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Animal Genetics, Inc. (n.d.). HYPP – Hyperkalemic Periodic Paralysis in Horses. Retrieved October 29, 2024, from https://avian2.animalgenetics.com/Equine/Genetic_Disease/HYPP.asp

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Hyperkalemic Periodic Paralysis (HYPP) is a genetic disorder affecting the muscles of horses, primarily in American Quarter Horses. This disorder leads to sporadic episodes of muscle weakness, stiffness, and sometimes paralysis due to abnormally high potassium levels in the blood (Aleman, 2008).

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HYPP traces back to a Quarter Horse stallion named "Impressive," who was popular in the 1970s for his strong build and muscle mass, traits favored in show competitions. However, it was later found that Impressive was the source of the HYPP mutation, which was transmitted to many of his descendants (Spier & Carlson, 1998).

Some searching revealed HYPP was a genetic mutation of the sodium ion channel gene (SCN4A), which impacts muscle contraction by affecting the flow of sodium and potassium in cells (Rudolph et al., 1992).

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Horses with HYPP experience episodes of muscle tremors, weakness, and, in severe cases, collapse. These episodes are triggered by fluctuations in potassium levels, especially following high-potassium meals (Aleman et al., 2004).

Due to the mutation in the sodium channel gene, potassium ions accumulate in the blood, disrupting normal muscle contractions, leading to symptoms like muscle spasms, excessive sweating, and respiratory issues (Spier & Carlson, 1998).

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Zoetis Equine. (n.d.). Mare & Foal Care Solutions. Retrieved October 29, 2024, from https://www.zoetisequine.com/learn/mare-foal-care-solutions

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Episodes of paralysis can be life-threatening, particularly if they occur during exercise or stress. Severe attacks may lead to sudden death (Rudolph et al., 1992).

HYPP poses ethical concerns within the breeding community. Although carriers often have desirable traits, breeding affected horses continues to spread the mutation (Aleman, 2008).

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Ramey, D. W. (n.d.). HYPP – Hyperkalemic Periodic Paralysis in Horses. Retrieved October 29, 2024, from https://www.doctorramey.com/hypp/

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Genotypes:

H/H (not seen here): Horses with two copies of the mutated gene exhibit severe symptoms.

N/H: Horses with one copy are also affected but may have milder symptoms.

Phenotypes:

The physical manifestation varies from muscle stiffness to full paralysis episodes.

HYPP is a dominant genetic disorder associated with a mutation in the SCN4A gene. A horse only needs one copy of the mutated gene (heterozygous) to exhibit symptoms (Rudolph et al., 1992).

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Genetice test via Laboratoire de Génétique Vétérinaire (LabGenVet). (n.d.). HYPP – Hyperkalemic Periodic Paralysis. Retrieved October 29, 2024, from https://labgenvet.ca/en/hypp-hyperkalemic-periodic-paralysis/

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Breeders now have access to genetic tests to identify HYPP-positive horses, allowing them to avoid breeding carriers (Aleman, 2008).

Many horse organizations, including the American Quarter Horse Association, have restricted the registration of horses with HYPP-positive genes to prevent the disease’s spread (American Quarter Horse Association, 2021).

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Aleman, M. (2008). Pathophysiology of hyperkalemic periodic paralysis in horses. Journal of Veterinary Internal Medicine, 22(5), 1173-1182.

Aleman, M., et al. (2004). Management of hyperkalemic periodic paralysis in horses. Journal of the American Veterinary Medical Association, 224(2), 249-253.

American Quarter Horse Association. (2021). HYPP Disease Information. Retrieved from AQHA

Rudolph, J. A., Spier, S. J., Byrns, G., Rojas, C. V., Bernoco, D., & Hoffman, E. P. (1992). Periodic paralysis in quarter horses: A sodium channel mutation disseminated by selective breeding. Nature Genetics, 2(2), 144–147.

Spier, S. J., & Carlson, G. P. (1998). Hyperkalemic periodic paralysis. Equine Veterinary Journal, 30(5), 380-385.