Imagine that you are a horse owner and the day starts off as most do; you finish your morning routine of feeding your horses and picking stalls, only to begin topping off water buckets as the heat index rises. You are singing along with the Keith Urban song that is playing on the radio when one of your horses noisily bangs against the stall wall. At first you yell a little warning to the troublemaker and continue your watering. But he bangs again and you suspect he may be cast. You set down the hose and calmly walk over, grabbing a lead rope since you might have to roll him over alone, something no one looks forward to.
As you approach the stall he is still, too still. He isn’t thrashing around when you get there; he is just lying there on his side with his muscles rippling and twitching. At first you think he might just be tired from getting himself rolled over, but then you see the unusual way his legs are sticking out and you are baffled. Thankfully, he isn’t dead, but he is looking at you, paralyzed, expecting you to do something, only you have no idea what is wrong. In another minute or so, he is trying to get up, but he can’t.
You are completely puzzled. It must be colic, you think, or maybe a seizure of some sort. Whatever it is, he has never showed it before. The vet is called and she comes over immediately. She instantly recognizes the stiff way the horse is laying on its side. She collects hair from his mane from the root and manages to collect some blood. She gives him an injection and tells the technician to get the Karo syrup from the truck. They give your horse the corn syrup and he begins to even out slightly. Thankfully, he hasn’t hurt himself and is just a little befuddled.
Why did this horse instantly drop like that? The cause is that four-letter word in the horse world, HYPP, and this horse had suffered an HYPP attack. HYPP, scientifically known as Hyperkalemic Periodic Paralysis, is a hereditary disease in certain Quarter horses that can trace their heritage back to one stallion, Impressive. It is also referred to as ‘The Impressive Syndrome’. The previously mentioned owner never even thought of the horse having an HYPP attack because his registration papers claim he is N/N, or ‘normal’.
According to the University of California at Davis Veterinary Genetics Library website, the disease is caused by a hereditary genetic defect that disrupts the sodium ion channel, a protein located in the membrane of muscle cells. This disruption causes uncontrolled influxes of sodium through the channel, producing the erratic voltage of current within the muscle cell that leads to muscle twitching, weakness and convulsing. This disruption may be triggered by high levels of potassium present in the blood stream.
“The sodium channel in the muscle cell is faulty by genetic design, and it gets stuck open,” said Dr. Tanis MacDonald, DVM, of Laurel, Delaware.
MacDonald compares it to leaving the back door open accidentally and coming home to a house full of flies and none of the resident cats are in sight.
“Normally, there is a perfect balance between the sodium and potassium in the cells and in the bloodstream. But when the sodium channels get stuck open, more sodium dumps into the cell, like the flies coming into the house,” MacDonald explained. This causes the muscle to twitch uncontrollably. The twitching of the cell causes the potassium to leak out of the cell into the bloodstream, like the cats that are let out into the backyard.”
Speaking biologically, HYPP has a number of genetic codes in horses. Much like hereditary genes in humans, the HYPP gene can be duplicated or not, depending on the horse and its genetic code. The UC Davis Veterinary Genetics Laboratory website states that “breeding an affected heterozygous horse (N/H) to a normal horse (N/N) will result in approximately 50% normal offspring, while 50% will carry the defective gene (N/H). Breeding an affected homozygote (H/H) will result in all offspring carrying the gene mutation, regardless of the status of the other parent.”
Characteristics of an HYPP attack are similar to the attack illustrated in the beginning of this article, but symptoms in positive horses differ depending on the individual horse. Traditionally, HYPP symptoms have been identified in horses as twitching uncontrollably and experiencing temporary paralysis, thus the sudden cease in movement and collapsing to the ground. But symptoms may not be as noticeable; the slight twitching of the skin can be overlooked as just a response to flies or some other bother. Weakness may also be mistaken for some other ailment.
There is nothing wrong with owning a horse that is N/H or even H/H, as long as the owner is aware of this at the time of the sale and is equipped to cater to the horse’s needs. When an owner or a handler does not know about the horse’s HYPP positive status, it can make decision making difficult and handling dangerous. Remember that the owner in the beginning of the article did not know what was wrong with her horse because his breed association registration papers indicated he was N/N. According to their website, the American Quarter Horse Association now requires foals born in 1998 and later with bloodlines connecting them to Impressive to have a statement on their registration papers indicating the possibility of HYPP and recommending a test.
So how did the aforementioned horse have this and why wasn’t the owner aware of the possibility of HYPP in her horse? The true reasons may never be known, but there are a number of potential answers that could be the key to this mystery.
Lab errors; faulty collection on a breeder or previous owner’s behalf; a paperwork error; the horse was older than the breed association mandates; or dishonesty are all very likely possibilities. The bottom line is that when buying a horse with bloodlines connected to the HYPP disease, a second blood and hair test may be just as necessary as the vet check. Because symptoms can be disguised as other ailments, a horse that is only mildly symptomatic can escape treatment or diagnosis before having a full attack.
“It is a well known fact that some breeders would simply substitute hair or blood from horses that were known to be N/N, for the testing of any horse that needed to be tested before the days of parentage verification,” said MacDonald. “The biggest reason for false samples submitted originally was because of resale value; the value of an N/H horse is less in most markets, not all, but most of the regular backyard people are not going to want to deal with the disease.”
Certain breed associations, such as the American Quarter Horse Association, require testing of horses with pedigrees that link them with Impressive and his offspring. If there is any doubt that your horse falls into this category, have it tested. Testing kits are available through AQHA and information regarding HYPP can be found on their website.
MacDonald owns three pleasure horses, two of which are N/H. She says that she does not mind caring for them despite the fact that they test positive for HYPP, but that it can make a difference in the decisions involved in issues such as a horse having emergency colic surgery, using sedation drugs, and choices of feed and medication, just to name a few.
HYPP affected horses have proven to lead extremely successful lives. Two of the more recognized Quarter horses of today; Vital Signs Are Good and Hope By Invitation, have achieved numerous levels of success despite their links to HYPP. Affected horses can have much success with the aid and care of educated and dedicated individuals. The catch, however, is that owners and handlers need to know if the horse is positive for the disease. Once owners and handlers know the status of the horse, they can set up a diet and medication regimen with the help of a trusted vet.
Get to know your breed association and its rules regarding HYPP. If you suspect your horse may be positive and the registration papers disagree, MacDonald suggests you re-test your horse. Consult your vet about a treatment program that is beneficial to both you and your horse. Pulling hairs with the roots intact and sending them with a kit to a reputable, approved testing lab is money well spent to put your mind at ease, especially when your family and their safety are riding on that horse.
For more information on Hyperkalemic Periodic Paralysis (HYPP) and information regarding a qualified testing lab, visit www.aqha.com or www.vgl.ucdavis.edu
Erin O. Stattel is a student at Monmouth University in Long Branch, New Jersey and is currently working on a Bachelor’s Degree in journalism and public relations. She has also attended the British American College of London in Regent’s Park, London, England. Erin resides in Marlboro, New Jersey with her family and enjoys the Jersey Shore and riding American Quarter horses. Be sure to follow Erin at Huntseathorses.com as she interviews riders for your reading pleasure.
Erin Stattel, Hunt Seat Horses.com Correspondent
