. In the horse industry, particularly in the quarter horse circles, HYPP genetic testing is essential for sales of halter quarter horses in order to control and eventually eradicate this dangerous genetic disorder.
HYPP is a relatively new disorder, making the history of the disorder concise. The origins of this genetic disorder can be traced back to one specific quarter horse sire, making the history of HYPP easy to trace and follow. Initial reports on the disease speculated that the disease was related to one particular quarter horse family, though the specific details were limited as to the original carrier was. It was officially released to the press in 1992 by the University of Saskatchewan that all of the affected equines in their research herd were descendants of a particular sire, Impressive (Naylor, 1994). The sire, known for his impressive muscling, sired more than 2,000 foals before his death in 1995 (Wood, 2012). The “Impressive Line”, once known for its incredible muscled foals and winning reputation in the halter show ring, became synonymous with this new genetic disorder. Though dealing a blow to the value of Impressive’s offspring, this information allowed for more targeted research on the disease. The research on HYPP following these findings soon led to the development of a genetic test for HYPP in 1994 (Wood, 2012). This test utilized DNA from blood or hair to determine whether an animal is a carrier of the disease or not. This DNA test is still utilized today in order to determine whether or not a particular horse is suitable for breeding purposes. The testing shall continue until the disease is eradicated in order to preserve and provide he heathiest horses possible for the industry.
HYPP is caused by a mutation in the alpha subunit of the sodium channel protein (Naylor, 1994). More specifically it is caused by a missense mutation, C to G substitution, resulting in the wrong substitution in the alpha-subunit of the skeletal muscle sodium channel alpha-subunit (Rudolph 1992). This mutation causes the sodium channels to malfunction and is a dominant trait. This malfunction causes the sodium channels to fail in the presence of excess potassium. As a result, an inward flux of sodium and outward flux of potassium begins, resulting in strong depolarization of muscle cells followed by temporary weakness (Finnow 2009). The DNA test utilized in diagnosing an animal with HYPP shows that both heterozygous and homozygous animals show symptoms of HYPP. The homozygous animal, denoted as H/H, will always pass on the disease to its offspring, thus creating a repaid spread of the disorder. 100% of the animals produced from an animal which is H/H will be a carrier of the HYPP disease. If bred to a homozygous animal, all offspring will be homozygous. This is a highly undesirable breeding pair. The heterozygous animals, denoted as N/H, are affected by HYPP but will not pass on the disease rapidly. A heterozygous animal bred to a heterozygous animal will produce a 25% possibility of a homozygous H/H animal, a 50% chance of a heterozygous N/H animal and a 25% chance of a N/N normal unaffected animal. A heterozygous animal bred to a homozygous animal will produce a 50% chance of a heterozygous N/H animal and a 50% chance of a homozygous H/H animal. Normal, unaffected animals are denoted as N/N and are ideal when breeding to produce an animal which will not spread the disease further (Finnow 2009). Animals which show a homozygous result are extremely undesirable in the breeding circuit and are generally castrated to avoid reproduction whenever possible. These animals are also unable to be registered as an AQHA horse.
The most severe portion of HYPP would be the clinical attacks which occur. Both heterozygous and homozygous animals experience clinical attacks of HYPP, though homozygous animals experience more severe symptoms more regularly. Foals which are homozygous tend to show symptoms a few days after birth, generally in the form of a respirator stridor. Horses which are heterozygous generally show symptoms at the beginning of training around ages 2-3 (Finnow 2009). A clinical attack of HYPP generally begins with muscles twitching or tensing with delayed relaxation. The animal generally sweats throughout the body, most severely on the shoulder, neck and flanks. In some cases, the nictitating membrane in the eye will prolapse, making it visible. As the attack progresses, the horse may begin to sway or lose balance (Spier 1990). A telltale sign of an attack is when an animal falls into the dog sitting position, as they are unable to stand and support their bodyweight normally. These attacks can last anywhere from fifteen minutes all the way up to sixty minutes. In the most severe cases, animals may die during an attack due to collapse or paralysis of the upper respiratory muscles (Spier 1990). Animals generally return to normal following an attack and do not show any adverse symptoms outside of an attack, though in some cases homozygous animals may still show signs of stridor or respiratory issues. After an attack, an animal will generally show abnormal fibrillation potentials and complex repetitive discharges in between clinical attacks (Naylor 1994). These results are an excellent factor in determining whether an attack of HYPP took place or if the symptoms were a result of a separate neurological disorder.
The attacks to not come suddenly and generally can be triggered by a variety of factors, the most important being diet. High levels of potassium in the diet of an HYPP affected animal can drastically increase the severity and regularity of a clinical attack of HYPP. Animals who are affected by HYPP should be given a diet which contains no more than 1% of potassium in their diet (Naylor 1994). Common feedstuffs for horses such as alfalfa hay, soybean meal, orchard grass, and molasses should be avoided as they are all known as high potassium feedstuffs for horses. Horses with HYPP should not be given electrolytes as they contain high levels of potassium as well. Animals who are sedentary are easier to maintain than active horses. Active horses require more potassium in their diets in order to perform. When dealing with an active HYPP positive horse, an animal nutritionist should be consulted so as to minimize the risk of clinical attacks for the animal (Finnow 2009). HYPP can also be managed through medication Medications which stabilize blood glucose levels by stimulating the secretion of insulin can aid in this process. The clinical attacks of HYPP can also be triggered by temperature. If an animal cools too quickly or gets too cold, the tensing of the muscles can cause an attack of HYPP (Naylor 1994). Owners of HYPP animals should take care to regulate the temperature of their animals as much as possible.
In the present day, actions are being taken to ensure this disease gets eradicated in a manner which is humane and efficient. This is largely headed by the American Quarter Horse Association or AQHA due to the breed specific nature of the disease. The AQHA no longer allows for any horse to be registered under AQHA whom is homozygous for HYPP. Furthermore, it is required that any horse who has “Impressive” bloodlines in their lineage at any point must be tested for HYPP upon birth. Horses which test heterozygous are still eligible for breeding, but it is recommended that the breeders be highly aware when breeding a heterozygous animal to another heterozygous animal so as to avoid the possibility of a homozygous offspring. This tactic has been instrumental in the prevention of the spread of HYPP disease. This, combined with the comprehensive knowledge researchers have found regarding the maintenance and care for HYPP positive equines has allowed for the disease to be greatly understood and contained through the selective breeding away from the HYPP animal. Avoidance of HYPP has changed the way that halter horses are bred as well. The traits which are valued in a halter quarter horse, are no longer the same as those synonymous with impressive bloodline. Focusing on traits other than large muscles has allowed for the reduction of HYPP as well. As selective breeding continues, it is predicted that the AQHA may one day not allow the registration of heterozygous animals to further encourage the selective breeding against HYPP. It is ultimately the goal to selective breed away HYPP completely.
Sources
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Finnow, Carrie J., et al. “Equine Diseases Caused by Known Genetic Mutations.” The Veterinary Journal, vol. 179, no. 3, 2009, pp. 336–347., doi:10.1016/j.tvjl.2008.03.016.
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Rudolph, J. A., et al. “Linkage of Hyperkalemic Periodic Paralysis in Quarter Horses to the Horse Adult Skeletal Muscle Sodium Channel Gene.” Animal Genetics, vol. 23, no. 3, 2009, pp. 241–250., doi:10.1111/j.1365-2052.1992.tb00136.x.
Spier, S J, et al. “Hyperkalemic Periodic Paralysis in Horses.” Journal of the American Veterinary Medical Association., U.S. National Library of Medicine, 15 Oct. 1990, www.ncbi.nlm.nih.gov/pubmed/2243032.