A very handsome horse caught our eye the other day (no, not as a prospective addition to our herd – we’ve reached our limit) – a big, muscled, Quarter Horse stallion. He was something else. Bill wondered why he wasn’t gelded, and if he might be used as a stud someday. I told him that the owner might go ahead and geld him, because he comes from the “Impressive” line. Bill, naturally, was confused by that: “Well, if he’s from an impressive line, why wouldn’t you breed him?” But unfortunately, it’s not an impressive line, it’s the Impressive line, as in “Impressive,” AQHA stallion and halter champion, famous for siring 2,250 foals. He is also considered to be the beginning of a bloodline carrying a debilitating and often fatal genetic mutation known as HYPP, for Hyperkalemic Periodic Paralysis. Sounds scary, huh? It really is.

I had to look up “hyperkalemic.” “Hyperkalemia” is an abnormally high concentration of potassium in the blood. This condition is caused in HYPP horses because their potassium regulation system is messed up. The controls get out of whack every once in a while (“periodically”) and cause a big surge of potassium. This causes the “paralysis” part of the name: potassium controls the voltage currents in muscle cells, so the potassium influx is sort of a “power surge” like the kind that can fry your electronics. The horse loses muscle control to some degree; the severity varies quite a bit, from little twitches to a fatal heart attack (the heart is a muscle too). Scary enough for you?

There are a few important things to know about HYPP. One is, it’s a genetic defect, so your horse cannot “catch” it; they are born with it. Also, the gene is a dominant one – only one parent has to carry the gene for it to be passed on to the offspring. Horses can have HYPP and show no signs of it; only a DNA test can tell whether your horse has it or not. As I said, HYPP is believed to have come from the Impressive line (the most recent count of his living descendants was 55,000 in 2003), so it’s most common in Quarter Horses, but is also found in the closely-related Paint Horse and Appaloosa lines, and of course can be found in other breeds if they have been cross-bred to Quarter Horses.

HYPP can be manageable, depending on the severity. Since it is a potassium issue, diet is very important. Also, stress can trigger an episode, so the horse’s environment and lifestyle are important factors. There are also medications that can ease symptoms and help prevent episodes.

A critical thing to consider if you or someone you know has an HYPP horse is that an attack can occur at any time, including when you are riding the horse. Any rider should be very experienced and always alert for signs, because your horse could literally collapse underneath you.

Now for the controversial stuff. How can HYPP be prevented or even wiped out? Obviously, since it’s an issue of genetics, if HYPP-positive horses don’t breed, the mutation can’t be passed on. That sounds simple, but while the downsides of HYPP are scary, the flip side of the Impressive line is that the horses bred from that line are highly muscled, very strong, show-winning horses. That’s how Impressive came to sire 2,250 foals – he was, indeed, impressive. So there will always be breeders willing to take that gamble for a desirably conformed horse. For now, the only control to prevent HYPP-positive foals is that when a horse is registered with the American Quarter Horse Association (AQHA), its HYPP status is included in that registration. Any potential breeder will know if the horse is a risk or not.

Well, there’s my very unprofessional, Internet-educated opinion of Impressive and HYPP. If this issue might affect you, I encourage you to do your own research. I, for one, may look into having Romeo (Appaloosa) and perhaps even Moonshine (part Quarter Horse) tested. Even though they surely would have shown signs by now, my web research has once again made me paranoid.