Breeds with HYPP; Quarter Horses, American Paint Horses, Appaloosas, and Quart Horse Crosses.
HYPP
"An inherited disease of the muscle, caused by a genetic defect. A mutation in the sodium channel gene. 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 become “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."
GENOTYPE
HYPP Positive= H/N and H/H
HYPP Negative= HYPP N/N
HYPP horses with H/N genotype means they are heterozygous carriers, carrying one copy of the HYPP gene. If you bred a mare who is H/N to a stallion who is N/N, you would have about a 50% chance the foal is H/N as well. However if you bred an H/N mare to an H/N stallion you will have about a 25% chance the foal being N/N, a 50% chance it will be H/N, and a 25% chance it will be H/H.
HYPP horses with the H/H genotype means it is homozygous, carrying both copies of the HYPP gene. 100% passing the HYPP gene to it's offspring.
I made a thing in case it helps the visual learners out there
IMPRESSIVE (but not really)
Let's talk about the QH stallion, Impressive. All horses that have a HYPP positive gene of any kind, all descend from this horse. Impressive sired 2,250 foals and they estimate he has around 50,000 living descendants. No dilution of lineage will remove HYPP, as it is a dominant trait and will show whether you are 5 generations back or 2.
HYPP SYMPTOMS
HYPP varies in severity, it's not always equal and every horse may have different reactions. HYPP can be confused for tying-up or even colic, they may have difficulty breathing, muscle tremors, sweating, weakness, tremors. In more severe cases the horse may collapse from paralysis of the muscles in the airway and can result in sudden death.
Not every horse who is HYPP positive may display symptoms, making this disease sometimes hard to detect.
IS IT CURABLE?
No. HYPP may be managed with diet and certain medicines for maintenance but it cannot be cured. H/H positive horses have poor prognosis and can be much more difficult to manage.
VIDEOS OF HYPP ATTACKS
CW: Horses in distress, videos may be hard to watch for some.
Video 1
Video 2
Video 3
Video 4
Video 5
Sources (and if you want to read more); AAEP , UCDavis, Tri-State
Is there anything I forgot? Anything to add? Just covering the basics of this disease and what it does to these horses.