HYPP...where to begin. I'm a diehard AQHA member and have been for decades, so I have to applaud AQHA for being the ONLY breed association to take a stand on it, a stand that started many years ago with denying H/H horses to be registered. SO the halter industry was forced to re-think their breeding practices. And HYPP positive, DOES...many times, come with more muscle. In HYPP positive horses, the muscles "fire" many times more than a non-HYPP horses due to the elevated potassium that leaks into the muscles. But because so many of the Impressive breds carry a lot of muscle anyway, adding some HYPP can accentuate the muscle load....so halter people sought it out. And because MOST attacks happen when the horse is "at rest"....you don't often see attacks happen when the horse is at work; actually exercise restriction is NOT what you do with HYPP positive horses, exercise is VERY important.
I've seen attacks that were scary where some just involved the skin rippling like fingers down the neck. Two horses I've had first hand dealings with died as a result of attacks. One of them was prone to "colic" when this horse was actually having attacks. The owner, a good friend of mine, refused to have him tested....said he was just colic prone. I knew otherwise because HYPP attacks can resemble colic). When she was in Florida, I was running the barn (my horse was there because I boarded at her place), he had an attack, got cast in his stall, was all busted up and I knew he was not going to recover from this one. The vet who was a very good friend of mine came out, we looked at each other and knew it was not good. 4 hours later, we had to put him down.
Another point....HYPP is not exclusive to just QH's, as we all know due to outcrossings to APHA and ApHC. But how many unregistered crossbreds are out there with HYPP? Backyard breds etc. This is why registration papers are so important, yet people say papers don't matter. Well, I sure think they do....and I will not own a non--papered horse because I have a bloodline history of that horse.
Someone mentioned Viital Signs Are Good. Yep, she is an incredible WP show mare and broodmare, however she is N/H. She's mostly Zip bred, but has the two lines that goes Impressive through Mr Conclusion (n/H) and Impressive Mint (N/H). While as worthy as she is in her own right to BE a broodmare, I still have an issue with it because of the N/H. She does have some N//H babies, one of the most famous being Invest N Vital Signs, a gorgeous and talented 2005 stud, N/H and dead at the age of 5, he was found dead in his stall.