Hyperkalaemic periodic paralysis in homozygous and heterozygous horses: a co‐dominant genetic condition

Abstract

Summary Historical, clinical and experimental data were collected from 9 horses homozygous for HYPP (H/H). All showed episodes of respiratory stertor, described as a rattling or honking sound, usually within the first week post partum. Five horses had one or more episodes of dysphagia, in 3 horses this was accompanied by drooling and in 3 by weight loss. In comparison, only one of 35 contemporaneous half siblings (of which approximately half would be expected to be of the H/N genotype and half N/N) was observed to have respiratory stertor prior to weaning and none had problems with dysphagia. One mature homozygous stallion was infertile secondary to urospermia. Six homozygous horses died or were subjected to euthanasia; 4 age less than a year, one age 20 months and one age 5 years. The remaining 3 cases were still alive at the end of the study. A comparison of homozygous and heterozygous horses using standardised potassium chloride challenge testing indicated that during attacks homozygotes showed significantly more frequent signs of drooling, prolapse of the third eyelid, respiratory stridor and weakness than heterozygous horses. Homozygotes also had significantly more total abnormalities (including myotonic discharges, high frequency repetitive discharges, and spontaneous activity) on electromyographic examination than heterozygotes. These data imply that HYPP is inherited as a co‐dominant genetic defect, because the homozygotes showed more severe clinical signs of disease than heterozygotes. Homozygous foals would be expected to be produced in 25% of matings in which both parents are heterozygous. Owners and veterinarians should be aware of the risks of this mating.

Clinical Approach to Commonly Encountered Problems

“…213,214,223,226,227,[295][296][297] Upper airway obstruction is another important cause of respiratory distress in horses, especially when oxygen demand is increased by exercise. 302,303 Infrequently, laryngeal paralysis has been associated with hepatic dysfunction. 257,[298][299][300] Intraluminal masses and arytenoid chondritis cause fixed upper airway obstruction and produce inspiratory and expiratory respiratory distress.…”

“…For example, American Miniature Horses are at increased risk of tracheal collapse. 302,303 Inadequate lung development, surfactant deficiency, persistent pulmonary hypertension, and meconium aspiration are problems unique to neonatal foals. 298,299 In Quarter Horses and related breeds, some foals affected with hyperkalemic periodic paralysis will show signs of respiratory stertor usually within the first week of life as a result of laryngeal dysfunction, especially if homozygous.…”

“…355,375,376,378 At this time the precise genetics have not been determined. 302 Other electrolyte abnormalities in addition to hyperkalemia may result in collapse including hypocalcemia, hyponatremia, and hypokalemia. 355,378 Other A number of other disorders can occasionally result in collapse.…”

Equine clinical genomics: A clinician's primer

“…Continuous depolarisation of myocytes ensues, manifesting clinically as transient paralysis (Cannon et al 1995; Rudolph et al 1992a; Rudolph et al 1992b). The mutation is inherited in an autosomal co-dominant pattern (Naylor et al 1999). …”

“…Severity of signs may range from asymptomatic to daily episodes to death. Homozygous animals may experience dysphagia and respiratory obstruction and may show more severe signs at an earlier age (Naylor et al 1999). Transient, infrequent episodes may resolve without treatment but oral corn syrup may be beneficial to initiate insulin release.…”