Should we be concerned about HYPP? You betcha. Is it one of the most discussed conditions facing western horses (quarter horses, paints, and appaloosas) today? Probably. And the reason is that many of today's stock breeds, especially those prominent in the show arena, can trace their ancestry back to one of America's premier champions . . . Impressive (registration #0767246). Along with his conformation and awe-inspiring musculature, it appears that this sire has also unknowingly passed along a genetic defect (listed in the American Association of Quarter Horses rulebook). And since Impressive descendants are so numerous, the prevalence of the genetic mutation is widespread.
This defect is found in the sodium ion channel gene of the muscle and its presence can cause havoc upon the horse's musculature leading to uncontrolled muscle twitching, profound muscle weakness, and possibly even death. It is found equally in males and females and is passed to offspring as an autosomal dominant trait. This means that breeding a horse with this mutation (N/H) to a horse that does not carry this gene (N/N) will result in a 50 percent chance of having a normal foal (N/N) and a 50 percent chance of a foal with the defect (N/H). Additionally, if you breed a horse with the defect (N/H) to another with the defect (N/H), you have only a 1 in 4 chance of a normal offspring. Not exactly good odds. The H/H horse will pass the trait on to their offspring 100% of the time regardless of the status of the other parent. Normal horses (N/N) (even if they have the genetic defect in their pedigree) can be bred with no fear of passing the genetic defect along.
Fortunately, HYPP can be successfully diagnosed and managed. So how is HYPP diagnosed? The American Association of Quarter Horses (AQHA) requires that a DNA test be performed on a hair sample of the horse in question. The samples can only be sent to licensed laboratories that the AQHA recognizes. They include those laboratories at the University of California at Davis, Oklahoma Blood Institute inn Oklahoma City, Oklahoma, Shelterwoods Labs in Carthage, Texas, Mann Equitest Labs in Guelph, Ontario, and NSW Agriculture in Wales, Australia. The test kits cost $35.00 and can be ordered from the AQHA. Only test kits with a properly signed identity waiver will be accepted.
HYPP positive horses may present with a wide range of clinical signs. Studies have shown that episodes usually being with a brief period of muscle tremors involving the neck and trunk muscles, which then spread out and produce a generalized shaking, trembling, and weakness. Can the clinical signs (symptoms) vary between episodes and horses? Yes, clinical signs may vary from one attach to the next and will vary from individual horse to horse. Homozygous horses (H/H) will have more severe clinical signs than heterozygous horses (N/H). Some heterozygous horses will have more severe clinical signs than others will but all have the same chance of passing the genetic defect on to their offspring.
Attacks may be associated with transportation, periods of stress, dietary changes, horse shows but it appears they are not associated with exercise (unless of course the exercise is at a stressful event, i.e., horse show). Attacks are also more commonly seen in younger horses but may occur in any age horse that carries the affected gene.
How can the disease be managed? The first step in managing the disease is to determine the status of your horse. If heterozygous (N/H) or homozygous positive (H/H), then there are certain steps that you can do to help prevent your horse from ever having an attack. First, make sure that you have a schedule for feeding and exercising and stick to it. Always make sure that your horse has an adequate supply of clean water. Don't keep your horse in a stall all the time. Affected horses do better when they are turned out to pasture or have access to a paddock. Feed grass or oat hay instead of alfalfa if at all possible. Alfalfa and orchid grass hays are high in potassium; thus, can contribute to the onset of an attack. Increasing the amount of time your horse is on pasture might help reduce the amount of hay they need. Feed hay and grain (avoid sweet feeds as the molasses is high in potassium too) in small frequent meals if at all possible. Avoid rapid changes in the diet and provide a source of white salt (loose or blocked).
When hauling be sure to stop and make frequent stops and water your horse frequently (about every two hours). You can also consult with your veterinarian about using acetazolamide (a diuretic) to control the disease during hauling and while at home. It should be noted that acetazolamide is considered a forbidden substance at horse shows by the AQHA and AHSA (American Horse Show Association). Thus, with common sense and good management HYPP horses can provide a lifetime of companionship and service to their owners.
