Genetic Defects and Model Horses - Hyperkalemic Periodic Paralysis

Genetic Defects and Model Horses - Hyperkalemic Periodic Paralysis
by Long Road Home

What is Hyperkalemic Periodic Paralysis?

Also known as HYPP, this condition is a dominant genetic disorder found in horses (and humans) that has serious health consequences for sufferers. Violent muscle twitching, and substantial muscle weakness or paralysis is characteristic of HYPP. During an episode, a horse may collapse due to muscle weakness, and if this happens while the horse is being handled or ridden, it can be very hazardous to the rider. It doesn't take a genius to understand that the heart and diaphragm are muscles. The diaphragm is responsible for the act of breathing itself; death by suffocation may occur if the diaphragm becomes paralysed during an HYPP episode.

Where does it come from?

This disease is thought to have been propagated through many of the stock horse breeds thanks to the very prolific sire, Impressive.

How is HYPP relevant to model horse hobbyists?

Because of the way HYPP works, the horse's muscles are constantly more contracted, rather than relaxed. This gives the horse the appearance of a big, beefy bodybuilder. An HYPP positive horse looks impressive. That's desirable in the show ring, so breeders have continued to breed for it, despite the health risks.

HYPP H/H horses look very drastically muscled, like Arnold Schwarzenegger in his Mr universe days. Homozygous HYPP horses have rarely won anything in the halter show ring, perhaps because their muscling is so big as to appear quite grotesque.

HYPP N/H horses are much more subtle. Where an H/H horse looks like Arnold Schwarzenegger, an N/H horse will have muscling similar to Vin Diesel.

How does this affect model horse judging?

To answer this question, we should first look at real horse organizations and their responses.

The AQHA mandates testing for the "Impressive" mutation, and does not register homozygous (H/H) foals. Discussion about the registration of heterozygous (N/H) foals is pending.

The ApHC likewise will not register homozygous foals.

Both primary palomino registries will not register any foal carrying the "Impressive" mutation, whether homozygous or heterozygous.

The APHA has yet to take action on the issue.

If we follow the real world example, horses other than Paint Horses, that are clearly H/H would be non-registrable. Ideally, they shouldn't place in model horse show breed classes, as the registry would not allow them to be registered.

N/H horses may still show in the real world. Here, the judge should use their best judgment. Personally, I don't like placing N/H horses over N/N horses, I feel horses with deadly genetic disorders do not represent the breed ideal.
          
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