Ideally, HYPP horses should consume forages that have been tested to ensure low potassium levels. Access to pasture might be beneficial, but make sure you know the pasture's potassium content. | Photo: iStock

Q: I have the opportunity to adopt a mare with HYPP. I would use her as a trail horse. Do you have any suggestions on crafting her diet to keep her healthy and relatively symptom-free?

A: Hyperkalemic periodic paralysis (HYPP) exists in horses descended from the Quarter Horse stallion Impressive. About 4% of Quarter Horses are thought to be affected. The condition is the result of a single point mutation in the sodium channel gene which causes the channel to become “leaky” when blood potassium levels fluctuate. Therefore, management centers around the horse’s diet in the hopes of maintaining a steady blood plasma potassium concentrations.

In affected horses high potassium levels cause sodium to leak in to muscle tissue through the defective channels. This causes muscle spasms and other signs, including muscle stiffness, third eyelid prolapse, involuntary recumbency (inability to rise), and sometimes death. When cellular sodium levels rise, cells push out potassium in an attempt to maintain the correct concentration gradient between the outside and the inside of the cell. Unfortunately, this causes the potassium concentration outside the cell to rise further resulting in a vicious cycle. The kidneys work to excrete the excess potassium, but if they’re unable to the horse’s heart rate slows and death is