Hyperkalemic periodic paralysis (HYPP) is a genetic condition in horses that affects how their muscles function and respond to signals from the nervous system. These horses have high levels of potassium in the blood and should be fed low potassium diets.

Horses affected by HYPP have pronounced musculature which is sometimes perceived as a desirable appearance. They may also perform better in halter classes. [1]

This aesthetic advantage could have contributed to the increase in affected horses through breeding practices that selected for these traits.

Horses that carry the genetic variant that results in HYPP typically begin to show signs around 2-3 years of age. Some horses may never show signs of the condition but are still genetic carriers that can have affected offspring.

The most common sign of HYPP is periodic muscle tremors and general muscle weakness. These horses have defects in how their muscle cells handle sodium and potassium resulting in high levels of potassium in the blood.

Horses with HYPP should be fed a low potassium diet and should avoid feeds such as alfalfa hay or soybean meal.

For help with formulating an appropriate diet for a horses with HYPP, submit your horse’s information online. Our nutritionists can help you design a feeding plan to minimize HYPP symptoms for free.

Hyperkalemic Periodic Paralysis

HYPP is a muscle disorder resulting from a gene mutation that affects ion transport in muscle tissue. Specifically, it impairs the normal function of voltage-gated sodium channels in muscle cells.

HYPP is most commonly seen in Quarter Horses, Paints, Appaloosas and crossbreeds. It is estimated that 4% of Quarter Horses alive today have HYPP. [2] [3]

This genetic condition originates from a popular Quarter Horse sire named Impressive. He was born in 1969 and is known to have sired over 350,000 Quarter Horse offspring.

It is now mandatory to do genetic testing of foals born to descendants of Impressive to determine whether they carry the known genetic mutation that causes this condition and to discourage breeding of affected horses. [2]

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Effects of HYPP in Horses

For movement to occur, your horse’s muscles receive signals from nerves to initiate the contraction of the muscle.

This results in brief activation of sodium channels in the muscle cell membrane that pump sodium into the cell and move potassium out into the blood.

In non-HYPP horses, these channels are only briefly open and move the appropriate amount of ions needed to stimulate muscle contraction.

In horses with HYPP these channels stay open for longer, resulting in excessive sodium moving into the cell and potassium moving out.

This causes prolonged depolarization of muscles, meaning they may be unable to respond to the next neural signal which causes temporary paralysis and muscle weakness. [4]

The resulting high levels of potassium in the blood can cause cardiac dysfunction leading to a potentially fatal heart attack.

Luckily this prolonged opening of sodium channels doesn’t happen all the time. Most HYPP horses will show signs of the condition intermittently with no obvious issues between episodes.

Diagnosis

Definitive diagnosis of HYPP is done by performing a genetic test that looks for the specific gene mutation.

Horses that are descendants of Impressive and show periodic muscle trembling, weakness or collapse are very likely to have HYPP and should be assessed by a veterinarian to get a firm diagnosis.

Horses that have two copies of the mutation, from both the mother and father (homozygous), will be more severely affected.

Those that only have one copy of the mutation from either the mother or father (heterozygous), will likely be less severely affected and may show no symptoms. [5]

Knowing whether your horse has one or two copies of the gene can help you understand the potential severity of the condition. Horses known to carry this genetic mutation (even without showing symptoms) should not be bred to avoid passing on this condition.

Signs of HYPP

Signs of HYPP can vary greatly between individual horses. Some horses that carry the gene mutation never show signs of the condition, while others experience daily muscle tremors and weakness.

Affected horses often go long periods without showing symptoms between episoides.

They may experience temporary episodes of muscle paralysis that can range from mild muscle tremors and cramping to severe symptoms that can affect their ability to breathe.

HYPP Episodes

An episode of HYPP paralysis commonly follows a similar sequence in most horses: [2]

  1. Brief muscle weakening, commonly with prolapse of the third eyelid
  2. Sweating and muscle tremors, especially in the flanks, neck and shoulders
  3. Progressively affects more muscle groups
  4. Most horses show mild to severe muscle weakness with some unable to stand; may exhibit staggering, swaying or dog sitting
  5. Some horses progress to more severe muscle cramping
  6. Horse becomes anxious and have increased heart rate and rapid breathing

Most episodes last 15 minutes to an hour. In severely affected horses, the muscles that control breathing can become paralyzed leading to respiratory distress which can be fatal.

Veterinary Care

Horses showing any signs of respiratory or cardiac distress should have immediate veterinary attention.

Although all HYPP horses should be carefully managed and monitored during an episode, those known to be homozygous for the genetic mutation should get veterinary attention during an episode.

They will be at higher risk of developing severe symptoms such as inability to breathe which can require a tracheostomy to assist breathing.

Exercising a HYPP Horse

Following an episode, horses generally appear normal and are able to walk unaffected or with minimal change to their gait.

Horses known to carry the HYPP mutation should only be ridden by experienced riders as episodes can occur rapidly and without notice, causing danger to both horse and rider

Treatment

Horses will often recover from an episode on their own. However, it is always a good idea to contact your veterinarian when you see symptoms of an HYPP episode arise.

If the horse is showing signs for the first time and only displaying mild tremors, light exercise can help stop the episode. However, excessive movement can make the cramping worse.

Feeding grain or a high sugar feed such as corn syrup can be useful. This will stimulate insulin secretion which helps move potassium out of the blood stream and back into cells.

Medications

Your veterinarian may choose to administer treatments to help abort the episode or decrease its severity: [6]

  • Administer an injection of epinephrine
  • Oral acetazolamide every 8-12 hours
  • Intravenous dextrose to stimulate movement of potassium back into cells
  • Intravenous infusion of calcium gluconate to depolarize the cell membrane and return it to the resting state

Oral acetazolamide can also be used on an ongoing basis to prevent episodes. It works by increasing potassium excretion by the kidneys to lower blood potassium levels.

Your veterinarian will decide which treatment or combination of treatments to administer based on the severity of symptoms and your horse’s medical and dietary history.

Always inform your veterinarian if your horse is known to carry the HYPP mutation. Anesthesia or heavy sedation can cause complications in these horses. [2]

Prognosis

Most horses with HYPP recover from an episode within 15 minutes to an hour and show minimal symptoms between episodes. However, severe cases that develop respiratory distress can be fatal.

Horses with HYPP may also have irregular heart function even without showing outward symptoms. [2]

Those with just one copy of the mutation can have a good quality of life as they are likely to have less severe episodes. They can be well-managed with an appropriate diet. Horses with two copies are more likely to develop severe episodes and can be more difficult to manage leading to a poorer quality of life.

How to Feed a Horse with HYPP

The main way to minimize the frequency and severity of HYPP episodes is to decrease dietary potassium intake. Potassium is easily absorbed from the gut and can result in a spike of levels in the blood after a meal. [7]

Forages are the main source of potassium in the equine diet.

This mineral is also found in high amounts in electrolyte supplements, molasses and kelp meal. [2]

High Potassium Feeds to Avoid

  • Sugar or Beet Molasses
  • Electrolyte supplements
  • Kelp meal
  • Alfalfa hay
  • Brome hay
  • Canarygrass hay
  • Orchardgrass hay
  • Soybean meal

Medium to Low Potassium Feeds

Horses with HYPP should only be fed low- or medium-potassium feeds including:

  • Later cuts of timothy or bermuda grass hay
  • Fescue hay
  • Soybean hulls
  • Beet pulp
  • Rice bran
  • Pure oils such as canola oil, flax oil or w-3 oil

It is important to consider both the concentration of potassium (%) in the diet as well as the total grams of potassium per meal.

Horses with recurrent episodes of HYPP should be fed diets that contain 0.6 – 1.5% potassium. A single meal should contain no more than 33 grams of this mineral.

If a low-potassium diet is not feasible, horses with HYPP can adapt to high-potassium diets.