Hyperkalemic periodic paralysis, or HYPP, is a genetically inherited disease that affects some horses descended from the Quarter Horse stallion Impressive. This bloodline is a proven source of the HYPP gene, and the gene is dominant, meaning it only takes one gene for the horse to have the disease.
These facts should make it easy for people to figure out who has to worry and who doesn’t — but they haven’t. It should also make people avoid breeding horses with these bloodlines — but they haven’t.
This genetic defect involves the cell wall sodium-potassium pump, which is supposed to keep normal (high) amounts of potassium inside the cell and sodium outside. This is part of the mechanism that keeps muscle cells primed to contract. With HYPP, too much sodium goes into the cell and too much potassium out.
When this happens, the high potassium levels outside the cells affect the muscle’s ability to contract. With moderate levels, the heart and muscles show fine tremors and twitching, a sign of weakening. In the heart, an arrhythmia (irregular beats) may develop. If the potassium continues to rise, weakness worsens and contractions can cause the horse to go down, be unable to swallow/breathe, and the heart may stop.
An HYPP horse may appear normal most of the time. However, because his muscles constantly leak more potassium into the surrounding tissues, he is “primed” to have an attack if the right trigger comes along. Simple clinical tests like tapping briskly on a large muscle (e.g. the rump) may induce it to show odd contractions (myotonia).
If the electrical activity of the HYPP horse is recorded with EMG needles — the machine used to study nerve and muscle function — the muscle recordings are clearly abnormal at all times, even when the horse himself appears to be showing no symptoms.
Many HYPP-positive horses go for years without symptoms, have long periods between actual attacks, or only have mild, easy-to-manage attacks. HYPP doesn’t mean the horse is worthless or headed for a fatal attack of HYPP. What it does mean is a lifetime of careful management that often includes avoiding stressful situations, such as shipping, showing and prolonged or strenuous exercise.
Homozygous horses (those with two HYPP genes) tend to be more severely affected than heterozygous (one normal, one HYPP). Mild symptoms include muscular fasciculation (twitching/trembling) that is beyond the horse’s control and some weakness or reluctance to move.
First-time attacks are often confused with tying-up, but HYPP horses don’t show the rigid stances and rock-hard muscles seen with tying-up nor have discolored urine. Blood work will show either normal or only mildly elevated muscle enzymes compared to the dramatic elevations seen with tying-up.
More severe attacks may involve arrhythmia, gurgling sounds when breathing, inability to swallow, difficulty breathing if the diaphragm is significantly weakened and an inability to expand the chest, profound weakness and collapse. With the most severe attacks, the horse may die from cardiac and breathing complications unless he receives emergency treatment (see sidebar at end of story).
Most HYPP horses are diagnosed by the time they are four. They probably show subtle symptoms long before the time of diagnosis, but it is not until they are regularly worked that someone happens to notice.
As a rule, attacks become less frequent as the horse ages, possibly because he becomes less reactive to day-to-day stresses. However, this is not an absolute, and management remains paramount. Frequent moderate-to-severe attacks that are not controlled by management and/or medication can lead to irreversible muscle weakness.
Attacks for most horses can be minimized with correct management. The key element is avoiding high dietary potassium (see sidebar at end of story), while maintaining normal levels (too little is also bad).
Any supplements containing potassium are forbidden. That’s easy. The problem is that all common feeds contain potassium, and you can easily end up with too much.
Beet pulp is the lowest in potassium of all the common feeds. Plain grains, such as oats, are also fairly low (not molasses-heavy sweet feeds, however). A common recommendation is that HYPP horses be fed half their required calories as grain and half as hay or hay and beet pulp. In fact, feeding a beet-pulp-based complete feed is a popular approach, but be careful. Many beet-pulp-based feeds contain 10% or more molasses. Molasses is extremely high in potassium. If the potassium level is not on the label, ask the manufacturer before you use it. Vegetable oil is good in formulating an HYPP diet, especially for the hard keeper. Oils contain no potassium.
A common myth is that alfalfa hay is higher in potassium than other hays. This only applies to hay that was cut and baled in very early stages of blooming. Later cuttings of alfalfa, recognizable by thick stems and obvious flowers, have no more — sometimes even less — potassium than some common grass hays. If you feed hay cubes, ask the feed manufacturer what the potassium content of the cubes are.
Your veterinarian or an equine nutritionist should be consulted when designing a diet for the HYPP horse. Your goal will be to get as close as possible to the actual daily requirement without exceeding it, keeping in mind HYPP horses have a somewhat higher requirement because of their higher losses.
Equally important in the HYPP diet is water — lots of it. The horse regulates his blood and tissue potassium levels by excreting excesses in the urine. Even a slight degree of dehydration will make an attack more likely. Free-choice plain white salt, block or loose, should also be available at all times to maintain normal hydration.
Designing A Diet
Keeping potassium low in the diet isn’t easy. A horse maintained on late growths of timothy pasture is getting about three times as much potassium as he needs if in moderate work, five times more than he needs for maintenance. The same horse kept on midbloom timothy hay gets twice as much as he needs for moderate exercise, three times too much for maintenance.
All hays are significantly higher than grains in potassium. Orchard grass hay is among the highest, higher even than mid and late cuttings of alfalfa. As a general rule, pick a “poor” quality (but clean) late-growth stage of hay and count on it only for the roughage your horse needs in his diet. To be safest, you should locate a steady source of the same hay (from same region) and have it analyzed first for potassium.
You want a potassium level no higher than about 1.4% (lowest you can reasonably expect). Feed this at 1% of the body weight, e.g. 11 pounds/day for an 1,100-pound horse.
Using National Research Council averages, if you use late-cutting timothy, you will end up with 71 grams of potassium, just 7 grams short of what the horse needs for the day if in moderate work but only about 1/3 of the required calories. For maintenance, this amount would give half the required calories but now be 36% over the required amount of potassium. Not a good start but it’s the best we can do given that the horse needs the fiber from hay.
The two lowest potassium options to use to boost calories are oats and beet pulp. However, using either one of these ends up with severe deficiencies/imbalances in both major minerals, calcium, phosphorus, magnesium) and trace minerals.
A better choice is to mix them in equal amounts (about four pounds of each as maintenance for an 1,100-pound horse). This gives the horse an excellent major-mineral profile, but you will need a good trace-mineral supplement.
We suggest either Vita-Key Equine Supplement (800/539-8482) or Base Mix P55 from Uckele (800/248-0330), both complementary to the mineral profile in this diet and neither of which has a vegetable material base. You might also consider an individu alized supplement program based on hair-mineral results.
Many horses with HYPP are maintained with the drug Diamox. When used in conjunction with a potassium-reduced diet, Diamox may make a big difference in the frequency and severity of attacks. This drug works by increasing the urinary excretion of potassium, helping to avert attacks in the event blood levels begin to rise.
There are other diuretics (e.g. thiazides or furosemide/Lasix) that also cause potassium loss, but some are not as beneficial. Diamox is believed to also improve the sensitivity of the muscle cells to insulin, helping to drive any excess potassium leakage right back into the cells where it belongs.
Moderate regular exercise also stabilizes hormone levels and blood sugars. The problem can be determining the difference between moderate and overdoing it, which varies with individual horses.
Any concurrent stress, such as a low-grade viral infection you may not even know about, will also potentially magnify the stress that exercise puts on the horse. The best approach is to keep exercise to a sensible level but always be alert for the development of HYPP symptoms for the first few hours after exercise.
In addition to constant grazing, turnout allows a low level of exercise, helping to stabilize blood sugar and insulin levels that results in a more stable potassium picture.
Whenever possible, avoid shipping the horse if you can. However, if that’s not possible, be sure you can stop at least every two hours during the trip to water the horse.
Also With This Article
Click here to view “HYPP Control And Hair Mineral Analysis.”
Click here to view “HYPP: Valid Questions.”
Click here to view “Dietary Potassium Management.”
Click here to view “HYPP Emergency Treatment.”