Hyperkalemic Periodic Paralysis (HYPP)

Hyperkalemic periodic paralysis (HYPP) is a hereditary disease occurring in Quarter Horses and their crosses.

HYPP was the first equine genetic disease for which the causative mutation was identified. Pedigree analysis revealed the first stallion that spread the mutation throughout the breed gene pool.

HYPP is inherited as an autosomal incompletely dominant disorder. Heterozygous affected horses generally have milder symptoms than homozygous affected horses, although the severity of clinical signs can vary considerably.

In homozygous horses, the first signs usually appear earlier, whereas in heterozygous horses they generally occur later, most often at 2–4 years of age.

The HYPP test is one of the six tests recommended as part of the AQHA panel.

In homozygous horses, clinical signs may appear as early as the first weeks of life and include weakness, muscle twitching and cramps, difficulty breathing or swallowing, paralysis, and sudden death due to respiratory or cardiac failure.

Attacks may be triggered by feed with a high potassium content, such as alfalfa, molasses, or soy, by rapid changes in the diet, stress, or rest following exercise.

After an attack, the horse shows no clinical signs.

In heterozygous horses, the onset of symptoms is later and the frequency of attacks varies.

Horses diagnosed with HYPP require a regular daily routine, all-day access to pasture, a potassium-free diet, medication and supplements. The prognosis and frequency of attacks may be related to the level of care provided.

The mutation responsible for HYPP is SCN4A g.15474228C>G. The SCN4A gene encodes a sodium ion channel. The mutation causes increased sodium permeability and potassium accumulation, resulting in abnormal, prolonged muscle contractions followed by paralysis.

Because HYPP is an incompletely dominant disorder, a single copy of the mutation is sufficient to cause clinical signs.

Homozygous H/H horses are not registered and must not be used for breeding. Heterozygous N/H horses may be used in breeding.

Genotypes:

N/N – healthy, no mutation.

N/H – affected, milder symptoms, transmits the mutation to offspring.

H/H – affected, severe symptoms, transmits the mutation to offspring.

If your horse has a risk genotype, contact your veterinarian and discuss the appropriate next steps and care.

More information about HYPP can be found on the AQHA website.

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References:

Bowling, A.T.; Byrns, G.; Spier, S. Evidence for a Single Pedigree Source of the Hyperkalemic Periodic Paralysis Susceptibility Gene in Quarter Horses. Animal Genetics 1996, 27(4), 279–281. DOI 10.1111/j.1365-2052.1996.tb00490.x

Naylor et al. Hyperkalaemic Periodic Paralysis in Homozygous and Heterozygous Horses: A Co-Dominant Genetic Condition. Equine Veterinary Journal 1999, 31(2), 153–159. DOI 10.1111/j.2042-3306.1999.tb03809.x

Tryon et al. 2009. Evaluation of allele frequencies of inherited disease genes in subgroups of American Quarter Horses. JAVMA 234(1), 120–125.