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HYPP is a genetic disease affecting Quarter Horses from the Impressive bloodline. This disease causes a dysfunction in how the electrolytes sodium and potassium move in and out of muscle cells. Horses can have one or two genes for HYPP, and thus can show a varying degree of signs, from mild to severe. Mild signs generally include sweating and twitching in the neck, shoulders, and flank, while more severe signs may consist of muscle trembling and cramping, which can cause the horse to sway, sit, or even go down. The entire event lasts anywhere from 15 to 60 minutes and horses can actually die during episodes due to cardiac or respiratory arrest. Due to the risk of personal injury, only experienced adults should ride and handle horses with HYPP.
Because an imbalance in the blood levels of sodium and potassium can only be seen during an episode, horses related to Impressive that demonstrate muscle tremors, weakness or collapse should be tested for HYPP. This involves having a DNA test of mane or tail hair performed on a quarter horse or quarter horse cross that either has Impressive in the pedigree or has shown any of the signs described above.
The medication most commonly used to prevent HYPP episodes is the human drug acetazolamide, which is a diuretic that helps filter potassium from the blood. If an owner recognizes that an episode is about to occur, they can gently exercise the horse or try feeding something with sugar in order to ward off the attack. Veterinarians that are present during an episode may give acetazolamide and other medications intravenously.
HYPP horses should not be given alfalfa hay, brome hay, molasses, soybean meal, or any other feedstuffs--such as electrolytes--that are high in potassium. Later cuts of timothy or Bermuda grass hay, grass pasture, beet pulp, and grains such as corn, oats, wheat, and barley contain low amounts of potassium and are generally safer to feed. Feeding multiple small meals throughout the day so that no one meal exceeds 33 grams of potassium is also advised. A veterinarian can further help you design a nutritional program that is complete and balanced yet contains the minimum amount of potassium necessary. A consistent exercise program—along with regular turnout--is also important in preventing bouts of HYPP.