Horse Health Feature of the Month: HYPP, A Genetic Disease
Identifying and Managing HYPP, a Genetic Disease Originating in One Line of Quarter Horses
by Tracy Williams
The madness began with a single horse. His name was Impressive, and he lived up to his name not only in appearance and show ring success but also by introducing an acronym that would shake the Quarter Horse industry to its core: HYPP. Impressive exploded on the halter horse scene with pizzazz, and it wasn’t long before his excellent conformation and near perfect musculature gave him the show record needed to 
launch him into stud success. Impressive passed along these stellar traits to his offspring, many of whom became strong influences in their own right in the breeding world. Thus, his genes spread quickly until owners began noticing strange muscle tremors in some of his many offspring. Along with his brilliance, Impressive had inadvertently passed along a genetic mutation that due to his popularity seemed to be everywhere.
It’s All in the Genes. Hyperkalemic Periodic Paralysis, more commonly known as HYPP, is strictly a genetic disease, meaning that it is not contagious and is only transmitted from parent to offspring. Unfortunately, it is a dominant trait; a horse needs only to carry one copy of the dominant allele to be affected and to pass the disease to its offspring. A horse that carries two copies of the dominant gene will pass the disease to its offspring 100 percent of the time, regardless of whether or not the other member of the breeding pair has the defective gene. Breeding a horse that carries only one copy of the mutation (heterozygous for the trait) to a horse that is free from the disease gives the breeder a 50/50 shot at having an HYPP foal. Breeding two heterozygous horses gives you only a 25 percent chance at having a healthy foal.
The disease began as a simple mutation, a normal step in the evolutionary process. This particular mutation causes a disruption in a muscle protein called a sodium ion channel. In a normal horse, the sodium ion channel opens and closes to allow sodium and potassium in and out of muscle cells, a process that is vital for the electrical activity responsible for muscle contraction and relaxation. In a horse affected by HYPP, the sodium ion channel malfunctions, allowing large influxes of sodium into the cell during an episode. This skews the sodium/potassium balance which in turn alters the voltage current in the cell causing uncontrollable muscle twitching, tremors and paralysis. Thus, high blood levels of potassium are usually present during an episode and can help pinpoint HYPP as the cause.
A Symptom Spectrum. Horses can exhibit a wide range of severity in their symptoms. Some affected horses appear clinically normal and have successful show careers, while others need daily medication to control attacks. A horse that carries two copies of the mutation seems to have more violent episodes, and these horses with severe symptoms also have numerous sodium channel mutations in their skeletal muscle. It is a muscle disease, so an episode can consist of weakness, tremors, paralysis, respiratory noises (from paralysis of airway muscles) and even collapse. In severe cases, horses can die from respiratory failure or cardiac arrest. These episodes tend to be unpredictable and can last anywhere from minutes to hours depending on the particular case.
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