Hyperkalemic Periodic Paralysis (HYPP) is a genetic disease that is especially unique because it can be traced back to a single quarter horse sire named Impressive. This disorder affects over 4% of all quarter horses.

It is also seen in other breeds with quarter horse lineage, all traced back to Impressive. During his career, Impressive was a championship halter horse that went on to lead a successful breeding career. He sired 2250 foals, many of which became champions themselves. However, within a few short years, many of Impressive’s offspring began experiencing muscle weakness, muscle twitching, complete paralysis and in some instances, collapse and death. This led to the diagnosis of HYPP. Other signs of this disorder include:

  • Appearance of “tying up” or suffering from colic
  • Body trembling
  • Third eye prolapse
  • Loud breathing noises
  • Exercise intolerance
  • Muscle stiffness

The HYPP disorder itself affects normal muscle function by disrupting the sodium ion channels within the muscle cells themselves. It also disrupts potassium balance in the blood. With this disease, the sodium ion channels to the muscle cells become “leaky.” This means the cells are more excitable and when triggered by fluctuating potassium in the blood, they will contract uncontrollably or it may even lead to muscle paralysis. This is a major concern as paralysis of the muscles controlling breathing or heart function can lead to death.

Many confuse HYPP with “tying up.” This is understandable, as tying up resembles HYPP with similar painful, continuous muscle contractions. However, tying up usually happens either during or after strenuous exercise. Symptoms of HYPP can appear almost anytime when the horse is at rest, or is often observed after a stressful event like exercise, after transport, or even after an illness. HYPP can also seem to appear out of nowhere to an unsuspecting horse owner. What we do know is HYPP is tied to when the horse has too much potassium circulating in their blood.

One of the most critical aspects of managing a HYPP afflicted horse is dietary management. Because potassium levels are strongly correlated with HYPP, the goal of any feeding program should be to feed low levels of potassium. Research studies suggest HYPP horses should be fed with no more than 1.5% potassium of the total diet and individual meals should not exceed 33 grams of potassium. This can be quite difficult as most hays are higher in potassium than most commercial feeds and cereal grains. Thus, feedstuffs that are higher in potassium should be avoided for a HYPP horse, and some of these include:

  • Alfalfa
  • Brome grass
  • Kentucky bluegrass

Pasture grazing for an HYPP horse is generally considered safe. Because of the high moisture content of the grass, it usually will offset any potassium excess.

While some hays are considered safer, they can exceed the low potassium threshold and trigger HYPP. This is because hays differ so much in quality and quantity of potassium due to where they are grown, rainfall or irrigation levels, fertility of the soil, and other factors. Thus, it’s always recommended to get your hay tested to determine potassium levels.

It is important to remember that commercial feeds will all vary in their total potassium levels. With an HYPP horse, owners should always look at the feed tag or contact the specific company for potassium levels in their feeds. At Tribute®, we include potassium as one of the listed nutritional values. Some of our feeds with less than 1% total potassium include:

Just feeding commercial feeds or hays lower in potassium are not guaranteed to prevent symptoms of HYPP. Thus, it is also highly suggested to reduce meal size and feed your HYPP horse more often and at minimum, four times a day. It is also recommended to allow as much time between meals as possible. This will help reduce circulating potassium levels in your horse’s blood. With more frequent and smaller meals spread throughout the day, less potassium will be circulating and reduce the chance of symptoms or severity of HYPP.

Genetic testing is the only way to verify if your horse is a HYPP carrier. Therefore, a veterinarian should evaluate your horse if you suspect HYPP. They may offer other treatments that can be helpful to your horse.  

All in all, HYPP is a dangerous condition for your horse if they are a carrier. It is critical that you remember nutritional management is imperative. As an owner, you must ensure your horse’s potassium levels remain low in their diet to reduce the instances of episodic HYPP. We highly suggest you contact us for advice in formulating a proper diet for your HYPP horse.

Nicole Rambo, Ph.D., Chris Mortensen, Ph.D.