Hyperkalemic periodic paralysis (HYPP)
Current estimates indicate that 4.4% of the Quarter Horse breed may be affected.
HYPP is a dominant disease (one copy of the mutation is sufficient to produce disease) affecting Quarter Horses, American Paint Horses, Appaloosas, and Quarter Horse crossbred animals worldwide. The genetic disease has been linked to the Quarter Horse sire Impressive. Current estimates indicate that 4.4% of the Quarter Horse breed may be affected.
Clinical signs consist of intermittent muscle twitching, shaking, trembling, weakness, and even collapse. Signs usually begin by 2 to 3 years of age. Severely affected horses may have difficulty swallowing or make loud breathing noises as a result of muscle failure in the throat region. Occasionally, sudden death can occur following a severe episode. In most cases, elevated potassium blood concentration occurs during clinical manifestations of the disease. HYPP horses may appear normal between episodes. HYPP is due to a mutation in a key part of the skeletal muscle sodium channel. The channel becomes “leaky” and causes the muscle to become overly excitable and contract involuntarily when potassium levels fluctuate in the blood. This may occur with stress and/or fasting followed by consumption of a high potassium feed such as alfalfa.
Horses that are homozygous for HYPP (horses with two copies of the gene mutation for HYPP) are affected more severely than horses that are heterozygous for HYPP (those horses that inherited only one copy of the gene mutation). Unfortunately, a heterozygous horse carrying the defective gene, but showing minimal signs of the disease, has the same chance of passing the gene to future generations as does the heterozygous horse with severe signs of the disease.
Owners of affected horses should be strongly discouraged from breeding these animals for the long-term health of the Quarter Horse breed and other related breeds. Breeding an affected horse to a normal horse will result in a 50% chance of producing a foal with HYPP. Beginning with 2007 foals, the American Quarter Horse Association no longer registers Impressive progeny who are homozygous (horses with two copies of the gene mutation).
Testing for the HYPP mutation is performed at the Veterinary Genetics Laboratory at the University of California, Davis on mane or tail hair. Quarter Horse foals born after 1998 that are offspring of an affected parent have a statement recommending DNA testing for HYPP on the Certificate of Registration.