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

Historical, clinical and experimental data were collected from 9 horses hom ozygous for HYPP (WH), All showed episodes of respiratory stertor, describe d 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 a ccompanied by drooling and in 3 by weight loss. In comparison, only one of 35 contemporaneous half siblings (of which approximately half would be expe cted to be of the H/N genotype and half N/N) was observed to have respirato ry stertor prior to weaning and none had problems with dysphagia, One mature homozygous stallion was infertile secondary to urospermia. Six h omozygous horses died or were subjected to euthanasia; 4 age less than a ye ar, 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 potas sium chloride challenge testing indicated that during attacks homozygotes s howed significantly more frequent signs of drooling, prolapse of the third eyelid, respiratory strider and weakness than heterozygous horses. Homozygo tes also had significantly more total abnormalities (including myotonic dis charges, high frequency repetitive discharges, and spontaneous activity) on electromyographic examination than heterozygotes. These data imply that HYPP is inherited as a codominant genetic defect, bec ause the homozygotes showed more severe clinical signs of disease than hete rozygotes. Homozygous foals would be expected to be produced in 25% of mati ngs in which both parents are heterozygous. Owners and veterinarians should be aware of the risks of this mating.