Hyperkalemic periodic paralysis (HYPP) is a genetic muscle disorder in Quarter Horses and related breeds, causing episodes of muscle weakness and paralysis that cause significant suffering in affected animals.
HYPP represents a case study in how selective breeding for cosmetic traits causes inherited suffering. Impressive was heavily muscled, a trait prized in halter competitions, and was bred extensively — spreading the HYPP mutation to tens of thousands of horses before the genetic basis was understood. Affected horses experience unpredictable episodes of paralysis and respiratory distress that are frightening and painful. Homozygous horses (H/H) experience more severe and frequent episodes. Management with low-potassium diets and medication reduces but does not eliminate episodes. The welfare solution requires refusing to breed HYPP-positive horses and selecting against the mutation — an outcome complicated by the economic value placed on heavy muscling in halter competition.