Rockin' G Ranch
What everyone needs to know about
HYPP- The Facts
Hyperkalemic Periodic Paralysis
It is heart breaking to purchase a horse only to find out it has HYPP.   We had a mare
that was HYPP/NH several years ago.  We have found it very surprising to find out how
many people still do not understand  or have the correct knowledge about the

This has prompted us to share some facts about
HYPP.   Two facts that are commonly
misconcepted  are:

1.) If a horse is HYPP/NH they are just a carrier and can not pass it on or exhibit any
symptoms of the disease its self.
           Unfortunately a horse that is HYPP/NH as well as HYPP/HH can exhibit symptoms
and can pass it on to their off spring.

Misconception #2)
If a horse has Impressive in their bloodline and test HYPP/NN it can still show up
somewhere in their off spring.
          Once a horse has tested HYPP/NN, it does not have the disease and there for
can not pass it on as long as they are not bred back to a horse that is NH or HH positive.

The information that we are supplying here was taken directly from Veterinary Genetics
Laboratory, UC Davis .  For laboratory tests or more information click on links in this
article.  I hope this information is helpful to those of you who are not familiar with this
Hyperkalemic periodic paralysis (HYPP) is an inherited disease of the muscle, which is caused by a
genetic defect. In the muscle of affected horses, a point mutation exists in the sodium channel gene and is
passed on to offspring.

Sodium channels are "pores" in the muscle cell membrane which control contraction of the muscle fibers.
When the defective sodium channel gene is present, the channel becomes "leaky" and makes the muscle
overly excitable and contract involuntarily. The channel becomes "leaky" when potassium levels fluctuate in
the blood. This may occur with fasting followed by consumption of a high potassium feed such as alfalfa.
Hyperkalemia, which is an excessive amount of potassium in the blood, causes the muscles in the horse
to contract more readily than normal. This makes the horse susceptible to sporadic episodes of muscle
tremors or paralysis.

This genetic defect has been identified in descendents of the American Quarter Horse sire, Impressive.
The original genetic defect causing HYPP was a natural mutation that occurred as part of the evolutionary
process. The majority of such mutations, which are constantly occurring, are not compatible with survival.
However, the genetic mutation causing HYPP produced a functional, yet altered, sodium ion channel. This
gene mutation is not a product of inbreeding. The gene mutation causing HYPP inadvertently became
widespread when breeders sought to produce horses with heavy musculature. To date, confirmed cases
of HYPP have been restricted to descendants of this horse.

Symptoms and Signs of the Disease
Homozygous horses are affected more severely than heterozygous horses. Under ideal management
practices, the defective gene does not appear to have adverse effects, but stress and/or increased
potassium in the serum can trigger clinical signs of muscle dysfunction. Why some horses manifest
severe signs of the disease and other exhibit little or no signs is unknown and currently under
investigation. Unfortunately, a horse carrying the defective gene but showing minimal signs has the same
chance of passing the gene to future generations as does the affected horse with severe signs.

HYPP is characterized by sporadic attacks of muscle tremors (shaking or trembling), weakness and/or
collapse. Attacks can also be accompanied by loud breathing noises resulting from paralysis of the
muscles of the upper airway. Occasionally, sudden death can occur following a severe paralytic attack,
presumably from heart failure or respiratory muscle paralysis.

Attacks of HYPP can take various forms and commonly have been confused with other conditions.
Because of the muscle tremors and weakness, HYPP often resembles exertional rhabdomyolysis
("tying-up" syndrome). "Tying-up" syndrome can be caused by many different circumstances, including
exercising a horse beyond the capacity to which it has been trained, as well as nutritional deficiencies and
metabolic diseases. A distinguishing feature of HYPP from "tying-up" syndrome is that horses usually
appear normal following an attack of HYPP. Horses with "tying-up" syndrome, on the other hand, tend to
have a stiff gait and painful, firm muscles of the hind limbs, rump and/or back. "Tying-up" syndrome is also
generally associated with some type of exercise. HYPP, by contrast, is not usually associated with
exercise, but occurs when horses are at rest, at feeding time, or following a stressful event such as
transport, feed changes, or concurrent illness.

Because a horse may be down and reluctant or unable to stand during an HYPP attack, many owners have
thought their horses were experiencing colic. HYPP has also been confused with seizures due to the
pronounced muscle trembling and collapse. Unlike seizures and other conditions that cause fainting,
horses with HYPP are conscious and aware of their surroundings during an attack and do not appear to be
in pain. Respiratory conditions and choking have also been confused with HYPP because some horses
make loud breathing noises during an attack.

Causes of an Attack
Environmental factors can actually cause an attack of muscle weakness. Owners of HYPP-positive horses
should be aware that external stimulus and events could increase the chance of paralysis onset. These
factors include dietary changes, fasting, general anesthesia, and concurrent illness and exercise

Prevention and Control of HYPP Attacks
Dietary management is extremely important in the management of affected horses. Dietary adjustments
include (1) avoiding high potassium feeds such as alfalfa hay, brome hay, canola oil, soybean meal or oil,
and sugar molasses and beet molasses, and replacing them with timothy or Bermuda grass hay, grains
such as oats, corn, wheat and barley, and beet pulp; (2) feeding several times a day; and (3) exercising
regularly and/or being allowed frequent access to a large paddock or yard. Due to the high water content of
pasture grass, a horse is unlikely to consume large amounts of potassium in a short period of time if kept
on pasture. If the horse is experiencing problems on its present diet, it is recommended to feed a diet
containing between 0.6% and 1.5% total potassium concentrations.

Several drugs have been used for prevention of clinical episodes of paralysis. Horses have been treated
with either acetazolamide (2-4 mg/kg orally, every 8 to 12 hours) or hydrochlorthiazide (0.5-1 mg/kg orally,
every 12 hours) with apparent success. These agents exert their effects through different mechanisms;
however, both cause increased renal potassium ATPase activity. Acetazolamide has been shown to
stabilize blood glucose and potassium by stimulating insulin secretion. Breed registries have restrictions
on the use of these drugs during competitions (some require a veterinary certificate).

Inform your veterinarian of the HYPP condition prior to any general anesthesia, as this may precipitate an
episode of paralysis. If your horse is receiving medication, maintain him or her on therapy before and after
surgery or anesthesia. Use common sense while hauling and be sure to stop and water horses frequently
(every two hours).

During a severe attack of HYPP, emergency treatment from a veterinarian is necessary. For long term
therapy, many horses can be managed by exercise and diet control alone. Regular exercise and access to
a large paddock or pasture is preferred over stall confinement. Maintain a regular feeding schedule,
preferably equally spaced, two to three times per day. Avoid rapid changes in feed, such as bringing a
horse off pasture grass and immediately switching to alfalfa hay. Most horses improve when the
potassium content in the diet is decreased.

Inheritance and Transmission of HYPP
HYPP is inherited as an autosomal dominant trait, which means it can occur in both males and females
and only one copy of the gene is required to produce the disease. The trait is inherited from generation to
generation with equal frequency; it does not get "diluted" out or skip generations. Breeding an affected
heterozygous horse (N/H) to an affected heterozygous horse (N/H) will result in approximately 50% carrying
the defective gene (N/H), approximately 25% will be normal (N/N) and approximately 25% will be
homozygous carriers (H/H). Breeding an affected heterozygous horse (N/H) to a normal horse (N/N) will
result in approximately 50% normal offspring and approximately 50% carrying the defective gene (N/H).

Expected Breeding Outcomes N/H x N/H have 25% chance of producing normal offspring, 50% chance of
producing heterozygous carriers and 25% chance of producing homozygote offspring N/H x N/N have 50%
chance of producing normal (NN) offspring and 50% chance of producing carriers (NH).
N H  N H
N 25% NN 25% NH N 50% NN 50% NH
H 25% NH 25% HH  

Breeding an affected homozygote (H/H) will result in all offspring carrying the defective gene
regardless of the status of the other parent.

Myths about HYPP
Some people have felt that the disease can be diluted out and not carried to distant generations. This is
false because an affected horse has just as much chance to pass on the trait as the affected parent which
passed the gene to him. Some people also believe the horse will "grow out of it." This is not true. For
unknown reasons, attacks of HYPP tend to occur most often at the beginning of intense training and fitting
for shows (age three to seven years old). It is important to realize that horses with HYPP are affected for
life. It is possible that older horses do not experience the same conditioning stresses as young horses or
owners have discovered the best management strategies for the older horses with HYPP.

Some people also think that if a horse does not show any signs up to a certain age, it does not carry the
trait. Unfortunately, this is not the case. Once again, horses with HYPP are affected for life. There was a
stallion and a broodmare with HYPP who did not show signs of the disease until age eight and 15, and
both horses only experienced one isolated attack.

Owners and breeders of affected horses should inform prospective buyers of the management constraints
these horses have and the potential for future episodes of HYPP.

Testing for HYPP
A DNA test is available at the Veterinary Genetics Laboratory to identify horses carrying the defective gene
causing HYPP. The test detects the presence or absence of the specific genetic mutation which has been
found in the extended pedigree of "Impressive" descendants. From hair samples with intact roots it can be
determined whether the horse does not carry this specific mutation (a normal horse), or whether it carries
one or two copies of this abnormal gene mutation (heterozygous or homozygous for HYPP, respectively).
Please see the submission instructions for forms, costs and sampling and mailing instructions.

Which Horses Should Be Tested for HYPP?
As noted above, the DNA based test for HYPP identifies the specific genetic mutation which we now know
exists in descendants of "Impressive". We presently do not know whether different genetic mutations in
other bloodlines also cause HYPP, and the DNA blood test will not identify other such mutations. Further
scientific research is required as to other bloodlines. We presently recommend that all descendants of
"Impressive" be tested for diagnostic, treatment and breeding purposes.

We hope this information ha been helpful for our readers.  I hope if you were considering breeding your
HYPP/NH or HH  horse you will reconsider.  

Still not convinced?
CLICK here for touching  HYPP experiences and related links.  Thanks for visiting this info page.
Hyperkalemic Periodic Paralysis (HYPP)