In fact because of them are negative horses are kept on a similiar diet - which does not contain processed foods, like many horse owners feed their horses. We breed these mares, because they are huge in muscle mass, and are producing just as huge babies some of which are positive and some which are negative. I would strongly suggest you talk to the breeders - the real professional breeders, not those people out there that have a pet horse in their backyard and breed to their neighbors stud. Yes I agree with you that as a pet horse this might be something that a layman without much experience and only wants to ride around occassionally, and feed whatever, an HYPP positive horse might not be a good choice. Amy Scovil Kelowna, BC, V1W 4C8, Canada Amy, Thanks for an amusing email.
Lets address some things.
Myopathic mutations affect differently the inactivation of the two gating modes of sodium channels. Hyperkalemic periodic paralysis caused by recurring mutation in the adult muscle sodium channel alpha-subunit gene. I think your word recent is correct. I don't think the rule change was in effect yet. That does not change the fact that it is a genetic mutation.