HYPP

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HYPP

• By Whitney Evans
• Christina Read

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Why is it called HYPP?

• Hyperkalemic Periodic Paralysis
• Hyperkalemia High level of potassium in the
  blood
• Periodic Horses have an irregular capacity to
  regulate potassium
• Paralysis Occasionally leaves the horses unable
  to rise

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Who is Affected?

• Descendants of the American Quarter Horse sire,
  Impressive
• Impressive, 0767246 was so strikingly muscled he
  never lost a halter class.
• He became a very important breeding stallion due
  to his ability to pass on his musculature to his
  offspring.
• Over 50,000 registered Quarter Horses are related
  to known carriers of the disease.

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Impressive, 0767246
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• Persists to this day in
• Quarter Horses and breeds with Quarter Horse
  blood
• Appaloosas
• Paints
• Palominos

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Inheritance of HYPP

• Began as a natural mutation that occurred as part
  of the evolutionary process.
• This point mutation which results in an
  alteration of a single amino acid in the alpha
  subunit of the sodium channel protein has
  continued to Impressives descendants.
• HYPP in the horse is a dominant trait.
• The possible genotypes are (H/H) homozygous
  carriers, (N/H) heterozygous carriers, and (N,N)
  normal.

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• Thus, the trait will show itself with both
  homozygous dominant and heterozygous phenotypes
  (N/H) and (H/H).
• Homozygous carriers are known to be more severely
  affected than heterozygous horses.

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HYPP at the Molecular Level

• While potassium is regularly found at high levels
  intracellularly, sodium in general, is at higher
  levels outside of the cells such as in the blood.
  
• This mutation in the sodium channel gene causes
  the channel to become leaky in conjunction with
  small fluctuations of blood potassium levels.
• As the sodium channels open, they begin to leak
  sodium which then causes potassium to flow into
  the blood stream.

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• With this increase potassium flow, the muscle
  first becomes hyperexciteable and contracts more
  readily than normal muscle.
• As increased amounts of potassium flow into the
  bloodstream, the muscles become fatigued and
  unable to contract.

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Signs of an HYPP Attack

• Muscle Tremors Causing Trembling/Shaking
• Prolapse of the Third Eyelid
• May notice third eyelid flashing across the eye
  or covering a larger majority of the eye than
  usual.
• Overall Weakness
• Increased Fatigue in the Hind Limbs
• Horse may be sitting back like a dog
• Complete Collapse
• Unusual Whinny
• Sudden Death is Possible

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Video of an HYPP Attack

• Link to YouTube Video of HYPP Attack

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Causes of an Attack

• Environmental Stimulus Can Increase Chance of
  Attack Including
• Changes in Diet
• Fasting
• Waking up from Anesthesia
• Illness
• Irregular Exercise Routine/Exercise Restriction

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Severity of the Attack

• Mild
• Little Muscle Trembling
• No signs of recumbence, vocal problems, or
  increased fatigue
• May be treated at home by a comfortable owner
• Moderate to Severe
• Full Symptoms are Expressed
• Veterinary Assistance is Required

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Diseases Commonly Mistaken for HYPP

• Exertional Rhabdomyolysis (Tying-Up Syndrome)
• Marked by aching, stiff muscles which continue
  for days to weeks following the episode,
  sweating, and high muscle enzymes.
• HYPP horses will appear normal following attacks
  and they will have normal muscle enzyme levels.
• Seizures
• Unlike with seizures, HYPP horses remain
  conscious and alert of their surroundings during
  an attack.

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• Cardiac Problems
• These horses will often present additional signs
  such as unusual jugular pulsations, peripheral
  edema, increased weakness, or cardiac murmurs or
  arrhythmias.
• Respiratory Conditions/Choking
• Horses often make loud breathing noises during an
  attack due to the relaxation of all muscles
  including the voicebox.

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Diagnosis

• Blood potassium levels will be extremely high at
  the time of attack.
• Specialized neurodiagnostic examinations can help
  confirm a diagnosis because the muscles are
  actually performing abnormally at all times.
• Not visible to the naked eye as during an attack,
  abnormalities can be detected using an
  electromyogram or EMG.

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• This tests senses spontaneous activity in the
  muscle due to miniscule potassium and sodium
  fluctuations.
• Definitive genetic tests are now available which
  looks for the gene for HYPP.
• Necropsy
• Histologically one can note the vacuolization in
  type 2B muscle fibers with mild degenerative
  alterations.
• An electron microscope will show dilations of the
  sarcoplasmicreticulum of myofibers.

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Treatment of Mild Attacks

• Lightly Exercising the Horse
• Carbohydrate Feeding such as Corn, Syrup, or
  Sugar
• Gets the body to release insulin which then
  encourages the movement of potassium back into
  the cells.
• If a veterinarian has already been consulted, the
  drug acetazolamide can be given.
• Diuretic drug which causes the horse to urinate
  thus expelling some of the potassium in the urine
  
• 2-4mg/kg orally every 8 to 12 hours

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Treatment of Moderate/Severe Attacks

• Intravenous Dextrose
• As will oral carbohydrates, this will promote the
  movement of potassium back into the cells.
• Dextrose and Bicarbonate Intravenously
• Bicarbonate has an additive effect with the
  dextrose.
• Intravenous Calcium
• May help contradict the consequences of the
  potassium.

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• Insulin
• Will boost what the horse is already producing in
  response to the dextrose.
• Acetazolamide
• Promotes urination in order to release excess
  potassium.

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Prevention

• Continuous Administering of Acetazolamide
• This drug has very few adverse effects, is quite
  inexpensive, and is proven to reduce the amount
  and severity of the attacks.
• Low Potassium Diet
• Oats, Corn, Wheat, Barley, and Beet Pulp
• A diet with 0.6 to 1.5 total potassium
  concentrations is recommended
• Avoiding High Potassium Foods
• Alfalfa, Canola Oil, Soybean Meal, Molasses,
  Timothy Hay

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• Regular Exercise
• Regular Feeding Schedule
• Use Common Sense
• Especially while traveling, make mandatory stops
  every few hours for water.

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Common Myths about HYPP

• The disease can be diluted out.
• False, each affected horse has the ability to
  pass the gene on to its offspring.
• Horses grow out of it.
• False, Attacks are most common in younger horses.
  However, this could be due to additional
  conditioning stresses or the fact that owners
  have the disease better managed in the horses
  older age.

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• If a horse has not shown the disease by a certain
  age, it is not affected.
• False, Horses with HYPP are affected for life and
  just because the gene doesnt show does not mean
  it is not there.

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Which horses should be tested?

• HYPP is listed as a genetic defect in the
  American Quarter Horse Associations (AQHA)
  rules.
• Foals born since 1998 that are traced to
  Impressive will have an account placed on their
  registration certificates which recommends
  testing for the condition.

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• AQHA will test any foals required to be parentage
  confirmed and who trace to Impressive prior to
  them being registered.
• AQHA accepts HYPP results only from a list of
  eight licensed laboratories.
• Home AQHA testing kits can be ordered for just
  40.00.

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Sources

• "AQHA's HYPP Fact Sheet." Hyperkalemic Periodic
  Paralysis. American Quarter Horse Association. 28
  Feb 2008 lthttp//www.aqha.com/association/registra
  tion/hypp.htmlabgt.
• Cawood, Kevin. "HYPP - Still With Us." Animal
  Disease Diagnostic Laboratory. 1995. Purdue
  University. 26 Feb 2008 lthttp//www.addl.purdue.ed
  u/newsletters/1995/hypp.shtmlgt.

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• "Hyperkalemic Periodic Paralysis (HYPP)." Tufts
  Cummings School of Veterinary Medicine.
  2005-2006. Tufts University. 26 Feb 2008
  lthttp//www.tufts.edu/vet/sports/hypp.htmlgt.
• "Hyperkalemic Periodic Paralysis (HYPP)."
  Veterinary Genetics Laboratory. 2008. University
  of California at Davis. 26 Feb 2008
  lthttp//www.vgl.ucdavis.edu/services/hypp.phpgt.