Introduction/Causative Agent:
HYPP is a genetic disease that affects horses that are descendants of the Quarter Horse stallion called Impressive. It is seen in most horses by the time they reach 2-3 years of age. This disease gets its name because of the excess levels of potassium (hyperkalemia) that are found outside of muscle cells in the body. In the normal horse, higher levels of potassium are found inside muscle cells and sodium levels are higher outside of the cells. In HYPP horses, the excessive potassium outside and the excessive sodium inside the cells causes the muscle to persistently depolarize and not respond properly to nerve stimulation. This results in a temporary weakness in the muscles of the body. In occasional cases the problem is stimulated by stress (a strenuous workout, fasting, etc.).Clinical Signs:
The signs noted in most horses are related to weakness and temporary paralysis of the various muscles of the body. This results in many different clinical signs and sometimes life threatening complications that can last anywhere from 15-60 minutes. Between different episodes, the horse may act completely normal. Most horses begin an episode with generalized weakness, muscle fasciculations (twitching), sweating, and prolapse of the third eyelid. This can progress into "dog sitting" or collapse, with the horse laying on its side (lateral recumbency). Some horses experience severe breathing problems due to the paralysis of the upper respiratory muscles and can die during an extreme episode.Diagnosis:
Most cases are diagnosed based on the clinical signs and any tie to the Impressive stallion in the genetics. If blood samples are taken during the episodes, elevated potassium levels can be found. Realize that a few HYPP horses can have normal levels of potassium, while some non-HYPP horses with kidney disease or under very strenuous training can have elevated potassium levels. Recent research advances can detect an abnormality in the DNA of a horse with HYPP. This test is performed by select laboratories using a blood sample collected from the suspect animal. In years past, a test called a potassium challenge was used to determine HYPP horses. Because of the potential for causing death in the animal, this procedure is no longer recommended.Treatment/Prevention:
In acute cases intravenous administration of dextrose, along with sodium bicarbonate or calcium gluconate, can help. Administration of a diuretic called acetazolamide is also of benefit. Reducing the amount of potassium consumed in the diet and avoiding things that cause stress (fasting, changes, etc.) can help in many cases. Diets low in potassium include some commercial pellets, oats/oat hay, barley, and corn. Because alfalfa is high in potassium, it should be avoided. Regular exercise and feeding smaller, more frequent meals can also help prevent problems. Many horses with HYPP live a normal life except for the occasional episode of HYPP; however, in most cases, the problem can never be completely removed. Because of this, horses with HYPP should not be bred.