Equine hyperkalaemic periodic paralysis (HYPP) is found in a large family of Quarter Horses (QH) tracing back to the stallion ‘Impressive’, a sire that has been the foundation of many QH, Appaloosa, Paint, Palomino, Buckskin and other breeding programmes for over 30 years (Bowling, Byrns and Spier, 1996). As a result, there are over 60,000 descendants of ‘Impressive’ potentially affected with HYPP (Reynolds et al., 1998b).
The condition closely resembles Gamstorp’s disease in man and is characterised by intermittent episodes of muscular fasciculations, weakness, myotonia, involuntary recumbency and dyspnoea all contributing to decreased exercise tolerance (Naylor, Jones and Berry, 1993; Spier et al., 1995b; Steele and Naylor, 1996). Episodes will occasionally be accompanied by respiratory stridor or upper-airway obstruction caused by laryngeal or pharyngeal paralysis (Carr et al., 1996; Maxson-Sage et al., 1998). Less commonly, sudden death can occur following a severe paralytic episode. Some HYPP cases have led to euthanasia due to increased frequency of attacks, or episode-induced injuries such as broken bones (Reynolds et al., 1998a).
It is now known that the gene for the predisposition for HYPP is inherited as an autosomal dominant disorder (Spier et al., 1992) caused by a single base substitution of guanine for cytosine in the DNA. This substitution results in a change in amino acid (leucine instead of phenylalanine), resulting in the physically or electrochemically generated leakage of sodium into muscle cells through the alpha sub-unit of the voltage-dependent sodium channel protein of membrane pores that would normally closed without nervous stimulation (Rudolph et al., 1992a,b).
HYPP exhibits incomplete penetrance, resulting in clinical symptoms ranging from nothing to periodic paralysis and death. This suggests that environmental factors can influence the clinical symptoms of the disorder in genetically predisposed horses. Regular exercise, turnout, and low potassium diets have been reported to result in decreased HYPP symptoms (Spier et al., 1992; Reynolds et al., 1998a,b).
Because individual reactions vary considerably, diagnosis of the genetic mutation for HYPP does not automatically mean serious consequences for the individual horse. Afflicted horses can lead productive, useful lives and may even grow out of their susceptibility to clinical attacks if properly managed. However, since ‘Impressive’ descendants are so numerous, and as HYPP is inherited as a dominant condition, it can and is being spread to other breeds. The American Quarter Horse Association (AQHA) has recognised that registration events are the ideal time and place to exert influence over the genetic condition of the breed, and has taken action to limit the detrimental influence of HYPP on its members.
As evidence suggests that homozygotes (H/H) are more severely affected than heterozygotes (Spier et al., 1995b; Maxson-Sage et al., 1998; Naylor et al., 1999), the AQHA’s decision to recognise HYPP as a genetic disorder and to increase awareness of the need for testing should be welcomed (Jones, 2004). Without this change, selection for the trait of HYPP might continue, the chance of mating two heterozygotes will increase, resulting in more homozygotes being born. It is therefore beneficial to selectively breed HYPP out of existence before it becomes so widespread that this becomes impossible.